CALIFORNIA
AGRICULTURAL EXTENSION SERVICE

CIRCULAR 97
February, 1936

WEED CONTROL

W. S. BALL, A. S. CRAFTS,
B. A. MADSON, AND W. W. ROBBINS

Cooperative Extension work in Agriculture and Home Economics, College of Agriculture,

University of California, and United States Department of Agriculture cooperating.

Distributed in furtherance of the Acts of Congress of May 8, and June 30, 1914.

B. H. Crocheron, Director, California Agricultural Extension Service.

THE COLLEGE OF AGRICULTURE

UNIVERSITY OF CALIFORNIA

BERKELEY, CALIFORNIA

CONTENTS

PAGE

Introduction 3

Losses caused by weeds 4

Loss of plant food, water, and

light 5

Increased cost of labor and

equipment 6

Increased cost of preparing crop

products for consumption. .. . 6
Impaired quality of farm pro-
ducts 6

Harboring of insect and fungus

pests 7

Injury to livestock and human

beings 8

Introduction and spread of weeds. . 8

Impure commercial seeds 9

Weed seeds in feed, packing,

and gravel 11

Other means of conveyance. .. . 11

Disposal of screenings 11

Weed characteristics 12

Principles of weed control 14

Annuals 14

Biennials 16

Perennials 16

Methods of controlling perennial

weeds 17

Clean cultivation 18

Flooding 18

Cropping methods 19

Smothering 21

Methods of chemical weed control. 21

Contact herbicides 21

Selective sprays 21

Translocated sprays 22

Temporary soil sterilants 22

Relatively permanent soil

sterilants 22

Seasonal control of weeds in un-

cropped areas 23

Oils 23

Sulfuric acid 23

Sodium arsenite 24

Control of annual weeds in cereals

with dilute sulfuric acid 24

The acid arsenical method 27

Preparation of the acid arsen-
ical stock solution 30

Preparation of large quantities

of stock solution 31

The spray solution 31

Precautions 32

Temporary soil treatments 32

Sodium chlorate 32

Precautions 35

Carbon disulfide 36

Soil sterilization 38

Sodium arsenite as a soil

sterilant 38

Dry white arsenic as a soil

sterilant 40

Nonpoisonous soil sterilants. . . 41

Other herbicides 42

Equipment required in applying

chemicals for weed control . . 43

Special problems 47

Weeds in uncropped areas 47

Mechanical control 47

Chemical control 49

Burning 50

Grazing 51

Weeds in grain fields 51

Common weeds occurring in

grain fields 51

Introduction and spread of

weeds in grain fields 51

Control of weeds in grain fields 52
Chemical control of weeds in

grain fields 52

Perennial weeds in grain fields. 53

Weeds in alfalfa 54

Weeds that interfere with es-
tablishment of stand 54

Weeds in an old stand of alfalfa 55

Parasitic weeds in alfalfa 56

Weeds in orchards and vineyards 57

Weeds in lawns and golf greens ... 58

Weeds in ditches and waterways . 61

Weeds in rice fields 64

Range weeds 66

Keeping firebreaks clear 66

Special weeds 67

Puncture vine 68

Germination of puncture-vine

seed 68

Control of puncture vine 69

Morning-glory 70

Characteristics of morning-
glory 70

Control of morning-glory 71

Control by cultivation 71

Control by smothering 74

Cropping 74

Flooding 74

Control with chemicals 74

Johnson grass 75

Camel thorn 76

Russian knapweed 76

Hoarv cress 77

Willows 77

Bermuda grass 78

Klamath weed 78

Poison oak 80

Artichoke thistle 80

Regulatory phases of weed control. 81
To prevent the introduction and

spread of noxious weeds 83

Eradication of limited infesta-
tions 83

General abatement procedure

against weed pests 84

Sending weeds for identification ... 86
Common and scientific names of
weeds referred to in this cir-
cular 86

WEED CONTROL'

W. S. BALL,2 A. S. CEAFTS/ B. A. MADSON/ and W. W. BOBBINS^

INTRODUCTION'

California agriculture with its diversity of crops, soil types, and cli-
matic conditions presents problems that challenge not only the farmer
but also the trained agriculturist and scientist. Among these problems
the control of weeds claims more and more attention. Even now, appar-
ently too little time and effort are spent in combating these pests.

Weeds, being inconspicuous in their habits, often go unnoticed ; after
having been present a few years they are frequently taken for granted.
This very characteristic, however, is dangerous, tending toward neglect
in their control and allowing their wide dissemination before the danger
is realized. An example is the Klamath weed (St. Johnswort), which
spreads over approximately 100,000 acres of range land in this state be-
fore organized control was started.

The situation is not hopeless, however, for many California growers
have so effectively combated weeds that now their fields and ranges are
clean, their products uncontaminated, and their losses from weeds re-
duced to a minimum. The time and labor expended annually for weed
control can be largely reduced if available methods are put to use.

With intensification and concentration of agriculture upon the more
productive soils, pest control is increasingly necessary. Not only is indi-
vidual effort demanded, but cooperation by community and state agen-
cies is needed to curb weeds.

Two examples show how wide may be the influence of weeds and how
complex the control program. Leafhoppers. which carry the curly-top
virus of the sugar beet and the western yellow blight of tomato, live over
on various summer and fall annuals in the south San Joaquin Valley
but do their damage in the Salinas Valley, the lower Sacramento and
upper San Joaquin valleys, and the intervening delta regions. Livestock,

^ This publication is a revision of and supersedes Extension Circular 54, "The Con-
trol of Weeds," by W. S. Ball, B. A. Madson, and W. W. Bobbins.

- Field Supervisor of Weed Control, California State Department of Agriculture.

^ Assistant Botanist in the Experiment Station.

* Professor of Agronomy and Agronomist in the Experiment Station.

= Professor of Botany and Botanist in the Experiment Station.

® A list of all weeds mentioned in this circular, giving both common and scientific
names, is found at the back of this publication.

[3]

4 California Agricultural Extension Service [Cir. 97

pasturing during winter and spring in the Klamath -weed -infested
ranges of Humboldt County, often spend the summer in Trinity Na-
tional Forest or the foothills of the upper Sacramento Valley.

Eradication of established noxious weeds is usually difficult. Opera-
tions must be prompt and persistent. Often, for complete eradication,
high initial costs are justified. The projects on camel thorn and artichoke
thistle now being supervised by the State Department of Agriculture
will, if successfully completed, mark an epoch in the history of weed
control. If allowed to spread, these two weeds would undoubtedly cost
the state many times the total expense of eradication. But many of our
noxious perennials are here to stay, and only through adequate control
measures may we continue to farm the infested areas. According to ex-
periments in progress, at least some of our deep-rooted perennials may
be controlled by methods that allow the simultaneous growing of crops
and incur little actual loss. Cultural methods, cropping sequences, and
rotation programs offer much promise in this field. As such methods are
undoubtedly the most practical for the average farmer, it is to be hoped
that adequate investigational work on this phase of weed control will
soon be provided.

This circular attempts to picture the weed problem in California, to-
gether with the most practical and promising remedies now employed.
Obviously some methods have not been sufBciently tried. We are not
ready to lay down arbitrary rules for controlling any one weed or group
of weeds. The different species, in fact, are so unlike in their character-
istics and habits, the climatic and soil conditions under which they grow
differ so greatly, and the effects of weeds on the various crops with their
different cultural requirements vary so much that methods universally
applicable will probably never be devised. The general principles pre-
sented and the specific cases cited should, however, be useful. This circu-
lar should answer many questions submitted almost daily to the College
of Agriculture and the State Department of Agriculture.

LOSSES CAUSED BY WEEDS

There are four groups of agricultural pests: (1) animal diseases, (2)
plant diseases, (3) insects, rodents, and predatory animals, and (4)
weeds. As shown by a recent report of the Agricultural Service Depart-
ment Committee, United States Chamber of Commerce, the annual losses
from weeds considerably exceed the combined losses of the other three
groups. This same organization listed thirty important items of waste
on American farms ; soil erosion ranked first and weeds second.

Pew realize what a burden weeds add to human existence. The produc-
tion of almost all crops is largely a battle with weeds. The preparation

1936] Weed Control 5

of many products of the soil for human consumption involves the elimi-
nation of weeds or their effects. Weeds cause losses in many ways :

1. They compete seriously with crops for plant food, moisture, and
light.

2. They increase the labor necessary for crop production.

3. They increase the cost of preparing many crop products for con-
sumption.

4. They impair the quality and destroy or reduce the value of many
products of the soil.

5. They harbor insects and fungus pests destructive or injurious to
economic plants.

6. They are sometimes poisonous and may endanger the health or life
of man and animals.

Weeds in the United States have been said to levy, in one way or an-
other, an annual tax on agriculture and industry of about three billion
dollars. Though no estimate has been made of the weed tax in California,
it is probably at least proportional to that of the country as a whole — a
minimum of sixty million dollars, the larger portion of which falls on
agriculture. Indiana in 1920 estimated its average annual loss through
weeds at $210 per farm ; Wisconsin in 1927 at $244.

Loss of Plant Food, Water, arid Light. — Probably the heaviest loss by
weeds, especially in California, results from their competition with
crops for plant food, moisture, and light. The crop-producing power of
our agricultural soil is usually limited either by the moisture, obtained
frequently only at high cost, or by the plant food available. When the
crop must share this limited supply with weeds, lower yields result.

Upon unirrigated land, water is usually the limiting factor in crop
production ; poor yields or total crop failures often occur because the
water supply has been exhausted before the crop matures. Weeds may
contribute greatly to this condition. Obviously, grain fields infested with
wild oats, wild mustard, wild radish, or other weeds will have their
yields reduced, partly, perhaps, through shading and loss of plant
foods, but chiefly through the use of water by the weeds. In fact, one
main reason for cultivating is that it eliminates the undesirable plant
growth which, except in the first few inches, causes most of the loss of
water.

On irrigated land, competition may be keener for plant food than for
water ; but the effect is just as striking. For example, alfalfa infested
with foxtail or Bermuda grass, usually makes but a sIoav, weak growth.
When the weeds are removed by cultivation or burning, the alfalfa
plants are immediately stimulated. In young orchards infested with
morning-glory, the trees are often noticeably affected by weeds.

6 California Agricultural Extension Service [Cm. 97

That morniiig-giory, creeping' mallow, and other perennials depress
or prevent the growth of summer crops like sorghum, corn, cotton, and
beans is too well known to need discussion.

hicreased Cost of Labor and Equipment. — The labor and equipment
necessary to keep weeds in check constitutes another tax upon the farm
income. The main reasons for cultivating annual crops are the prepara-
tion of the land for planting and the keeping down of weed growth. In
vineyards and orchards the principal, if not the sole, purpose of cultiva-
tion is weed control. The average cost of such tillage on cultivated lands
has been estimated at one-twelfth the value of the crop or, for California,
about $40,000,000 annually. These figures, however, do not include the
very considerable expense of eradicating weeds in ditches, roadways,
and waste lands near the cultivated areas, nor the public money ex-
pended specifically on campaigns against the more noxious weeds.

Also chargeable against weeds is an enormous investment in equip-
ment. Almost every farm possesses one or more implements used pri-
marily for weed control.

Increased Cost of Preparing Crop Products for Consumption. — After
the crop is grown, weed contamination may involve further expense in
handling and processing, especially with the seed crops and with cereals
such as rice, barley, and wheat. Millers must install costly equipment for
removing weed seeds and other material of weedy origin.

Most seed crops grown in California are contaminated with weed seed.
In fact, one main reason for the relatively high price to the farmer is
that such seed, as grown, is usually foul with weeds, which the dealer
must remove by expensive methods.

An excellent example of the extent to which weeds increase the hand-
ling costs may be found in our cereal crops. The average annual produc-
tion of wheat, oats, barley, and rice in California is about 1,444,000
tons, with an average dockage of well over one per cent, consisting mostly
of weed seed or material of weedy origin. In the harvesting and market-
ing of these crops, therefore, more than 11,000 tons of weed seeds and
residue must be handled and transported. Further, as already indi-
cated, much of the cleaning cost to which most cereals must be subjected
should be charged against weeds.

Impaired Quality of Farm Products. — Weed contamination of many
crops reduces their quality and market value. Weedy alfalfa hay, for
example, brings from $1.00 to $2.50 less a ton. Since much of the first
cutting and part of the second are liable to be weedy, at least one-fifth
of the crop may be assumed to contain objectionable quantities of weeds.
With an average reduction in value of $1.50 a ton, the annual loss to
the alfalfa industry in California is approximately $1,200,000. Some

1936] Weed Control 7

weeds are more objectionable tliaii others. For example, liay infested
with puncture vine may be practically worthless ; in fact, many counties
prohibit its importation.

The market value of wheat may be greatly reduced by the presence of
certain weeds. Even a few seeds of sour clover, for example, will render
a sack of wheat unfit for milling.

Harhoring of Insect and Fungus Pests. — "Weeds harbor many fungus
and bacterial diseases and many insect pests. Thus they aid in propa-
gating such crop enemies, which they render more destructive and more
difficult to control. The bacterial organism causing bean blight lives on
some of the wild legumes; the organism causing blackleg of cabbage
also thrives on wild mustard. Certain wild mustards may harbor the
fungus that causes clubroot in cabbage. The downy mildew of lettuce is
caused by a fungus that may live on several weeds belonging to the com-
posite family, including prickly lettuce and sow thistle. Harris and
Stout^ list 29 species of California weeds known to be hosts for one or
more virus diseases; aster yellows, for example, is spread by the six-
spotted leafhopper, an insect common on several wild hosts, especially
the broad-leaved plantain. The curly top of sugar beets, another virus
disease, is spread from wild hosts to cultivated plants by the beet leaf-
hopper; its virus has been found present in California in 19 weeds,
especially Atriplex species and Russian thistle. Lockwoocf cites the
following insects that hibernate in trash and weeds : grape leafhopper,
apple leafhopper, field crickets, tarnished plant bug, big-eyed bug, and
harlequin cabbage bug. In addition, several important insect pests build
up their numbers on weeds and migrate to commercial crops. Lactuca
species will act as a host to the bean thrips ; Russian thistle not only to
the beet leafhopper mentioned above, but also to the western beet leaf
beetle and the young of the tiger moth; wild aster to the vegetable
weevil ; shepherd's purse to such pests as the sugar-beet leafhopper, the
clover aphid, the leaf-curl plum aphid, the cotton or melon aphid, the
bean aphid, the cabbage aphid, and the green peach aphid; lamb's-
quarters to the cottony cushion scale, the harlequin cabbage bug, the
banded flea beetle, the beet and spinach-leaf miner, and the apple skin
worm. Runiex species are hosts to the citrus thrips, the grape leafhopper,
the apple leafhopper, the sugar-beet wireworm, the strawberry-root
weevil, and several species of destructive aphids ; Jimson weed to the
red spider, the cotton or melon aphid, the potato flea beetle, the tomato

' Harris, M. R., and G. L. Stout. Weeds as a factor in the spread of plant diseases
in California. California State Dept. Agr. Mo. Bui. 22(6) : 273-277. 1933.

^ Lockwood, Stewart. The relation of weeds to insect pests. California State
Dept. Agr. Mo. Bui. 22(6) : 279-282. 1933.

8 California Agricultural Extension Service [Cir. 97

sphinx, and the potato-tuber moth; several species of plantain to the
rosy apple aphid, the cotton or melon aphid, the tarnished plant bug, the
potato flea beetle, and the serpentine leaf miner ; species of wild mus-
tard, for a time at least, to the sugar-beet leafhopper, the grape leaf-
hopper, black grass bug, the harlequin cabbage bug, and other injurious
insects ; wild morning-glory to many crop pests, including the common
red spider, several species of thrips, the cotton or melon aphid, and the
soft brown scale ; and lupine to the pod borer, so destructive to baby
lima beans. J. C. Elmore, of the United States Bureau of Entomology,
states that in the pepper-growing districts of southern California ap-
proximately 85 per cent of the first weevil infestations in the spring
originate from one species of nightshade {Solanum dmiglasii), which is
a host to the pepper weevil, especially in winter. This weed, therefore,
should be eradicated from areas devoted to pepper growing. Nematodes
and grasshoppers, destructive to many crop plants, live and multiply
on weeds. Though other examples might be cited, those given indicate
how weeds assist insect pests. In fact, if uneconomic plants could be
eliminated, some of our worst crop pests could be easily controlled.

Injury to Livestock and Human Beings. — Many domestic animals are
lost annually from weed poisoning. The most important poisonous weeds,
such as larkspur, water hemlock, whorled milkweed, Klamath weed, and
death camas, occur extensively on some ranges, under conditions diffi-
cult to control. On cultivated lands and in pastures seeded to cultivated
plants, many animals are lost through their owners' failure to eradicate
poisonous weeds. Certain species of lupine, poisonous when in seed,
cause annual losses of sheep.

The health of human beings may also be affected. Deaths occasionally
occur from the eating of poisonous seeds, berries, or tubers. Poison oak
causes much suffering and distress, and many victims of hay fever can
trace their affliction to the pollen of weedy plants.

INTRODUCTION AND SPREAD OF WEEDS

Weeds as we know them today were no problem in the lives of early Cali-
fornia settlers. They were, with few exceptions, introduced into this
country with the settlers and their crops. To this day, in fact, specific
crops tend to have their special pests ; and, with the introduction of new
crops, new weeds are certain to occur. Wild oats and many weedy range
grasses, apparently, arrived with the Spanish invaders, probably in hay
accompanying livestock. Some of these weedy annuals were so thrifty
and so prolific in seeding that they have practically supplanted the native
vegetation of our foothill range areas.

With the westward march of intensive agriculture came the weeds

1936] Weed Control 9

that had attended its development for ages. The pigweeds, thistles, morn-
ing-glories, and others came with the American farmer and have not
only persisted to tax his labors but thrived as in no other situation. In
the deep alluvial soils and temperate climate of California many weed
species find better conditions than in their native lands. They have
tended, consequently, to drive out economic plants and have resisted the
control measures effective in the East and in Europe.

The introduction of new weeds has continued up to the present, and
only constant vigilance can check their establishment and spread. Arti-
choke thistle was first brought in as a vegetable, restricted to a few
gardens. Escaping, it has covered thousands of acres of valuable range
land in Solano and other Bay counties. Camel thorn, an extremely per-
nicious weed, was first brought into a southern county, presumably from
southwestern Asia in Turkestan alfalfa seed. From this small start it
spread to ten California counties, infesting approximately 600 acres. At
present Marlahan mustard, or dyer's woad, a biennial weed from Europe,
is confined to Scott Valley in Siskiyou County. Austrian field cress, a
perennial, also from Europe, is limited to a region at the headwaters of
the south fork of the Pit River in Modoc County. Many of the early in-
festations were limited in area and, by proper handling, could have been
eliminated; but ignorance, indifference, neglect, or improper methods
have permitted most of them to spread, till today some are beyond hope
of eradication.

If an infestation involves a small patch of a very noxious weed, espe-
cially a new introduction, strenuous methods and considerable expense
are justified in preventing that weed from gaining a foothold. An ex-
pense that might be inordinate if required over a larger territory
becomes, in such an emergency, highly practical.

To prevent the introduction and spread of new noxious weeds in Cali-
fornia requires vigilance, not only by quarantine officials, county agri-
cultural commissioners, and farm advisors, but also by growers. When
new weeds are found, prompt and concerted action should be taken. If
eradication is too difficult for the individual, then county and state
should cooperate. All infested areas should be adequately marked and
periodically inspected to insure complete eradication.

Impure Commercial Seeds. — Probably the most common and effective
means of introducing weeds is the sale and distribution of impure com-
mercial seeds. Hoary-cress and Russian-knapweed infestations in many
localities in California can be traced to impure Turkestan alfalfa seed
brought in during the World War. These tAVO pests are now so widely
distributed that there seems to be little hope of eradicating them.

During the winter of 1927-28 many consignments of Sudan grass seed

10

California Agricultural Extension Service

[CiR. 97

grown in Texas and Oklahoma were shipped into California. The State
Seed Laboratory of the Bureau of Field Crops, California Department
of Agriculture, noted a rapid increase in the percentage of Johnson
grass seed beginning with the receipt of samples of this imported Sudan
grass seed. The high percentage of Johnson grass continued through

'^m^''

Fig. 1. — Left, rhizome of nut grass penetrating a potato ; right, nutlets formed

within a potato.

1929 in samples containing typical California weed seeds, indicating-
many new infestations of this noxious perennial. These examples illus-
trate the danger of noxious weed seeds that are all too prevalent in un-
clean commercial seed submitted for analysis.

According to the State Seed Laboratory, alfalfa seed may contain the
seeds of barnyard or water grass, dodder, Russian thistle, creeping
mallow, yellow star thistle, Johnson grass, morning-glory, bull thistle,
Russian knapweed, and hoary cress. Found in barley are yellow star
thistle, morning-glory, blessed thistle, barnyard grass, and Johnson
grass. Seed oats and wheat may contain these same impurities. In Sudan
grass the common weed seeds were barnyard grass, morning-glory,
Russian thistle, Johnson grass, and yellow star thistle.

Though these cases were observed by the Seed Laboratory, the ex-
change of seed from farmer to farmer usually involves inferior grades
never submitted for analysis. This uncontrolled sale largely explains the
thousands of acres of morning-glory, yellow star thistle, Russian knap-
weed, and other noxious weeds in California today.

A new means of weed dissemination has recently been noted by the

1936] Weed Control 11

State Department of Agriculture. While seed potatoes from the Delta
region were being planted at Shaf ter in the south San Joaquin Valley,
a worker called the grower's attention to some foreign bodies in the cut
seed pieces. The bodies turned out to be nutlets of nut grass {Cyperus
rotundus) , a weed not prevalent in that region (fig. 1).

Federal legislation regarding weed introduction is not adequate prop-
erly to protect our agricultural lands. The present California seed law,
for instance, allows a tolerance of 89 weed seeds per pound in clover and
alfalfa without special labeling. Almost all commercial seed is allowed
certain tolerances, any of which are sufficient to permit scattered infes-
tations.

Weed Seeds in Feed, Packing, and Gravel. — Though the practices de-
scribed above are dangerous, other means of dissemination are equally
offensive. Weeds are often introduced in hay, in the packing about trees,
in various feedstuffs, and in dirt, sand, or gravel used in construction.
During transportation these materials are often scattered along the way,
as b}^ the loss of hay from trucks, the kicking of bedding from stock cars,
the passage of weed seed through the digestive tracts of animals, and the
scattering of manure, gravel, and sand. Puncture-vine and camel-thorn
seed can pass unharmed through farm animals. One dairyman, having
grazed his animals in a pasture infested with camel thorn, later moved
them to the foothills. Their path was marked by scattered infestations
of camel-thorn seedlings. Another stockman purchased several loads of
hay contaminated with hoary cress. He fed this by scattering it on his
own clean range. Hundreds of dollars were later spent in eradicating
the resultant infestations.

Oilier Means of Conveyance. — Weed seeds are often distributed in
mud and dirt on the feet of animals or the wheels of vehicles ; in soil
adhering to plows, cultivators, and other implements ; and, within recent
years, on the rubber tires of automobiles and airplanes. All kinds of har-
vesting machinery may carry weeds from farm to farm, particularly the
combined harvester, which has spread many pests in California grain
fields.

Disposal of Screenings. — The disposal of screenings from seed-clean-
ing establishments is a matter of grave concern. In the cleaning of com-
mercial seed large amounts of weed seeds are accumulated, and their
disposal often becomes a problem. Although burning or fine grinding is
the only proper procedure, some unscrupulous dealers sell this material
to sheep or cattle growers for feed, or dump it into streams. Seedsmen
who tell the farmer at the front door to sow clean seed will sell screenings
through the back door to sheepmen, who dump them out on the open
range. Sheep thus fed may later be driven for miles through farming

12 California Agricultural Extension Service [Cir. 97

areas and into the hills on summer ranges. No more perfect means of
spreading weeds could be devised.

Seed-cleaning establishments have been known to dump their screen-
ings into the Sacramento River, to be carried down stream and spread
by irrigation. Whether perpetrated through ignorance or malice, such
practices should be forbidden by law and prevented wherever supervi-
sion is possible.

The Seed Committee of the State Agricultural Commissioners' Asso-
ciation attempted to determine how objectionable the seed screenings
are and how effectively the present grinding methods destroy weed seeds.
Samples of screenings, ground and unground, collected by county agri-
cultural commissioners, yielded the following information : Nine sam-
ples from alfalfa all contained weed seeds. These 9 samples contained 45
different plant species — mostly weeds. A large-seeded alfalfa dodder
was present to the extent of 344 seeds per pound of sample, Russian
thistle 27, common field dodder 333. Also found in quantit}^ were Ber-
muda grass, yellow star thistle, sow thistle, chicory, seaside heliotrope,
and creeping mallow.

Eighteen samples of mixed screenings from wheat, barley, and oats
contained 77 different plant species — mostly weeds. Johnson grass some-
times occurred in numbers as high as 8,175 seeds per pound of sample,
morning-glory 882, yellow star thistle 2,178, and cat's-ear 1,200. Also
present were chicory, sow thistle, spiny sow thistle, milk thistle, and sea-
side heliotrope.

One sample of sorghum screenings averaged 18 seeds of Johnson grass
and 45 seeds of puncture vine per pound of sample, and showed a total
of 16 different weed species.

Of the ground screening samples examined, 11 had been run either
through a bur type grinder or a Hammermill. Of these 11 samples, 81
per cent showed the presence of weed seeds. Thirty-one different plant
species, almost all weeds, were found in the ground material, having
actually gone through the grinding process without visible injury.

WEED CHARACTERISTICS

Of the countless plants in the world, relatively few are weeds. Most of
our native plants do not have weed characteristics ; they do not spread
from adjacent hills and valleys to cultivated areas and establish them-
selves as pests. But the world contains some plants with pernicious char-
acteristics : they tend to grow where not wanted ; they resist man's at-
tack ; they may survive frost, high temperature, and drought ; they can
grow under varying soil and climatic conditions ; they produce enormous
numbers of seeds that may live for years in the soil ; and they usually

1936] Weed Control 13

multiply and spread rapidly. Of course, no one plant necessarily has all
these undesirable characteristics ; but it may have enough to become a
pest. Any plant that seeds prolifically, or reproduces vegetatively from
underground parts, or is poisonous to livestock or human beings, or
causes mechanical injury, may become a noxious weed. Furthermore, a
weed from one locality may not be a weed in another section with differ-
ent soil and climatic conditions. Johnson grass, for example, is noxious
only where winters are mild, and Klamath weed where the rainfall is
relatively high.

Annual weeds must reestablish themselves each year ; to survive they
must have a means of perpetuation. Most of these pests either produce
enormous numbers of seeds or have some effective way of spreading. For
example, certain individual tumbleweeds have been known to produce
over a million seeds; and the common annuals usually produce many
thousands.

Some weeds are adapted for dispersal by wind, by water, or by ani-
mals. The seeds of thistles, milkweeds, dandelions, sow thistles, willow,
and wild lettuce have feather-like or cottony attachments that enable
them to be blown about. Many weed seeds will float on water ; in Cali-
fornia irrigation is known to be an important means of spreading them.
One of the writers, working in Colorado, found in 156 weed-seed catches
from three different irrigation ditches a total of 81 different species,
those most frequently met with being prostrate pigweed, tall pigweed,
sedge, lamb's-quarters, tall marsh elder, doorweed, black bindweed,
curled dock, and dandelion. The seeds passing a given point on a 12-foot
ditch during a 24-hour period may number several millions. Early irri-
gation waters were found to be most heavily loaded with weed seeds,
which were often observed to rest during the nonirrigating season in the
mud of the ditch. Ditch banks, iapparently, are more to be feared than
roadsides as effective sources of infestation.

The seeds of numerous weeds such as sandbur, puncture vine, cockle-
bur, and beggar-ticks attach themselves by barbs or hooked prickles to
the hair or wool of animals, or the clothing of man, and are thus carried
from place to place.

The noxious perennials spread not only by seed but by roots and root-
stocks. Although vegetative spread of undisturbed infestations is rela-
tively slow, plows, harrows, and cultivators dragged through these weeds
may carry seeds and portions of the underground organs rapidly over
the field, so that the areas soon increase.

Cultural operations as a means of weed dissemination are often over-
looked by farmers. In the clean cultivation of morning-glory, Johnson
grass, and similar perennials, a very satisfactory cleanup has often been

14 California Agricultural Extension Service [Cir. 97

followed by reinf estation by seedlings, and the labor mostly lost. Often,
in fact, large increases in the area infested may follow this practice.

Many seeds retain their vitality for years, especially when buried in
the soil. As experiments have shown, the seeds of shepherd's purse, mus-
tard, purslane, pigeon grass, pigweed, mayweed, dock, and chickweed
may survive thus for more than 30 years. Prickly pigweed and morning-
glory seed live 25 years or more ; mallow over 5 years ; ragweed, corn-
cockle, cheat, wild oats, and plantain, from 1 to 5 years. Plowing may
turn weed seeds under, placing them at depths where the soil atmos-
phere is unsuited for germination ; then, after many years, another plow-
ing brings them to the surface where they germinate. Thus a field that
has long been relatively free may suddenly develop a crop of weeds. The
seeds of wild oats, tall pigweed, cocklebur, and others seldom germinate
the year they are shed, apparently requiring a rest period in the soil.
Others, like puncture vine, have hard coats that must undergo a certain
amount of decay before germination.

PRINCIPLES OF WEED CONTROL

The methods used in controlling any weed must be related to its habits of
growth and reproduction. Since these vary widely, as already indicated,
control practices differ. The first problem, always, is to determine the
mode of attack best suited to the weed and to the conditions in question.
Sound judgment is needed for often success depends upon adapting the
methods to the situation.

Annuals. — Annual weeds live but one year; they produce seed but
once and then die down entirely, root and all. Having no parts under-
ground by means of which they can spread, they propagate themselves
by seeds alone. Obviously, all methods of controlling them have one
principal object — the prevention of seeding — which may be attained by
mowing, cultivating, burning, or spraying. If seed production is con-
sistently prevented over a series of years, and if the introduction of
weed seeds from neighboring areas is largely eliminated, the annual
weed population will gradually decrease. Of course, weed seeds of many
annuals may live for years in the soil and may then be brought by cul-
tural operations to the surface, where conditions for their growth are
favorable. The germination of such seeds should be encouraged by irri-
gation or cultivation before or soon after the crop is planted, to insure
killing of the young plants before they injure the crop. Annuals in the
early stages are easily and cheaply destroyed by cultivation, plowing, or
chemicals ; and, once the top has been killed, the root has no power of
rejuvenation.

Summer and winter annuals differ considerably in their habits. The

1936]. Weed Control 15

seeds of summer annuals germinate in the spring ; and the plants grow
to maturity during the same season, develop a crop of seeds, and die
during the winter. Shallow cultivation and sprays, either oil or acid,
afford the best control. It is to check summer annuals that an inter-
tilled summer crop is usually placed in a crop-rotation plan.

Winter annuals usually start in the fall or early winter when soil-
moisture conditions are favorable. The young plants live throughout the
winter in a vegetative condition, often forming a rosette-like growth. In
the spring they grow rapidly, flower, and produce seed. Such plants are
more difficult to control in California. The seeds usually germinate
throughout the winter ; and shallow cultivation, to be effective, would
have to be repeated several times — a procedure often impossible during
the rainy season. Two effective methods, however, are available.

The first method is plowing. If plowed under fairly early in the spring,
these weeds are effectively controlled, leaving the land free for crops.
Plowing which is done primarily for the control of annual weeds should
be shallow, in order that seeds be kept in the surface layers of soil where
they will germinate readily. Spring plowing is common practice in Cali-
fornia, both in cereal production and in preparing the land for later
summer crops. In cereal production a good job of plowing often means
the difference between a clean, heavy crop and a weedy unprofitable one.

The second means of checking winter annuals, principally used in
growing cereals, is a selective spray such as sulfuric acid. This method
will be described in detail in the next section.

Annuals often grow luxuriantly during the mild winters in California
and are sometimes used for pasturage. In this case they are usually
plowed under in the spring, and they serve the two-fold purpose of pro-
viding green feed for cattle or sheep and a soiling crop for the land. They
should be turned under before seed is produced, and the plowing should
be clean and thorough. Annuals are similarly used in orchards, provid-
ing a valuable covercrop at minimum cost. Where used in this way, they
are perpetuated by allowing the seeds to ripen before plowing. This prac-
tice always presents the hazard of severe competition for moisture if
irrigation water is not available, and it also may menace surrounding
fields if seeds scatter. Where such a program is followed, the orchardist
should be extremely careful that noxious perennials do not invade his
land and spread undetected. Great caution should also be exercised in
the planting of screenings for covercrop production.

Although mowing will prevent annual weeds from seeding, some
species, such as wild lettuce and wild radish, will send up new shoots
from buds in the axils of the lower leaves and may produce a second
crop of flowers and seed. Such operations as mowing and spraying must

16

California Agricultural Extension Service

[CiR. 97

be done at the proper time ; delay may mean failure. The easiest way to
dispose of annual weeds is to kill them in their early stages.

Common California annuals are mustards, wild radish, tumbleweed,
pigweeds, wild oats, shepherd's purse, barnyard grass, chess or cheat,
lamb's-quarters, Russian thistle, puncture vine, dodder, spring sow
thistle, yellow star thistle, sunflower, cocklebur, and crab grass.

Fig. 2. — Showing the distribution in the soil of the roots and rootstocks
of the morning-glory plant.

Biennials. — A biennial weed lives for two years, passing through the
first year in a low rosette form and producing seed the second summer.
In California the biennials do not form a distinct or important group.
Many weeds that are annuals in regions of freezing winters have a
biennial habit in this state. True biennials are few, examples being pur-
le star thistle and burdock. They are controlled like annuals, the treat-
ment being made during the first year.

Perennials. — The most troublesome weeds are perennials, which re-
quire special methods and systematic, painstaking endeavor for control.
Plants in this group, as contrasted with annuals and biennials, live three
years or more and spread not only by seed but also by underground roots
or stems.

Perennials fall into three classes, according to their methods of repro-
duction :

The simple perennials have either a large taproot, like the dandelion.

1936] "Weed Control 17

or a fibrous root system, like certain bunch grasses ; in either case there
is a well-developed perennial crown. Under natural conditions these
perennials propagate only by seed ; but if the roots of such plants as the
dandelion, or the crowns of the bunch grasses, are broken into pieces,
each piece is capable of rejuvenating the plant.

The creeping perennials are the most difficult type to control because
they reproduce by creeping underground stems (rootstocks or rhizomes)
as well as by seed. Among the most common are Johnson grass, hoary
cress, morning-glory (fig. 2), Canada thistle, and Russian knapweed.
These not only spread horizontally for appreciable distances under-
ground but have deeply penetrating roots. Morning-glory roots have
been found over 20 feet deep in the soil.

Bulbous perennials reproduce by bulbs or nut-like structures and by
seeds as well. Their reproductive organs are often but weakly attached
to the mother plant and are easily dragged loose and scattered through
the soil during plowing and cultivation. Some common bulbous peren-
nials of California are wild onion, wild garlic, and nut grass.

METHODS OF CONTROLLING PERENNIAL WEEDS

In combating perennial weeds, two problems are involved. First, to
check spread or reinfestation by seedlings the old plants must be pre-
vented from ripening seed, and seedlings developing from seed already
present in the soil must be killed. Second, the existing stand of old
established plants must be eradicated.

In handling the seedling situation, one may use methods similar to
those already described for annuals, except that the greater hazard in-
volved makes it much more important to prevent the spread of seed.
Mowing in the blossoming stages, sjjraying with contact sprays, or burn-
ing will accomplish this purpose. Probably early spraying or mowing,
followed by spraying of the stubble after removal of top growth, is the
best way to prevent seed formation. Plowing or shallow cultivation, if
done early enough, will kill seedlings of perennials. Morning-glory seed-
lings may be controlled in the two to five-leaf stage by two cultivations
to a depth of 3 inches on unirrigated land or four cultivations after each
irrigation on irrigated land. If allowed to become more mature, the plants
assume a perennial habit, and eradication becomes much more difficult.

In controlling or eradicating established perennial weeds, several
methods are effective. Certain requirements limit the vigor of such
plants and their ability to perpetuate themselves. All, for example, need
sunlight, oxygen, water, and certain mineral nutrients from the soil. If
any one of these factors is limited, either by cultivation, smothering, or
immersion under water, or by competition with other plants, the vigor

18 California Agricultural Extension Service [Cir. 97

of the plant is reduced ; and if any one is rendered permanently un-
available, the plant must ultimately die.

In addition, the perennial plant goes through a more or less normal
annual cycle of vegetation, reproduction, storage, and dormancy ; and
any disturbance of this cycle lowers its vigor. A plant may be induced,
furthermore, under certain limited conditions, to take up a chemical
through cut or injured tops ; the poison is then translocated to the roots,
killing all tissue with which it comes in contact. And finally a plant,
under other conditions, may absorb toxic chemicals from the soil and
die when these accumulate sufficiently. With these points in mind pres-
ent methods for controlling perennial weeds will be discussed.

Clean Cultivation. — If a perennial plant is not allowed to send leaves
up into the sunlight and air, food manufacture is prevented, and the
roots are killed. This principle underlies the old practice of clean culti-
vation. Although excellent in some places, this method has drawbacks in
California. On our deep alluvial soils the roots of such perennials as
morning-glory are so extensive that two or more years are often required
for depletion of food reserves. In unirrigated soils, as the soil moisture
runs low the roots tend to become dormant ; thus their food consumption
is reduced, and their life prolonged. This difficulty may be overcome by
occasional irrigation where possible. Any seeds formed on the infestation
may later be brought to the surface and in certain crops, such as cereals,
may reinfest the areas. In fact, cultivation tends to spread the seed so
that if reinf estation occurs the areas are larger than ever. A clean culti-
vation program, once inaugurated, involves watching the areas closely
for years after the old plants are gone. In California the actual areas of
infestation should be kept free of crops during the first year of cultiva-
tion, because it is almost impossible to keep down growth around crop
plants.

In dry-land areas, summer fallowing not only disturbs the roots to the
depth of plowing but, if followed by a weed-cutting program, accom-
plishes the same result as clean cultivation when conscientiously carried
out. Although pasturing of fallowed land tends to check such perennials
as morning-glory, it does not appreciably reduce the stand.

Flooding. — A practice increasingly popular in California for con-
trolling perennial weeds is flooding. This is accomplished by surround-
ing the areas with dikes and covering them with 6 to 10 inches of water
for several weeks in the summer. The infested area should be plowed
before immersing, and no growth should be allowed to appear above
the water.

Flooding first attracted attention when areas infested with morning-
glory and other noxious perennials were planted to rice. Such weeds

1936] Weed Control 19

were usually eradicated. This method has been used rather extensively
with camel thorn and hoary cress (fig. 3) and yields excellent results in
sandy soils. In heavy soils flooding has not been so satisfactory.

White horse nettle occurring in several of the camel-thorn areas has
been killed in nearly every instance. In Stanislaus County a 13-acre area
of Russian knapweed, flooded for 60 days, was completely killed. In

'^C4'-

^*'

Fig. 3. — An area in Imperial County, infested with hoary cress,
flooded to eradicate the weed.

several other places knapweed has given the same response. Occasionally
submergence for three to five weeks has given satisfactory control.

As results of flooding have varied on different soils, soil type is evi-
dently a factor. Flooding is effective only where the area is completely
immersed for the whole time of treatment. Many failures have resulted
from allowing the water level to lower so that regrowth occurred. Not
all weeds react in the same way to flooding and our information is not
yet complete enough to warrant general recommendations.

As flooding kills plants by excluding the air, it is effective only when
the plant is covered and the roots completely surrounded with water.
Under certain soil conditions some plants apparently become dormant
and thus, through lowered oxygen consumption, are enabled to survive.

Cropping Methods. — In several cases recently noted in California,
morning-glory, creeping mallow, and other perennial weeds have dis-
appeared in certain crops. After the serious outbreak of curly top in the
sugar-beet area of the Salinas Valley in 1925, lettuce culture was in-
augurated ; within three years the extent and vigor of morning-glory in-
festation markedly decreased. Two years ago the Agronomy Division at

20 California Agricultural Extension Service [Cm. 97

the University Farm, Davis, found that morning-glory had almost dis-
appeared in certain of their alfalfa plots. A series of alfalfa plots plowed
in 1934 proved to be almost free of this pest, although the area had been
heavily infested four years previously when it was planted. A grain-
sorghum crop grown there during the summer of 1934 was practically
free of weeds. Encouraged by these results the Agronomy Division,
which had previously spent hundreds of dollars annually in spraying
and hoeing morning-glory, has initiated a 5 to 6-year plan of crop
rotation, with alfalfa serving the dual role of legume for soil improve-
ment and competitor for weed control.

Studies now under way explain the effectiveness of alfalfa as a weed-
control crop under California conditions. Though no definite report is
yet possible, several factors are apparently involved in this response. In
the first place, alfalfa grows vigorously and competes effectively with the
weed for soil moisture and nutrients. Aboveground it shades the sun-
loving morning-glory. Furthermore, it takes the available nitrates from
the soil rapidly in the spring and then utilizes nitrogen fixed by the
bacteria in its root nodules. Thus it can maintain a low nitrate level in
the soil and yet thrive in competition with a nonlegume. By the second
crop in the summer following spring planting of alfalfa at Davis, the
morning-glory becomes spindling and light green, evidencing nitrogen
deficiency. Tliis condition progresses throughout the season and, al-
though less apparent in the first crop of the following season, becomes
very pronounced during the second summer. Meanwhile, if the alfalfa
is vigorous, the stand of morning-glory becomes notably weaker and
thinner. By the third season only a fraction of the original infestation
remains and by the fourth it is practically gone. Finally, morning-glory
likes to grow vigorously without competition in the spring, blossom,
fruit, and mature seed during the summer ; and then, with the depletion
of soil moisture, die down and go into dormancy until the next spring.
When this cycle is disturbed the plant suffers. In both lettuce and alfalfa
culture, in addition to frequent cutting, which tends to keep the plant
vegetative, the soil is kept moist, inducing continued growth. This long
vegetative period reduces starch reserves in the roots to a minimum and
then fails to provide for replenishment. The roots consequently start
the winter in a depleted condition and fail to survive, especially if the
rainy season is long and wet. Apparently, then, several factors operate
in this cropping method ; and when they favor the crop in comparison
with the weed, excellent results are obtained. The benefits are three-
fold : not only are the weeds eliminated, but the soil is improved in fer-
tility and physical condition, and a valuable crop is produced.

Besides alfalfa, ladino clover offers some promise as a control crop for

1936] Weed Control 21

weeds. Although it does not provide so much shade nor such keen root
competition, it may be grown on a wide variety of soils not suited for
alfalfa. It may take longer, but it should eventually give the same
results.

In addition to these two legumes, several combinations will be tested
for their value in weed control. Most of them consist of an annual legume
and a nonleguminous truck or field crop. Crop rotation has not been
sufficiently stressed in this state, where large areas of the most fertile
lands are still under a system of bonanza farming.

Smothering. — The control of perennials by smothering with straw,
manure, or paper, though not practicable for large infestations, may be
successful on small patches. Weeds with indeterminate growth, like
morning-glory, are extremely difficult to keep under cover. On others
more easily managed, like Bermuda and Johnson grass and Russian
knapweed, smothering has proved successful. If continued long enough,
it should be effective on any plant that can be successfully covered.

Smother crops are not so generally useful in California as elsewhere.
Millet, Sudan grass, sweet clover, sunflowers, rape, rye, and sorghums,
unless combined with some other agency, can do little except hold the
weed in check. Any crop that offers competition, however, tends to re-
duce the vigor of perennial weeds and to check their spread ; and it is
always advisable, if nothing else can be done, to plant one of the crops
just listed rather than let noxious weeds thrive undisturbed.

METHODS OF CHEMICAL WEED CONTROL

Within the past few years, progress has been made in chemical weed con-
trol. Not only have new chemicals been added to the list of herbicides,
but research has increased our knowledge of the reagents previously
used. Although no revolutionary methods have been introduced, recent
information permits a much more intelligent application of chemicals
than was possible in the past.

The types of herbicides being used with success in California are as
follows :

Contact Herbicides. — These chemicals kill only the tissues to which
they are applied. They include oils, dilute sulfuric acid, dilute sodium
arsenite, 2 per cent sodium chlorate, iron sulfate, copper sulfate, and
sodium metaborate. They are used principally on annual weeds to pre-
vent seeding, to reduce fire hazard, and to destroy weeds which harbor
insect pests and plant diseases.

Selective Sprays. — These solutions, applied to mixed vegetation, kill
the broad-leaved annuals having exposed growing points but do not
seriously affect cereals and grasses. Dilute sulfuric acid, iron, copper,

22 California Agricultural Extension Service [Cir. 97

and ammonium sulfate solutions are the most important. Although
Kainite and calcium cyanamide are used as dusts for the same purpose,
the fertilizing elements that they carry are not usually required in Cali-
fornia soils ; they cannot, therefore, be generally recommended.

Translocated Sprays. — These sprays not only affect the tissues to
which they are applied but, under certain rather specific conditions, are
carried through the conducting system of the plant deep into the roots
or rootstocks. For this purpose a solution containing % per cent arsenic
trioxide in the form of sodium arsenite and 5 per cent sulfuric acid by
weight has been found most satisfactory. Other chemicals that will have
the same effect on plants are arsenic acid, arsenic trichloride, and sodium
chlorate. Translocated sprays are used on certain deep-rooted perennials.

Temporary Soil Sterilants. — Sodium chlorate applied in the fall or
early winter, either dry or in solution, will be leached into the soil by
rains. After plant roots have absorbed a sufficient quantity of this com-
pound, the entire root system may be killed. By decomposition and leach-
ing, the chemical is gradually lost from the soil, so that in time crops
may be grown. The success of a given treatment depends upon factors
to be discussed later in detail.

Carbon disulfide injected into the soil and sealed in by a compact or
moist soil layer will volatilize and fill the air spaces with a vapor poison-
ous to plant roots. It diffuses, usually in 3 to 6 weeks, leaving the soil
unaffected for subsequent crops.

Relatively Permanent Soil Sterilants. — These chemicals, applied di-
rectly to the soil, render it sterile for several years. They act as poisons,
killing the emerging seedlings. Among them sodium arsenite, arsenic
trioxide, sodium chlorate, and various sodium borates are the most prom-
ising. They are used on ditchbanks, roadsides, fence lines, and other
waste areas to prevent all plant growth.

As shown by the discussion of weed characteristics, there can be no
universal method of controlling these pests. Methods must be adapted
to the specific weeds and to the attendant conditions as these factors
relate to the problem at hand. In any intelligent practice the properties
of the weed and their relation to the environment largely determine the
reagent to be used and the method of application. General recommenda-
tions, therefore, must be interpreted and amended. Their use should be
tempered with judgment and, if possible, based upon local experience.

With these points in mind, the various methods for using herbicides
in practical weed control will be discussed in detail.

1936] Weed Control 23

SEASONAL CONTROL OF WEEDS IN UNCROPPED AREAS

Under this heading will be considered the contact sprays used princi-
pally on annual weeds, as contrasted with the more permanent method
of soil sterilization. One should note at the outset that, wherever possible,
mechanical means rather than chemical are more economical for con-
trolling annual weeds. This matter will be further considered in a later
section. On some areas, however, chemicals can be used economically;
and there is then considerable question as to the most suitable reagent.

The principal agencies now engaged in seasonal control of weeds in
California are the railroads and the State Highway Commission. Broadly
speaking, their purpose is threefold : fire prevention, pest control, and
elimination of unsightly weed growth. Many farmers and industrial or-
ganizations have followed their example and adopted their methods.
Although these activities have served as a country- wide demonstration
of the effectiveness and practicability of chemical weed control, improve-
ments in method could probably be made at the present time.

Oils. — The Highway Commission has used Diesel oil almost exclu-
sively in its control program. This material, being relatively cheap in
California, can be applied at a cost of 5 to 6 cents per gallon. Application
rates vary from 300 to 500 gallons or more per acre, according to the
height and density of the vegetation. The oils have sometimes been used
at the time of burning the dead vegetation. Oil that penetrates the seed
coat almost invariably kills seeds.

The cost of such treatment is not prohibitive and, in view of the prop-
erty saved, has been thoroughly justified. In some seasons, however, late
spring rains stimulate a second crop of weeds on the sprayed areas and
two or more applications become necessary. Diesel oil, furthermore, has
little cumulative effect and must be used every year. For these reasons
the method of chemical soil sterilization could perhaps well be applied to
this problem of roadside weed control, at least in regions where climatic
and soil factors would favor effective results.

Diesel oil is, at best, strictly a contact killer. It spreads very well on
all vegetation and is particularly effective on grasses. It kills most
rapidly during hot weather and, if properly applied, will destroy all
vegetation down to the ground level.

For a comparison of oils and mechanical methods in the control of
roadside weeds, see page 49.

Sulfuric Acid. — The increasing use of Diesel oil as fuel and the at-
tendant rise in price make it advisable to consider a substitute. In this
field dilute sulfuric acid seems to offer the greatest possibilities. It does
not spread well, but certain commercial spreaders now available may

24 California Agricultural Extension Service [Cm. 97

solve this problem. Improved equipment described in a later section
also favors the use of this material. Experiments now in progress look
toward the use of sulfuric acid both in aqueous solution and in an oil
emulsion as a herbicide for annual weeds. No recommendations for other
weeds than mustard and wild radish can yet be given ; but possibly, as
Diesel oil is required in greater amounts as a fuel, sulfuric acid will
take its place in weed control.

Sodium Arsenite. — For years the railroads have used a dilute sodium
arsenite solution as a weed killer on their roadbeds. This practice, though
inexpensive and fairly effective, has led to numerous lawsuits over
poisoned cattle and has impaired the popularity of chemical weed con-
trol. The continued use of arsenic in this way usually results in an
accumulation ; as a partial soil sterility is built up, the applications are
required less often.

In view of the results recently obtained by using arsenic in heavier
dosages as a soil sterilant, and the difficulties resulting from the present
practice, the railroad companies would probably do well to apply the
arsenic less frequently and in larger doses, making the application
directly to the soil during the winter. Thus they would avoid many of
the stock-poisoning cases ; they would have clean roadbeds at all times ;
and they would be making more efficient use of the arsenic applied. The
same could be said of all others who use dilute sodium arsenite as a con-
tact spray. There seems to be little excuse for the continued use of
arsenic solutions as "weed killers" on growing vegetation in view of our
present knowledge of mechanical weed control and chemical soil sterili-
zation.

CONTROL OF ANNUAL WEEDS IN CEREALS WITH DILUTE
SULFURIC ACID»

The differential killing of broad-leaved plants in grain by chemical
sprays has been known since 1896. Though experimental work with iron
sulfate in the early years of this century indicated the possibilities of
the method, not until investigations in Europe with dilute sulfuric acid
proved the feasibility of the method did it become common. According
to reports, 70,000 tons of sulfuric acid were used in weed control in
France in 1933, and over 20,000 acres of cereal crops were sprayed in
England in 1934. Much of this work, however, was done with knapsack
or horse-drawn equipment on the small farms of those countries.

The large grain farms of California require boom equipment capable

® This section was prepared by Mr. W. E. Ball, Associate in the Experiment Station
and Investigator for the Crop Protection Institute. The equipment used is more fully
described in California Exp. Sta. Bui. No. 596, The control of weeds with sulfuric
acid, by W. E. Ball and O. C. French.

1936]

Weed Control

25

of covering many acres per day. Cooperative work aiming toward devel-
opment of such equipment lias been carried on by the divisions of Botany
and Agricultural Engineering at Davis during the last two years. A

Fig. 4. — Equipment used in applying dilute sulfuric acid in the control of
mustard in grain fields. The water tank has a capacity of 400 gallons ; the drum
containing concentrated sulfuric acid is attached behind the tank ; the boom has
a spread of 20 feet.

Fig. 5. — Mustard plants in the best stages of development for treatment
with sulfuric acid. The corresponding sizes of grain plants are shown at the
right. (From Bui. 596.)

machine (fig. 4) designed during the past winter, was used in the field
throughout February, March, and April, 1935. At present this equip-
ment appears satisfactory in the use of dilute sulfuric acid in California.
It departs in design from the customary practice of pumping the herbi-
cide directly. In it, water from the pump under a pressure of about 200
pounds per square inch is passed through a Penberthy ejector, which
causes concentrated sulfuric acid to be drawn in directly from a con-
tainer and mixed thoroughly at a fixed concentration. The amount of
acid drawn into the water stream is controlled by an orifice in a thin

26

California Agricultural Extension Service [Cir. 97

nickel washer held by a union in the suction line. The diluted acid from
the ejector is distributed through a brass boom equipped with special noz-
zles. The nozzles on the boom are so arranged that the plants are sprayed
twice from different angles as the boom passes over them, giving a very
satisfactory coverage. All parts of the equipment coming in contact with
the acid are of brass, and little corrosion could be found after the first

Fig. 6. — The effect of dilute sulfuric acid in the control of wild radish in
wheat. The unsprayed strip at the right is indicated by the radish in bloom.
(From Bui. 596.)

season's run. Many standard orchard sprayers may be readily converted
at little cost into equipment of this nature.

With this method best results are obtained by spraying mustard from
4 to 7 inches high (fig. 5). Control approximating 90 to 95 per cent has
followed such treatments. A solution of 10 per cent sulfuric acid by
weight at a rate of 130 gallons per acre has proved best. The concentra-
tion of the acid discharged may be measured with a hydrometer, which
gives the specific gravity of the solution. A 10 per cent solution by weight
at 25° C has a specific gravity of 1.064. Concentrated sulfuric acid
weighs approximately 15 pounds per gallon. A pressure of 75 pounds
per square inch at the nozzles gives excellent coverage. Additional
pressure causes undesirable drifting of the spray.

At present prices the acid costs about $1.40 per acre. Cost of applica-
tion depends largely upon the size and efficiency of the spraying oper-
ations. "With the equipment mentioned above, operating at $1.50 per
acre, the total cost was approximately $3.00 per acre.

This method, though used principally to control mustard, is also
effective against wild radish, Amsinckia, various Chenopodium and
Atriplex species, thistles, clovers, and other annuals occurring in grain.

1936] Weed Control - 27

In the experimental work, control of some of these other species has
brought greater increases in yield than when only mustards were killed.
Another unforeseen virtue of the sulfuric acid spray is that the treated
grain is less subject to lodging ; thus the harvest yield may be further
increased. In heavily infested fields the treated areas have yielded 50 to
60 per cent more grain than the untreated (fig. 6) .

In mixing acid and water, always add the acid to the water, not the
water to the acid. Wear old clothing, for the acid is corrosive ; and choose
rubber or woolen clothes rather than cotton. A solution of bicarbonate
of soda will neutralize any acid splashed on the hands or face.

To summarize, dilute sulfuric acid has many advantages as a weed-
control agent : (1) it is economical, a gallon of 10 per cent solution cost-
ing approximately one cent ; (2) it is neither poisonous to livestock nor
combustible; (3) it reacts with plant tissue very quickly and is there-
fore not seriously affected by climatic conditions ; and (4) it is not detri-
mental to the soil as are certain common weed sprays.

Dilute sulfuric acid has one unfortunate characteristic : it is highly
corrosive to most metals and to clothing. This difficulty has been largely
overcome, however, by the special equipment described above.

THE ACID ARSENICAL METHOD

The control of deep-rooted perennial weeds requires a herbicide that
will kill the roots and rootstocks of plants as well as the tops. An acid
arsenical solution developed at Davis will do this with fair success on
certain of our noxious weeds. This type of spray depends for its effect
upon translocation ; the chemical moves within the plant from the tops
to which it is applied, deep into the underground organs. This movement
of the herbicide within the plant depends in turn upon a physiological
response to the spray. For this response to occur, certain conditions are
required ; in fact, they largely determine the results of a given treat-
ment.

The mechanism responsible for the movement of the arsenical herbi-
cide into the roots after its application to the leaves is as follows : Dur-
ing the summer days in California loss of water from the leaves usually
exceeds uptake by the roots. When this happens, pressure within the
water-conducting tissues of the plant becomes reduced or subatmos-
pheric, and water is drawn from turgid living cells throughout the plant.
When this process takes place very quickly the plant promptly wilts ;
but when the condition develops slowly, the plant may adapt itself
somewhat and a large water deficit may exist with little visible evidence.

When a plant with a large water deficit in its tissues is sprayed with
a strongly acid solution, the living cells of the leaves and stems are

28 California Agricultural Extension Service [Cir. 97

rapidly killed and rendered permeable, and all moisture free to move is
forced into the water-conducting tissue and carried down into the roots.
If the spray solution contains arsenic, this poison will diffuse into the
killed tissue and, mixing with the sap, will also be drawn into the roots.
The depth of its penetration and the thoroughness of its distribution
into the root system depend upon several factors. Among these water
deficiency in the plant tissues has been mentioned. In addition, the
water-supplying power of the soil must be low, or the deficit will be
satisfied with water absorbed by the roots. The plants furthermore must
have a large top growth to allow application of sufficient arsenic ; spray-
ing must be done at night so that the chemical can act before the solution
dries on the leaves ; and the effectiveness can be further enhanced by
making the total application in two separate sprayings, followed next
morning with one or more applications of water.

To summarize : the acid arsenical spray should be applied after dark
in the summer or fall to mature plants having ample top growth, which
have been allowed to grow undisturbed until the available soil moisture
is practically depleted. Spraying should not be done during unusually
hot, dry, or windy weather.

Success largely depends upon the type of growth of the plants. With
morning-glory in central California the best results are probably ob-
tained by allowing the first flush of growth in the spring to mature and
then spraying in June or early July. If this plan cannot be followed, the
areas may be irrigated at any time during the summer before August 15
and then allowed to mature and to deplete the soil moisture before being
sprayed. The best index to the soil moisture supply is the condition of
the plants ; lush, green foliage with rapid end growth on all shoots de-
notes high moisture. When moisture becomes limiting, end growth
ceases, and the foliage tends to reach an even state of maturity. Only
when this condition is reached should the spraying be done.

Where morning-glory has been cultivated several times during the
summer and later allowed to mature, the results with the acid arsenical
have been less satisfactory. Only when an irrigation is used to insure a
uniform, abundant growth of foliage is this plan successful. Where irri-
gation is not available, the early summer growth should be normal to in-
sure satisfactory treatment. During the winters of 1932-33 and 1933-34,
the rainfall was deficient at Davis, and the spring growth of morning-
glory was sparse. The soil had been moistened to less than 3 feet, and the
roots of plants treated with the acid arsenical during the subsequent
summers were only superficially killed. This method, therefore, is inad-
visable on morning-glory in seasons when the rainfall has been notably
deficient unless irrigation water is available to produce a heavy growth.

1936]

Weed Control

29

The average stand of morning-glory can be covered with about 250
gallons of spray per acre. For satisfactory results about twice this
amount is required. To conserve the solution and provide maximum

Fig. 7. — Showing use of the "jar method" in treatment of camel
thorn. Upper, inserting the top of the plant in the jar; lower, filling
the jar with a 1 per cent arsenic solution. The plants are left in the
solution for 24 to 48 hours.

penetration, the areas should be sprayed at a rate of 250 gallons per acre
and then, after an interval of 20 to 40 minutes, sprayed a second time.
This insures maximum coverage and gives the first application time to

30 California Agricultural Extension Service [Cir. 97

act on the foliage so that the second penetrates rapidly. Where the foli-
age is very heavy the treatment can well be increased to two applications
of 350 gallons per acre, or three of 250 gallons. Proper spraying equip-
ment will be described later.

The results thus far obtained with the acid arsenical have been best
on Russian knapweed, variable on morning-glory, even less successful on
alkali mallow, and negligible on hoary cress. Success is probably related
to the root structure ; plants having abundant storage tissue as compared
with the woody water-conducting cylinder give the most satisfactory
response.

A modification of the acid arsenical method has been useful on some
perennials. In this "jar" or "dipping" method, as it is called, the sulfuric
acid may be omitted, although there are some indications that its pres-
ence is not detrimental; the tops of the plants are simply bent over,
forced into a glass jar or tin can (fig. 7), and covered with a solution
containing 1 per cent arsenic trioxide by weight. To make a 1 per cent
arsenic solution, add 2 pounds of the stock solution described on page 31
to 100 pounds of water. As the foliage is killed and rendered permeable
by the arsenic, the solution is drawn into the root system until all water
deficiency is satisfied. Even better results are obtained on weeds that
are interconnected through their roots. In this case, if one plant is
treated all the connected plants draw on the solution and the entire
system is killed. This situation occurs in camel thorn, where excellent
results have been obtained. The method is also useful in killing indi-
vidual morning-glory plants in gardens or strawberry beds. Although
effective on camel thorn, morning-glory, and white horse nettle, it gives
less favorable results on Russian knapweed, hoary cress, or alkali
mallow. This method is usually practicable only in scattered infestations
of perennial weeds.

Other chemicals used as translocated sprays are arsenic acid, arsenic
trichloride, and sodium chlorate. The first two are no more effective than
arsenic trioxide used according to directions to be given, and they are
more expensive. Although sodium chlorate is nonpoisonous to livestock,
as a translocated spray it has all the limitations of the acid arsenical,
besides being much more expensive. If followed by an application of 5
per cent sulfuric acid, the chlorate spray is effective at a dosage of 1
pound per square rod on morning-glory. In this form the method might
find some use in pastures. Under most conditions, however, this chemical
gives best results when used in a soil treatment.

Preparation of the Acid Arsenical Stock Solution. — The spray solu-
tion used in the acid arsenical method is easily prepared and applied. It
consists of a stock arsenic solution, concentrated sulfuric acid, and water.

1936] Weed Control 31

The stock solution is composed of 4 parts by weight of arsenic trioxide,
1 part of sodium hydroxide, and 3 parts of water. When mixed in these
proportions the chemicals react, evolving; heat, and dissolve rapidly.

Enough stock solution for one-half acre or more can be conveniently
mixed in a bucket or can of about 5 gallons' capacity. For this small
amount mix thoroughly, in the dry condition, 20 pounds of arsenic
trioxide (AsoOo, white arsenic, arsenious oxide) and 5 pounds of sodium
hydroxide (NaOH, caustic soda, soda lye) in the bucket. Weigh out 15
pounds of water and add about half of it to the mixed chemicals, stirring
to loosen the arsenic and lye. The mixture will heat rapidly ; as it ap-
proaches the boiling point, the arsenic goes into solution, forming sodium
arsenite. If boiling becomes violent, add a little cold water. Continue to
stir, loosening the chemicals from the bottom until they are entirely free
in the solution. When boiling ceases, any arsenic that remains undis-
solved should be mashed against the container with a paddle. A piece of
wire screen can be used to hold the lumps of arsenic against the side
while mashing. When the chemicals are completely dissolved, add the
remaining weighed water, stir thoroughly, and strain through doubled
cheesecloth. This stock solution is now ready to use. It can be kept a
few weeks but will crystallize if allowed to evaporate. It can be redis-
solved by boiling.

Preparation of Large Qnaiitities of Stock Solution. — Equipment for
mixing 160 pounds of stock solution at a time can be made by mounting
a 50-gallon oil drum horizontally upon wooden rockers. A large opening
in the center on top and a gate valve or molasses spigot in one end at the
bottom allow for adding the chemicals and withdrawing the solution.
There should be a cover for the top opening. To prepare a batch, weigh
out 60 pounds of water, pour 30 pounds into the drum, and add 20 pounds
of sodium hydroxide. Mix by rocking the drum so that the solution
splashes violently against the ends. Weigh out 80 pounds of arsenic
trioxide ; as the solution warms, add it a few pounds at a time, using a
sugar scoop or can. Mix well between additions. If boiling becomes
violent, add more water. Keep the opening covered between times. When
the arsenic is all in solution, add the remaining water, mix well, and
draw off through a fine screen or cheesecloth filter. By using a 100-
gallon drum, one can prepare even larger quantities and save consider-
able time. This stock solution contains 50 per cent AsgO^ by weight and
weighs approximately 16 pounds to the gallon.

The Spray Solution. — Experiments have shown that a satisfactory
spray solution must contain at least % per cent arsenic trioxide by
weight and 5 per cent sulfuric acid. To make 3 gallons of this solution,
for a knapsack sprayer, add to 3 gallons of water % pound of stock

32 California Agricultural Extension Service [Cir. 97

arsenic solution, stir well, pour in 1% pounds of concentrated sulfuric
acid, mix, and apply. For a 100-gallon lot pour 8 pounds (approximately
% gallon) of the stock arsenic solution into 96 gallons of water in the
spray tank, agitate thoroughly, add slowly 40 pounds (approximately
21/2 gallons) of concentrated sulfuric acid, agitating constantly. Best
results follow application of at least 500 gallons per acre of solid infes-
tation, using 75 to 100 pounds' pressure with Chipman nozzles. When
the temperature is low, more acid may be used to increase the rate of
penetration. A boom outfit with fixed nozzles gives more complete cover-
age than hand application. Some advantage may be gained by dividing
the total treatment into two separate applications of 250 to 300 gallons
each with an interval of 20 to 40 minutes between them. If there is con-
siderable wind or the foliage is unusually dense, at least three such ap-
plications should be made.

Precautions. — Use extreme caution in mixing and applying these
solutions. Do all mixing in the open air, avoiding fumes. The stock solu-
tion heats rapidly and spatters. Keep to one side if mixing it in an open
container. The boiling solution, being extremely hot, will cause severe
burns and poisoning. Soluble arsenic is extremely poisonous. Keep stock
and spray solutions out of cuts or open sores ; avoid all contact as much
as possible. In making up the spray solution, pour the acid into the
water with constant stirring. Avoid slopping and spilling. Sulfuric acid
will cause severe burns and will rapidly dissolve cotton clothing. Even
the 5 per cent acid of the spray solution will ruin clothing. Keep a bucket
of bicarbonate of soda (baking soda) solution available on the spray rig
and immediately wash hands, face, or clothes that come in contact with
the spray.

Arsenic is poisonous to livestock. One week after application of the
spray the weeds may be burned, disked, or plowed under. If they are
burned, avoid the smoke, as it carries arsenic and may cause serious
poisoning.

TEMPORARY SOIL TREATMENTS

Sodium Chlorate. — Sodium chlorate, a white crystalline substance, is
obtainable from any large chemical supply house, usually in a finely
divided form not unlike table salt. Though relatively nonpoisonous, it
involves a fire hazard, which will be discussed later. The proprietary
herbicide '^Atlacide" is a mixture of sodium chlorate and calcium
chloride.

Workers on weed control have been much confused as to the way in
which sodium chlorate kills plants. It is becoming apparent, however,
that the principal action, under most conditions, is through the soil.

1936] Weed Control 33

As described above, the plants absorb this chemical ; and, when enough
has been accumulated, they sicken and die. Recent work in California
has determined the main factors involved in the successful use of
chlorate through the soil.

Before going into the more specific effects of chlorate, we may con-
sider briefly the inherent differences in plants with respect to injury.
Many plants are easily killed. When sprayed with a contact herbicide
or cut off at the ground level, they die. Others will resprout from the
stump or crown ; and still others, including many noxious weeds, will
regenerate from underground stem or root tissue and are eradicated
only when all vestiges of these tissues are killed. In treating a variety of
plants with sodium chlorate, one is sure to find differences in the degree
of control ; and unless he has previously determined the response to
mechanical injury, he cannot interpret accurately his results. Plants
differ widely, furthermore, in their susceptibility to injury by chlorates.
In one instance reported, a certain parasitic plant was controlled by the
use of 12 pounds of sodium chlorate per acre. In other cases, over 1,000
pounds per acre have failed to control certain resistant species. This
problem of species' susceptibility must be considered in prescribing the
dosage for any particular situation.

The influence of soil type upon chlorate toxicity is another important
factor. Although experiments are still in progress and final determina-
tions have not been made, it now seems that, within a soil series, toxicity
of chlorate runs higher in the coarser textural grades. Among series,
alluvial soils of recent origin show the lowest toxicities ; old weathered
soils the highest. A recent survey of several California soils shows one
exception to the first rule. Extremely heavy clays and adobe clays show
high toxicities. This is very apparent in primary clay soils and in heavily
leached or water-laid clays. Soils high in organic matter, such as peats or
semipeats from the Sacramento River basin, show low toxicities com-
parable with those of the highly fertile recent alluvial soils.

Another factor in the use of chlorate through soils is that certain soils
retain this chemical so that it does not move freely with soil moisture. In
general, soils showing the lowest toxicity have the strongest ability to
retain chlorate, and in them the vertical distribution of applied chlorate
is a problem. In Davis, morning-glory must be killed to a depth of at
least 4 feet or it will resprout and survive. In a Yolo loam soil, chlorate
has been leached to that depth with 6 surface inches of water. In a Yolo
clay, double this amount was required. Where the moisture comes as rain-
fall with opportunity for evaporation between storms, even more water
Avould be needed. This one factor probably explains many of the failures
with chlorate treatments in the arid regions of the West.

34 California Agricultural Extension Service [Cir. 97

Chlorate decomposition is conditioned largely by moisture and tem-
perature. In the humid sections where soils are moist during the summer,
little difficulty is experienced with residual chlorate. In the West, where
the soil is dry during the summer heat, sterilization has sometimes per-
sisted for three years or longer. As experiments have shown, leaching
with water is the easiest way to rid the land of chlorate. Where irrigation
water is not available, chlorates must be used with considerable dis-
cretion on heavy soils.

The proper season for chlorate application has been frequently dis-
cussed, and the issue has been confused by results of applications made
in the West according to recommendations formulated in the humid
East. In California, experiments indicate three feasible methods: (1)
fall spraying where rapid absorption and root killing, after transloca-
tion of part of the chlorate, are followed by leaching by the winter rains
and absorption from the soil; (2) a straight soil treatment during the
winter; (3) spring soil treatment followed by proper irrigation. All
three methods aim to provide a lethal concentration of chlorate in the
soil solution during the spring for absorption by the roots during the
period when they are depleted of organic foods and engaged in rapid
absorption from the soil. The methods are subject, however, to differ-
ences in growth conditions and soil type ; and the first two, which depend
upon rainfall, must be used with utmost care.

Evidently, then, there can be no universally successful procedure with
chlorates. Because of the extreme variations in species' susceptibility^
and the numerous factors affecting toxicity and absorption, methods
must be adapted to conditions.

In so complex a situation, recommendation of dosages is hazardous.
The old formula of 3 pounds per square rod applied in 3 gallons of Avater
is a good average figure. For morning-glory at Davis it represents a bare
minimum ; 4 pounds would be a better general dosage. For hoary cress
in a Yolo type soil 6 and even 8 pounds were not enough. In the same
locality 4 pounds were satisfactory on a Montezuma adobe clay. Whereas
2 pounds per square rod killed Bermuda grass in Fresno sandy loam at
Turlock, 8 pounds were not enough in Yolo clay at Davis. As these few
figures show, experimentation by trial-and-error under local conditions
is the only reliable method for determining dosages. In this case a little
local experience is worth much more than a large amount of expert
advice.

In the soil-treatment method, concentration of the applied solution
has little significance and becomes mainly a matter of convenience. In
the first method listed above, where translocation plays a part, the con-
centration should be equivalent to one or more pounds of sodium chlorate

1936] Weed Control 35

per gallon of water to insure prompt penetration. For the straight soil
treatments the chemical may he applied dry provided uniform distri-
hution can be had.

Precautions. — Many warnings have been sounded as to the dangers
of sodium chlorate. Though the soil-treatment method tends to minimize
the hazard, one should remember the properties of this chemical and be
cautious. Sodium chlorate added to organic material, such as cotton
cloth, wood, or straw, forms a mixture similar to black gunpowder, being
highly inflammable and easily ignited. It burns with great violence and
cannot be smothered, since the chlorate supplies the oxygen. The safest
policy is to handle it with care and avoid spilling or loss from the con-
tainers. In case of fire, use water and flood the burning material.

Men should preferably not Avork alone in using chlorates. If two or
more are together, assistance is at hand in case of fire. A bucket of water
may extinguish the blaze and prevent a serious accident. Spray equip-
ment should carry a fire extinguisher.

In handling chlorates, as during loading and unloading, or in prepar-
ing the spray solution, any crystals or solution accidentally spilled on
floors, trucks, or machinery should be washed off with a hose or otherwise
drenched with water.

As chlorates are particularly combustible when in contact with sulfur,
great caution should be taken not to store sulfur, or spray materials con-
taining sulfur, near chlorates. Moreover, a spray machine once employed
in applying sulfur spray should be thoroughly washed inside and out-
side before being used for chlorates.

Accidents can occur after spraying operations are completed, if the
clothing is allowed to dry on the body. This danger can be eliminated by
having two pairs of overalls. At noon one pair can be rinsed out in clean
water to remove all chlorate, and the dry pair put on for the afternoon ;
this pair, in turn, should be washed in the evening. Chlorate should not
be allowed to dry on the operator's rubber boots, and they should be
washed every evening.

Keep spray equipment well painted so that the solution does not
saturate the wood. Metals are better than wooden containers for the
chlorate solution. Wooden containers, if employed, should be washed
out thoroughly after use and even allowed to stand for several days
filled with water. A wooden barrel, for example, used for chlorate solu-
tion, when emptied and dried out, becomes highly combustible. As an
extra precaution, mix solutions away from outbuildings, barns, or straw
stacks.

Chlorates should be purchased preferably in metal containers. If
shipped and stored in sacks they should be kept dry, for they absorb

36

California Agricultural Extension Service

[CiR. 97

moisture from the atmosphere ; and the chemical, in close contact with
the fiber of the sack, presents, when dry, a fire hazard.

In emphasizinof these precautions, one should remember that chlorate
sprays are no more dangerous than j>asoline and other chemicals used

Fig. 8. — Mechanical applicator for carbon disulfide. Along the
shank of each siibsoiler is a tube carrying the chemical. Di-um capac-
ity, 550 pounds of carbon disulfide.

daily on the farm as insecticides and fungicides, and less hazardous
than many chemical compounds handled constantly in the industries. If
chlorates are effective herbicides, the care necessary should not be a
serious drawback to their use.

Carbon Disulfide. — Another chemical that can be used as a temporary
soil sterilant is carbon disulfide. A clear, volatile liquid, it readily vapor-
izes to give an extremely toxic gas, which is heavier than air. When

1936] Weed Control 37

injected into the soil and sealed in to prevent excessive loss, it kills all
roots within the treated soil mass and is therefore extremely effective in
eradicating- noxious perennial weeds.

Experimental work has been done which seems to show that the effec-
tiveness of carbon disulfide is influenced by soil factors. The recommen-
dations as to rates of application are sufficiently high to cover any small
variations in toxicity and species' susceptibility and can usually be fol-
lowed with reasonable assurance.

Fig. 9. — Equipment used in applying carbon disulfide. The mechanical appli-
cator is followed by a heavy roller which compacts the soil, which prevents the
ready escape of carbon disulfide fumes.

The usual method of treating small areas is to go over them with a
weed cutter or hoe and then apply the liquid by making holes in the
soil 18 inches apart each w^ay in staggered rows and pouring from 2 to 4
ounces into each hole. The depth of application is important. If there is
ample moisture in the surface soil the chemical should be applied at a
depth of not over 6 inches, and then the soil thoroughly sealed. If, on
account of lack of surface soil moisture, it is necessary to apply deeper,
crown growth may occur because the fumes, being heavier than air, do
not come into contact with the crowns in sufficient amount to effect a
kill. As soon as the chemical is applied, the holes should be filled with soil
and carefully tamped.

The manufacturers of carbon disulfide have a mechanical applicator
(fig. 8) consisting of a 3-standard subsoiler equipped with a container,
valves, and fittings. This machine, drawn by a caterpillar tractor, will
apply a drum of the chemical in 20 minutes — under average conditions,
a rate of one acre per hour.

In the treatment of large areas, care should be exercised in the prep-

38 California Agricultural Extension Service [Cir. 97

aration of the soil prior to the application of the chemical. This usually
means a shallow disking or plowing which will make possible a good
sealing of the soil by rolling (fig. 9) after the chemical is applied.

The principal drawback to carbon disulfide is the expense. Treat-
ments using equipment described above will cost from $200 to $250 per
acre, according to the dosage and the method of application. The chemi-
cal has the advantage of leaving the soil rapidly so that crops may be
grown ; and being applied through the soil, it offers a higher degree of
control than a translocated spray.

Carbon disulfide vapor is inflammahle and, whe7i mixed with air, is
explosive. The liquid must be handled with care. Empty drums are par-
ticularly dangerous.

On small areas of shallow-rooted perennials, such as Bermuda grass
and nut grass, carbon disulfide has been successfully used by sprinkling
it on the surface of the soil and quickly covering it with damp canvas.
The covering should be kept wet so that the vapor will not escape, and it
must be left on for at least 24 hours.

SOIL STERILIZATION

With the intensification of agriculture on the more fertile soils of the
state, pest control is increasing in importance. Considering the relation
of weeds to insects and plant diseases, and the steady, persistent ingress
of noxious species, complete control of all weed growth over waste areas
in our more important agricultural districts appears highly desirable. In
many areas, furthermore, weeds are now being controlled by inefficient
methods with contact sprays and hoe labor. In these cases soil steriliza-
tion of all areas that cannot be handled by mechanical means seems
highly advisable. For this work sodium arsenite has proved most effec-
tive, though arsenic trioxide, sodium chlorate, and certain borate ores
offer promise.

Sodium Arsenite as a Soil Sterilant. — In the use of sodium arsenite in
soils, toxicity is again affected by the type of soil. The chemical is most
toxic in light, sandy soils and least in heavy clays. The adobe clays show
high toxicities, which, however, decrease rapidly with time. In field-plot
tests, 2 pounds of arsenic trioxide per square rod, applied in the form of
sodium arsenite, has proved as effective on Fresno sandy loam as 8
pounds on Yolo clay. Over a fairly wide range of soil types, 4 pounds
per square rod is usually recommended and gives excellent results. In
the fertile recent alluvial soils and organic soils found in California, at
least 8 pounds per square rod is required ; and even this amount is soon
rendered unavailable to plants.

The retention of arsenic in an available form in high concentration is

1936]

Weed Control

39

a common property of all soils studied. The concentration held depends
upon soil type, being highest in the heavier soils; it also depends upon
the dosage, being higher where applications are heav.y.

i*^4-i

f

T fh- T ■%

Fig. 10. — Soil sterilization on a cement-lined ditch. Three-foot
strips on each side of the ditch have been treated with sodium
arsenite.

Arsenic in the soil is fairly resistant to leaching. Passage of 64 surface
inches of water through 3-foot columns of soils failed to remove all the
arsenic from medium and heavy-textured soils, whereas 16 inches re-
moved it from Fresno sandy loam. In the field, arsenic applied at a rate
of 4 pounds per square rod to Bermuda grass has been effective on the

40 California Agricultural Extension Service [Cm. 97

Fresno soil but not on Yolo loam. Tests on the soil showed arsenic to be
present in the sandy soil to a depth of 12 inches or more after the first
winter rains. In the Yolo soil it was confined to the top 4 inches.

In field plots, 4 pounds of arsenic per square rod on Fresno sandy
loam rendered the soil sterile for four years (fig. 10) . Although enough
rain fell during this time to leach the arsenic from the top soil, had it
been applied continuously, the fact that it fell intermittently made it
less effective in moving the chemical. This factor must be considered in
interpreting the effects of rainfall on sterilized soils. Light, intermittent
rainfall will not wash away the sterilant nearly so effectively as steady
rains or flooding. For this reason, dosage and type of application of
sodium arsenite for soil sterilization must be adapted to climatic condi-
tions as well as to soil types. Where rainfall is heavy, excessive loss from
leaching may be avoided by light annual applications. In drier localities,
one heavy application may be effective for as many as four successive
years.

Although sodium arsenite may be purchased in a stock solution known
as "weed killer," if more than a few pounds will be required it may be
economically prepared by the formula for the stock arsenic solution
given on page 31. With the steel-drum equipment and with proper load-
ing and emptying facilities, one man can make up to 4 tons of stock
solution per day. This stock solution containing 50 per cent arsenic tri-
oxide by weight may be diluted with water to any convenient volume
for application. For the usual application of 4 pounds per square rod,
a satisfactory solution is made by measuring out 8 pounds of the stock
solution and diluting it with water up to a total volume of 3 gallons.
Since the stock arsenic solution weighs approximately 16 pounds to the
gallon, this would be, on a volume basis, a half-gallon of stock solution to
2% gallons of water, making a 1:5 mixture. Though this dilution is
recommended for general work, the proportions of stock solution to
water may be varied between wide limits for any particular equipment
or conditions. The important factor is the dosage in terms of arsenic
trioxide in pounds per square rod. This is determined by soil type and
environmental factors and should not fluctuate for any other reason.

Dry White Arsenic as a Soil Sterilant. — Arsenic trioxide or white
arsenic, as it is commonly called, is much less soluble than sodium
arsenite. For this reason it does not sterilize the soil during the first year
after its application. By the second year, however, enough becomes avail-
able in the soil to affect plants ; and from that time on its seems equal in
effectiveness to sodium arsenite applied in solution. In fact, because of
its low solubility, it seems more lasting than the arsenite and therefore
more economical.

1936] Weed Control 41

There are two possible ways of using this dry white arsenic in soil
sterilization. It may be applied directly to the soil, with enough sodium
chlorate added to give sterilization during the first year. This plan is
particularly useful with perennials, for the chlorate will destroy many
of the deeper-rooted weeds and the arsenic will be set free and kill an-
nuals thereafter. Another method is to apply part of the total dosage of
arsenic in the solution form and then to follow this with the remainder
in the form of the dry white arsenic. The dry treatment is also recom-
mended to follow a sterilization with sodium arsenite solution that is
beginning to fail. It should be more permanent and economical than a
second solution treatment.

The extremely poisonous nature of arsenic has been pointed out. In
the large quantities used in soil sterilization it is particularly hazardous.
To avoid killing livestock it should be applied to the bare soil in the early
winter, after the first rains but before green vegetation has made any
appreciable growth. All treated areas should he posted with poison no-
tices, and livestock kept away until 2 or more inches of rain have fallen.
Special care should he taken to avoid applying arsenic to vegetation,
either alive or dead, which may retain the poison and increase the hazard.

The following precautions, prepared by a pharmacologist, should be
heeded by all who use this chemical :

Avoid all contact of arsenic with the skin, for repeated contacts may
cause a rash. Where unavoidable contact with arsenical sprays may
occur, protect exposed skin, eyelids, and lips with white petroleum
(vaseline).

Arsenical solutions are highly poisonous when absorbed into the body.
Symptoms are nausea, vomiting, diarrhea, intestinal pain, scanty burn-
ing urine. In case of accidental swallowing of arsenic, take eggs, milk,
and flour by mouth, and cause vomiting. Call a doctor immediately.

The accidental repeated ingestion of small amounts of arsenic over a
long period may cause the slow appearance of symptoms of chronic
poisoning, which are muscular weakness, scanty burning urination, and
intestinal disturbance. If these symptoms are suspected, consult a doctor.

Nonpoisono\is Soil Sterilants. — While arsenic may be used on ditch
banks and other waste areas on privately owned land with relative
safety, there are many places, such as public roads and schoolgrounds,
where nonpoisonous chemicals are required. The permanence of a sodium
chlorate treatment depends largely upon soil type and rainfall. In heavy
clay soils and in regions of light rainfall, chlorate will sterilize the soil
for two or more years. In other localities it still may be used by making
annual applications in the late winter before vegetative growth becomes
excessive.

42 California Agricultural Extension Service l^^^- ^^

Borax and certain borate ores not only have a lower solubility than
sodium chlorate, but are retained in the soil in an available form against
the leaching effects of moving water. This retention, though not so pro-
nounced as with arsenic, makes the borates more persistent than chlor-
ate. The toxicity of boron compounds in soils has long been known.
Pound for pound, anhydrous borax is approximately as toxic as sodium
chlorate or sodium arsenite. Its one drawback is a tendency to be fixed in
the soil in an unavailable form rather rapidly. On the other hand, crude
borate ores are much less expensive than chlorate or arsenic.

Borates are subject to the same wide variations of toxicity with soil
type and species' susceptibility as are chlorate and arsenic. Used by
themselves, they lack the intense killing power of these other chemicals.
Combined with sodium chlorate they usually form very satisfactory
sterilants.

According to experiments, a mixture containing 1 pound of sodium
chlorate to 8 pounds of borax, Colemanite, or Kramer ore forms the best
combination for soil sterilization. Applied dry at rates of 4 to 16 pounds
per square rod, according to soil type and vegetation, it gives excellent
results. One should not expect a single heavy application to last for
several years; annual treatments with minimum effective dosages are
more economical. Using this mixture dry during the winter on the bare
soil reduces to a minimum the hazard of the sodium chlorate involved.
One should, however, observe the precautions listed on page 35.

Although not on the market at present, Colemanite and Kramer ore
are in production and could be supplied if the demand existed. Judging
from tests, Colemanite is the most effective of the three boron com-
pounds used, probably because of its lower solubility.

A warning should be sounded concerning the danger of boron com-
pounds to valuable ornamental plants or commercial orchards. The
common varieties of citrus fruits, deciduous fruits, grapes, and nuts are
all so sensitive to boron that it should not be used near them. Certain
ornamentals also come within the list of sensitive species. On the other
hand, where there is no danger to valuable plants, the sterilizing mixture
mentioned above can be used with considerable satisfaction on graveled
drives and parking areas, schoolgrounds, or roadsides. Hours of hand
hoeing and weeding can often be avoided by a simple chemical treatment.

OTHER HERBICIDES

Many chemicals have been used in weed control at various times, and
new ones are frequently proposed. The most useful have already been
discussed, but a few others tested will be considered briefly.

Ammonium thiocyanate has recently been advocated for weed con-

1936 J Weed Control ' 43

trol. In our tests it proved ineffective as a translocated spray. In the
soil it was less toxic than chlorate and did not leach readily. Though
more expensive than arsenic, it was less persistent for soil sterilization.
There appears to be no logical place for it in chemical weed control in
California.

Calcium Chloron, a proprietary mixture introduced about three years
ago, proved worthless as a weed killer.

Ethylene dioxide, proposed in the East for weed control and tested
in barberry eradication, is expensive, extremely difficult to handle in
the field, and not available in quantities.

Atlacide and other proprietary mixtures containing sodium chlorate
are, according to workers, effective in proportion to their sodium chlor-
ate content. In no case reported have the diluting or adulterating chemi-
cals marked herbicidal properties. In soil treatment they have no known
value. The cost to the user of any one of these materials should be related
to the percentage of sodium chlorate in it.

Many arsenical "weed killers" are sold. In almost every instance these
are sodium arsenite solutions whose value is determined by their content
of arsenite trioxide. By comparison on this basis they will often be found
to be excessively expensive. The simple formula given on page 31 can
always be used in making up a sodium arsenite solution and usually in-
sures considerable saving.

EQUIPMENT REQUIRED IN APPLYING CHEMICALS FOR
WEED CONTROL

The common herbicides, applied by means of spraying equipment, may
be divided into two groups according to their corrosive nature. Such
noncorrosive liquids as Diesel oil, sodium arsenite solution, and sodium
chlorate solution may be applied with any pressure spray machine. For
small operations the diaphragm-pump knapsack sprayer of the Ver-
morel type is satisfactory (fig. 11). The wheelbarrow and barrel types
used in orchards are better where two men are available, and the power
orchard sprayer is most suitable for large jobs (fig. 12). Many special
equipments have been built for weed work. These are usually fitted with
an all-bronze pump and mounted on a truck. They may be fitted with
hand guns for spot work and boom equipment for roadsides or open
field (figs. 13 and 14).

Although the acid arsenical spray solution contains 5 per cent sul-
furic acid, the inclusion of sodium arsenite so greatly reduces its cor-
rosive action that this chemical may be handled in ordinary orchard
spray equipment. The pump should have bronze-lined cylinders ; since
porcelain is etched by the acid, pumps lined with it may be ruined. All-

44

California Agricultural Extension Service

[CiR. 97

-i, .Jli ,...::*

1:; m m *:li^fc

nmmiK "Vj

;: SV-^«,^«1

SSSSSSSS.-- : -.■

^^1 ||ip^^^^

i^tSI

i^

^S'P

m

pi'

^^;::::. ■ :;||' ' ' ' -*||? f ■^■Ift^M -.:!:

A

^^^HK^'i ^^^^^S

liaii«fciJ

1 .:€'! ! v^^^HniMl

i

H^ ^^^^^^

.^

p % ^

#'^M^nHg

^H '

'1

■n

^

^HB'ft'

1 ,*^

pw

»■■■■■■■'

W^m"^'

Fig. 11. — Vennorel knapsack sprayer; equipped with BroAvn pop valve, 22-
incli brass extension, and Chipman nozzle, for weed work. The pressure gage
is not needed for ordinary spraying.

1936]

Weed Control

45

Fig. 12. — Po^^e^ ^piay rig, showing two lines of hose with multiple nozzle.
(FromExt. Cir. 54.)

Fig. 1.'5. — Power sprny rig, built for i-o.-idsidc si)r;iyiug. Will spniy a strip
15 feet wide. Capacity, 900 gallons. (From Ext. Cir. 54.)

Fig. 14. — Power spray equipment adapted for spot work along roads.
Two lines of hose. (From Ext. Cir. 54.)

46 California Agricultural Extension Service [Cm. 97

brass plumbing, too, is best, though not absolutely necessary; a boom
equipment made of ordinary steel pipe was used for three years at Davis
without corroding through.

For corrosive chemicals, like dilute sulfuric acid, the special equip-
ment mentioned before (page 25, and fig. 4) has proved satisfactory. It
can be attached to any ordinary spray rig capable of supplying a steady
flow of water at a pressure of 200 pounds per square inch. It consists of
a container for the acid, the ejector for mixing acid and water, a boom
for applying the spray solution, and the framework and plumbing neces-
sary to fit it to the spray machine. All plumbing should be of brass ;
steel pipe is rapidly corroded by the dilute acid.

Some time has been devoted to a study of spray nozzles and a nozzle
commonly used for spraying potatoes was adapted to weed spraying."^"
It delivers a flat, fan-shaped spray, giving uniform coverage.

As supplied by the manufacturers, these nozzles were not particularly
uniform, especially as to the angle or spread of the fan. Since an even
coverage is essential in boom spraying, slight changes were made in the
design of the disks used in these nozzles. The improved disks deliver the
spray at a uniform spread of 60° and have been calibrated for various
standard drill sizes so that any desired delivery rate can be produced
at will.

The market offers at present no special equipment for applying dry
chemicals in soil sterilization ; but any fertilizer drill that will spread
the necessary amounts evenly over the ground should suffice. All plots
used thus far have been treated by hand, a method fairly satisfactory
for small areas. For somewhat larger areas a lawn-fertilizer spreader
has been used, though difficulty was experienced in obtaining a uniform
coverage, especially on sloping ground.

Equipment for both small-scale and large field application of carbon
disulfide has been designed by the manufacturers of this material. They
sell hand equipment at a nominal cost and rent machine-drawn equip-
ment for larger areas. They also offer advice on the proper time and rate
of application of this material.

^° This nozzle, termed the "Hummer," is manufactured by the Friend Mfg. Com-
pany, Gasport, New York, and may be purchased from this company or from the
Chipman Chemical Company, Palo Alto, California. Similar nozzles may be obtained
from the John Bean Mfg. Co., San Jose, California.

1936] Weed Control 47

SPECIAL PROBLEMS

California has many special weed problems resulting from infestations
by species not common to other localities, from special effects related to
our deep, fertile soils and mild winters, and from conditions related to
our speciality crops and irrigation farming. Some of these problems are
peculiar to California agriculture and have never before been subject
to scientific investigation. Others have been studied in more detail here
than elsewhere. A number of these problems are presented here along
with such information, aiming toward their solution, as has been gained.
Although each represents a special situation requiring special technique,
it would be well for the reader to consult the sections on weed character-
istics and principles of weed control in connection with the discussion
presented.

WEEDS IN UNCROPPED AREAS

Throughout the state, weeds grow in profusion on ditch banks, on
levees, along fence rows, roadsides, and driveways, and on vacant lots.
Growers cannot expect to gain the benefits of cultural or chemical
methods of weed control in farmed fields unless a concerted effort is
made to prevent the growth and seeding of weeds in the uncultivated
areas surrounding their farms.

Weeds of all classes — annual, biennial, and perennial — may be found
in these noncultivated areas ; but, as a rule, annuals predominate. This
fact should somewhat simplify the problem of control. The pests must
be destroyed before they mature seed. All too frequently, along road-
sides or elsewhere, they are mowed when in seed and left on the ground
to dry. Then entire plants or seeds are blown in great numbers to adja-
cent cultivated fields or orchards. Often farmers erroneously suppose
that if the plants are mowed when in flower all danger of seed produc-
tion is past. Many plants form flowers over a period of weeks; and,
although the plant may appear to be in flower, it may mature seeds even
when cut near the ground line. Consequently, early destruction is ad-
visable if seeding is to be prevented.

Mechanical Control. — On all uncropped areas where a tractor or cul-
tivating implements can be moved, clean cultivation with disks or plows
will usually give the cheapest and most effective control.

Mr. A. E. Mahoney, Agricultural Commissioner of San Joaquin
County, who has demonstrated the practicability of roadside weed con-
trol by means of tractor disks, makes the following statement :

48

California Agricultural Extension Service

[CiR. 97

Eoadside weed control with tractors and disks is now an established operation in
8an Joaquin County. Control of weed pests and fire prevention are the primary pur-
poses. The cost is much lower than the cost of oiling and burning.

The disking of roadsides was first started in the spring of 1931, with a caterpillar
tractor and a 5-foot offset disk. Since then tAvo more tractors and disks have been
purchased, one by this office and one by the county highway-maintenance depart-
ment. The work has proved so satisfactory that the purchase of three additional
tractors and disks for the 1936 season is contemplated, making a total of six imple-

Fig. 15. — A puneture-vine-infested roadside after disking, San Joaquin County.
(Photograph by A. E. Mahoney.)

ments for roadside weed control. When not used for this purpose the tractors are
employed in other road-maintenance work.

The original purpose of roadside Aveed control was x)rimarily the eradication of
puncture vine. Weedy roadsides make a harbor where puncture vine may grow and
reseed unobserved, to be picked up and scattered by automobile tires. When road-
sides are weed-free, however, the puncture vine, which starts growth after the winter
and spring annuals are turned under, can easily be seen, sprayed with oil, and burned
or gathered before the burs mature.

In 1935, 244 miles of roads were disked in the Manteca-Ripon-Escalon districts
where puncture vine is prevalent (fig. 15). A second disking was made on 151 miles
to destroy many weeds, especially summer annuals, which had started after the first
treatment.

Eoadside weed control also keeps down or eliminates infestations of Eussian
thistle, yelloAv star thistle, Johnson grass, Bermuda grass, and many other species of
weeds that creep and blow over on the farmer's lands. Since the interest of the grow-
ers adjoining these roads has been aroused, much eradication has been accomplished.
Farmers are removing miles of fence line to permit the control of weeds generally
found growing in such places.

Farmers in grain districts, where puncture vine and other noxious weeds are not
a serious problem, demand the same roadside disking for fire protection. This work
is done early in the season, the main object being to turn under the winter and spring

1936'

Weed Control

49

annuals growing between the road and the fence line. Its effectiveness is shown by
the fact that last year no fires originated on disked roadsides.

In 1935, 487 miles of roads were disked for fire protection and weed control
(fig. 16), starting February 4 and finishing May 15. Because of an unusually wet
spring, 183 miles of roads were disked twice, the second disking being done after a
second growth of weeds.

Aside from weed control, the condition of our roads has been vastly improved from
a highway-maintenance viewpoint. Unsightly piles of tin cans, discarded automobiles

Fig. 3 6. — The disk is used to make a firebreak along a roadside, San Joaquin
County. (Photograph by A. E. Mahoney.)

and their parts, broken-down fences, and other eyesores have been removed to facili-
tate disking. The general appearance of the countryside denotes thriftiness.

A total of 670 miles of county roads was disked in 1935 at an average cost of $7.57
per mile. Many of the roadsides were disked twice, and on some of the wider roads
as many as nine rounds were made. For each 5 -foot strip the cost was 72 cents per
mile. This cost is considerably less than when the work was started, thanks to a gen-
eral improvement of roadside conditions and added experience in the work.

It might be well to compare the cost of disking with that of oiling and burning.
To apply Diesel oil, possibly our cheapest weed eradicator, at 4 cents per gallon and
300 gallons per acre, would cost $24.00 a mile per 8-foot strip for oil alone. To this
amount must be added depreciation of equipment and cost of labor required in the
application and burning. To treat the roadside from road shoulder to property line,
as in disking, would cost approximately $75.00 per mile for the oil treatment as
against an actual cost of $7.57 per mile for disking.

Chemical Control. — On areas difficult to reach with large equipment,
chemical methods may be practical. Here consideration should be given
to oils and sulfuric acid. These sprays, if used, should be first applied
when the growth is only a few inches high, for then complete coverage

50

California Agricultural Extension Service

[Cm. 97

can be made at minimum cost. Moreover, the plants then are young and
tender. During the season, several applications may be needed. The first
year will naturally require more material than subsequent years because
the soil contains innumerable seeds that will germinate. If, however,
after germination the seedlings and other growth not killed by the first

Fig, 17. — ^A aiTciioaiiK grazed with sheep, iiiis is an eiTective and economical
method of keeping weeds down along irrigation canals and ditches. (Photograph
by A. E. Mahoney.)

spraying are sprayed again, the weed population is rapidly reduced. In
most sections of the state, where the dry summers permit very little
growth, one or two sprayings in a season will effectively keep down
weeds on noncultivated, unirrigated areas. The cost should be somewhat
smaller each year, for the vegetation to be covered constantly decreases.

Where more permanent results are desired, soil sterilization as de-
scribed on pages 38 to 42 may be used. This is usually the most economi-
cal practice on areas which cannot be reached with cultivating tools.

Burbling. — Since burning of dry weeds does not kill all the seeds, par-
ticularly those that have been shed, it produces no permanent benefits.
Though useful in reducing fire hazard and in freeing ditches of weeds
that hinder the flow of water, it accomplishes little from the standpoint
of weed control.

Burning of green weeds is too slow and costly to justify the work
required. In the first place, grasses can scarcely be killed thus on a prac-
tical scale. Whatever killing is done could be accomplished by other

1936] Weed Control 51

methods with considerable saving of time and labor. Burning of areas
treated with contact oil is recommended to remove the unsightly growth
and reduce fire hazard.

Grazing. — A very effective and economical way of keeping weeds
down along irrigation canals and levees is by grazing. Sheep are espe-
cially useful for this purpose. The animals not only keep the banks free
of weeds, but they compact the soil, making it difficult for gophers to
work. For four successive years a main irrigation canal on the Univer-
sity Farm has been grazed with sheep (fig. 17). In 1931 this canal was
grazed 3,514 sheep-days, the total gain in weight of animals being
765 pounds ; in 1932, this same canal was grazed 3,929 sheep-days, the
total gain in weight being 908 pounds (during 9 days, no weight taken) .
No data were taken for 1933 and 1934. This particular ditch in years
past has been covered with a heavy growth of weeds, the seeds of which
were scattered to other parts of the Farm. Grazing has kept the banks
clean, and eliminated this area as a source of weed seed.

WEEDS IN GRAIN FIELDS

The weeds that infest grain fields are many. In number, as well as in
kind, they vary with the district and the conditions of culture. They con-
sist largely of winter annuals, though some perennials occur.

Most of the grain in California is grown on unirrigated land ; and the
weeds, competing with the grain crop for moisture, plant food, and
light, invariably reduce the yield. In addition, weeds increase the cost
of harvesting, cleaning, and preparing for ultimate consumption.

Common Weeds Occurring in Grain Fields. — Among the weeds most
common in grain fields are wild oats, several species of mustard, wild
radish, common spikeweed, and buckthorn or fiddleneck. Thistles, such
as the spiny sow thistle and the milk thistle, occur in many sections.
Napa thistle or tocalote has spread throughout the state and yellow star
thistle is present in the central coast region and in the Napa and Sacra-
mento valleys. The few perennials that occur, such as morning-glory
and creeping mallow, rarely cause much trouble except on land fre-
quently fallowed.

Introduction and Spread of Weeds in Grain Fields. — Aside from the
natural agencies of dissemination, there are two other potent means of
spreading the weeds of grain fields. One is the common practice of buy-
ing seed "over the back fence" or from a neighboring district without
ascertaining that it is free from weeds or cleaning it before sowing ; the
other is the custom harvester that moves from ranch to ranch, from in-
fested field to weed-free field, without being cleaned.

Annual weeds, once started in a field, spread rapidly, especially on

52 California Agricultural Extension Service [Cm. 97

land cropped every year. Most winter annuals, such as wild oats, mus-
tard, and wild radish, ripen their seed before the grain crop ripens, or
at the same time, so that their seed is shed or scattered by the combine in
harvesting the grain. Others, like the star thistle and the Napa thistle,
do not usually mature their seed until after the harvest ; such seeds on
maturity drop to the ground and maintain the infestation.

Control of Weeds in Grain Fields. — Control of the annual weeds in
grain fields is mainly a cultural problem. Some farmers keep their fields
fairly free by delaying plowing until January or later ; this gives the weed
seeds a chance to sprout, so that the young plants may be plowed under.
This practice, if followed consistently, will normally keep the crop free
from weeds ; but late seeding in many of the grain-growing sections often
results in low yields. Another practice, common where the land is fal-
lowed every second or third year, is to delay plowing for the fallow until
March, when all weeds and volunteer grain have started and can readily
be destroyed by plowing. Since little rain falls after the land is plowed,
the subsequent growth of weeds is usually negligible. This practice, how-
ever, is a poor one because it largely nullifies the benefits of the fallow —
namely, moisture conservation and the stimulation of nitrification.

Occasional fallowing is fairly effective, as a rule, in controlling or
eradicating wild oats, the seeds of which remain viable for only two or
three years; other weeds, however, such as mustard, wild radish, and
fiddleneck, whose seeds may remain viable for many years, are less
easily eradicated. Late winter or spring seeding, though undesirable,
may be necessary for weed eradication if the infestation is heavy.

A satisfactory procedure is to delay plowing or cultivation in the fall
until most of the weed growth has started. Then, after plowing, two or
three weeks should elapse before the seed bed is prepared and the crop
seeded. This delay permits germination of the weed seeds that have been
turned up, and the seedlings can then be destroyed with surface tillage
implements just before seeding. Replowing just before seeding is un-
desirable, for it brings to the surface a fresh supply of weed seed to
sprout with the grain. During the fallow year tillage should be followed,
although the first plowing might be delayed until January or February
in order to give the weeds a start. After plowing, two or three cultiva-
tions during the spring will destroy large numbers of weeds and will
stimulate germination of the seed in the plowed layer. This practice,
though more costly than delaying the plowing till spring, destroys weeds
more effectively.

In one section of the state (fig. 18) Russian thistle has been combated
by cutting and burning on stubble or summer fallow.

Chemical Coyitrol of Weeds in Grain Fields. — In the eastern and mid-

1936]

Weed Control

53

die-western states, as in certain grain-growing sections of Europe, nuis-
tard and other broad-leaved annuals in young grain are often sprayed
with chemicals. In this country iron sulfate ("copperas") solutions,
varying in strength from 5 to 20 per cent, have been used. In tests this
spray was most successful if applied when the weeds were small, with
not more than four to six leaves, and when the atmospheric humidity
was high. Injury to grains or other grasses was negligible. The sulfuric

^fitijj^^^:^ Z.

Fig. 18. — Equipment ciuploycd on Cari.sa jjlains in the campaign to control
Kussian thistle in grain fields. The tractor-drawn equipment carries a set of
knives which cut the thistles at the ground-line, and a cradle which drags the
weeds into windrows. Later the thistles are burned. Used principally in stubble
or summer-fallowed fields.

acid method of controlling weeds in grain fields, and practical equipment
for its use, are described on pages 24—27. It seems likely that this method
will have wide application.

Peramial Weeds in Graiii Fields. — As already stated, morning-glory
and other perennials that make their growth during the summer usually
spread more rapidly on fallowed land than on land continually cropped,
because in the fallow year the moisture conditions are ideal for the
growth of the weeds. On land fallowed every second or third year such
perennials, once started, may soon occupy the whole field.

Perennials must be prevented from producing seed, for while some
spread does occur because of the rootstocks, the greatest spread comes
from dragging the mature vines, loaded with seed, over the field. When
grain land is badly infested, its value will perhaps not justify the at-
tempt to eradicate deep-rooted perennials by present expensive methods.
Where small areas occur, weeds can and should be prevented from

54 California Agricultural Extension Service [Cir. 97

spreading. As already stated, perennials should be kept from going to
seed, then allowed to make a good vegetative growth in the fall, and
sprayed heavily with a chlorate spray in October and November. The
treatment should be repeated each year until the pest is eliminated. It
will probably sterilize the soil for some time ; but if the area is not large
and the weed is finally eradicated, this temporary sacrifice is justified.

WEEDS IN ALFALFA

As to their effects upon the alfalfa crop, weeds fall into three groups :
(1) Weeds that appear in the new seeding of alfalfa and threaten to
choke out or compete seriously with the young plants. These weeds are
mainly the annual, herbaceous, broad-leaved plants such as pigweed,
mustard, Russian thistle, bur clover, and certain grasses. Such peren-
nials as morning-glory and creeping mallow may also interfere with the
growth of young alfalfa, especially that of late spring seeding. Alfalfa
seeded in the fall or early spring ordinarily suffers but little. (2) Weeds
that appear in the older fields and compete with the crop for plant food,
moisture, and light. These may materially reduce the quality of the hay.
Of greatest concern to the grower are foxtail, Bermuda grass, brome
grass, crab grass, and water grass. Johnson grass, too, where prevalent,
may be troublesome. In certain older fields, hoary cress and Russian
knapweed may appear in dense patches ; they are especially objection-
able in hay. Although other weeds may occur in certain districts, those
mentioned are most prevalent. (3) Parasitic weeds, such as dodders, that
prey on the alfalfa and reduce or completely destroy the plants. Of sev-
eral species, the small-seeded dodder is most common in California.

Weeds that Interfere with Establishment of Stand. — Control of weeds
that may interfere with the establishment of a new stand consists pri-
marily in eliminating as many as possible before the alfalfa is seeded
and in providing favorable conditions for the growth of the alfalfa. Pre-
ceding the alfalfa with a cultivated crop will eliminate many weeds.
Sudan grass, if irrigated, will smother most annuals before they have
an opportunity to produce seed. A weedy field should be plowed several
weeks before the alfalfa is seeded and should be irrigated, if necessary,
to provide favorable conditions for weed growth. Then, just before seed-
ing, the seed bed should be prepared, and the weed growth destroyed
with as shallow tillage as possible. Shallow cultivation at this point is
important in order to provide a good seed bed for the alfalfa and also
to avoid bringing a fresh supply of weed seeds to the surface. Handled
in this way, a clean stand of alfalfa can usually be obtained by seeding
in early spring, when«most of the winter annuals have started and are
readily destroyed, and when moisture and temperature conditions favor

1^^^] Weed Control 55

the rapid growth of the alfalfa. The same procedure is advisable for fall
seeding, though elimination of weeds is then usually not so easy, be-
cause the time available for germination and destruction of the weeds is
shorter, and the conditions are less favorable to alfalfa.

If a heavy growth of weeds starts with the alfalfa, keep it clipped
back, allowing sunlight to reach the alfalfa plants. Weeds on a new field
of alfalfa should always be cut high, 3 inches or more from the ground ;
young alfalfa plants grown in the shade are slender and spindling, with
but few leaves. If cut off close to the ground, with most of the leaves
removed, many of them will die.

Weeds in an Old Stand of Alfalfa. — When a good stand of alfalfa has
been obtained and the conditions for growth are favorable, weeds seldom
cause much trouble for the first two or three years. Thereafter, however,
the stand becomes thinner, and weeds fill the vacant spaces. The general
belief that the weeds cause the thinning is only in part correct. A dense,
vigorous growth of alfalfa can normally hold its own, and only when
space has been provided do the weeds come in and cause trouble. This
may not be true of Bermuda grass and hoary cress, which appear
actually able to crowd out the alfalfa. On the other hand, when soil-
moisture conditions are good, excellent crops of alfalfa are sometimes
obtained despite a dense undergrowth of these weeds.

The winter annuals, once started, spread rapidly and soon occupy
all vacant space, making their most rapid growth while the alfalfa
is practically dormant. A bad infestation may greatly stunt the spring
growth by robbing the crop of plant food and moisture, and may in
addition render the first cutting of hay almost worthless.

Many alfalfa growers have attempted to reduce the winter annuals
by cultivation, the only known practical method. This procedure has
usually met with but little success, because the job has not been done
thoroughly and at the proper time. The spring-tooth harrow is the best
implement, and the cultivation should take place in late January or
February after the weed growth is well started. The field must be gone
over repeatedly until all the weeds are dug out. Going over it once, as is
usual, will rarely remove all the weeds and is often wasted effort. The
drastic cultivation necessary to destroy the weeds will, obviously, injure
many alfalfa plants ; but given favorable conditions most of them will
recover and grow much more vigorously than with the weeds present.
The only condition under which cultivation may not be advisable is
where such diseases as bacterial wilt and crown wart are present, for
the cultivation, by injuring the plants, facilitates the spread of these
diseases. Recently attempts have been made to control winter annuals
with chemical burners ; the results promise well if suitable equipment is
developed, though the method is still in the experimental stage.

56 California Agricultural Extension Service [Cir. 97

Most of the summer annuals, such as crab grass and water grass,
occur only where the soil is continually moist. They are troublesome,
therefore, only in fields where unfavorable soil conditions necessitate
two or three irrigations per cutting. Attempts to eradicate such weeds
by heavy summer cultivation have been unsuccessful, being apt to injure
the alfalfa. The best remedy is crop rotation, or correction of the un-
favorable soil condition, to lessen the need of frequent irrigation.

Bermuda grass is the weed pest most troublesome to alfalfa in many
sections of the state. From small beginnings it spreads in ever widening
areas until in a few years it may occupy the whole field and greatly re-
duce the yield of alfalfa, which can compete with it successfully for
plant food and moisture only under the best conditions. The Bermuda
grass cannot be removed economically; the only practical remedy is
crop rotation. An alfalfa field badly infested with Bermuda grass
rarely repays cultivation ; the best procedure is to plow it up and eradi-
cate the pest. After shallow plowing in August or September, all the
Bermuda plants being carefully cut off and the exposed roots left to
dry and perish, the field should be left without further treatment until
late fall or early winter, when it should be deeply plowed in preparation
for future crops. Two or three years should elapse before again seeding
to alfalfa, and the summer crop should be one that permits cultivation
and the destruction of such Bermuda as reappears.

Parasitic Weeds in Alfalfa. — The dodders are typical plant parasites.
Starting from seeds, the yellow, leafless vines soon attach themselves to
the alfalfa plant ; then their roots die, and they grow entirely by extract-
ing nourishment from the host. From a local infestation, the parasite
spreads rapidly by the elongation of its stems. Appearing usually in
rapidly spreading spots, it greatly reduces the growth of the alfalfa
plants, seriously injuring the stand.

Of the various species of dodder, the small-seeded is the most prev-
alent in California, although others occur. One species, prevalent on
native vegetation, will also attack alfalfa ; but apparently it does little
damage.

Dodders are good examples of seed-borne weeds. The pernicious spe-
cies are nearly always introduced into the fields at a time of seeding.
For this reason dodder is emphasized as an impurity in alfalfa seed. The
parasite, after appearing in fields, produces abundant seed, which may
remain viable in the soil for many years. Rotation for several seasons
is, therefore, usually necessary to eradicate bad infestations but lighter
ones are easily remedied, preferably by spraying the areas with oil or
kerosene, and burning. This is better practice than mowing before treat-
ment, for it involves less danger of spreading the dodder with the mower

1936] Weed Control 57

or scythe. In young stands the alfalfa plants may also be destroyed in
this way, but in stands a year old or more they will not be injured. The
spots should be carefully watched for reinfestation. In bad cases the
best procedure is to plow up the alfalfa and rotate with other crops for
two or three years.

WEEDS IN ORCHARDS AND VINEYARDS

Weeds in orchards and vineyards may not be evidence of careless farm-
ing, nor are they always an unmitigated evil. In fact, certain annual
weeds are employed as covercrops and may even wisely be allowed to
come into seed. Pernicious annuals, however, such as Russian thistle and
star thistle, should not be allowed to come into seed in orchards or vine-
yards, but should be plowed under or disked in. Such annuals employed
as covercrops, though relatively harmless to the orchard, may endanger
adjacent cultivated fields. Perennials, on the other hand, particularly
morning-glory, hoary cress, Russian knapweed, Bermuda grass, and
Johnson grass, should not be allowed to gain headway in an orchard or
vineyard nor be employed as covercrops ; they provide a too dangerous
source of infestation.

The most important purpose of cultivating orchard and vineyard soils
is to remove weed competition, as Veihmeyer and Hendrickson have em-
phasized."

Different experiment stations have attempted to measure yields on
tilled and untilled plots. Areas in which weed growth has not been a
factor show no increase that can be attributed to stirring the soil. In
fact, the conclusion that cultivation is needed only to remove weeds is
almost unanimous.

Many fruit growers, therefore, have materially changed their meth-
ods of soil management during the past several years and have allowed
weeds to grow for long periods. Considerable saving has resulted because
much less frequent cultivations are given and tillage is shallower than
formerly. It must be pointed out, however, that weeds in an orchard or
vineyard may compete so seriously for water and soil nutrients as to
make their removal imperative. In the eastern states, as well as in Eng-
land, sod has sometimes proved very injurious to orchards, probably
through the depletion of nitrates, which should in such cases be added
to the soil.

Weeds often harbor insect and fungus pests and may, in orchards and

vineyards, cause infestation of neighboring crops ; consequently clean

cultivation, even though unnecessary to conserve moisture, may be

needed to destroy the weed hosts of pests.

"Veihmeyer, F. J., and A. H. Hendrickson. Essentials of irrigation and cultivation
of orchards. California Agr. Ext. Ser. Cir. 50:1-24. 1930.

58 California Agricultural Extension Service [Cir. 97

Evidently, as far as weeds are concerned, cultivation in orchards is
advisable and perhaps necessary when weed growth becomes a serious
competitor for soil moisture, when pernicious weeds threaten neighbor-
ing fields, and when the weeds harbor dangerous insect or fungus pests.

For the control of morning-glory and other perennials, see the sec-
tions covering these specific topics. The acid arsenical solution, either as
a spray or in jars, may be used in orchards without injury to trees, vines,
or soil. The jar method is particularly applicable to small patches of
morning-glory. Wild blackberries in orchards often give trouble. Usually
the cheapest control method for these is to remove the top growth, plow
or grub the plants, and work the area over carefully for several seasons.
The top growth may be killed by spraying with Diesel oil, and burning.
The chlorates and carbon disulfide, if used at all, must be applied cau-
tiously for they may seriously injure trees or vines.

WEEDS IN LAWNS AND GOLF GREENS

The eradication or even the control of weeds in lawns and golf greens is
exceedingly difficult. At the outset, extreme care should be taken to
plant the lawn in a well-prepared seed bed and with the best and purest
grade of lawn-grass seed available. Often many weed seeds are intro-
duced into a lawn at the time of seeding. The variety or mixture of grass
should be well adapted to the soil and climate of the locality. Except in
very fertile soils, a light application of nitrogenous fertilizer should be
made during the preparation of the seed bed to help the young plants
become established. Once a solid turf has been formed, if it is kept thrifty
by proper watering, regular mowing, and fertilizing, weeds have diffi-
culty in invading it.

Many of the weeds that are troublesome in lawns have special char-
acters that enable them to grow and spread in spite of competition by
the grass and frequent cutting. They either have very short stems,
forming rosettes of leaves near the surface of the soil, or they spread
by rootstocks or runners near the surface. Common weeds of the first
type are : dandelion, plantain, shepherd's purse, and chicory. Spreading
lawn weeds are : crab grass, Bermuda grass, knotweed, chickweed, speed-
well, oxalis, creeping buttercup, field madder, bur clover, and spotted
spurge.

If any of the above-named weeds appear in a newly planted lawn they
should be pulled while the soil is moist so that all roots are removed. Any
of them, if allowed to become established, will crowd out the grass and
eventually ruin the turf. The best time to eliminate them is while they
are young.

Because of their excellent means of seed dissemination, most lawn

1936] Weed Control 59

weeds are able to invade relatively clean and well-established lawns. The
maintenance of a lawn in a weed-free condition therefore is a constant
battle. Two methods are used in combating these pests in an established
lawn. The first, and undoubtedly the best, is to keep the lawn so thrifty
that only occasionally are weeds able to secure a foothold. Certain rules
must be followed in proper care of a lawn. Mowing should be fairly fre-
quent and regular but not too close. If too much leaf surface is removed
the perennial roots of the grass become depleted of organic nutrients
and annual weeds are favored.

Irrigation or watering should be ample to provide for the needs of
the grass but not in excess. Frequent heavy watering leaches nutrients
from the soil and favors the growth of plantains, dandelions, and annual
grasses. Plant nutrients should be amply provided. In many places it is
becoming the practice to leave the clippings on the lawn. They are soon
hidden from view in a thrifty lawn and depletion of the sod is pre-
vented. Where this practice does not induce other problems, such as
fungus diseases or insect pests, it seems to be thoroughly sound.

Under the most favorable conditions, however, a lawn needs occasional
fertilization. A good grade of well-rotted manure, free of weed seeds,
is undoubtedly the best material for this purpose. The viability of weed
seeds in manure is destroyed if it has been well rotted. If such manure
cannot be obtained, use some other organic fertilizer such as cottonseed
meal, fish meal, blood meal, or guano. These materials must be applied
sparingly and are best used during the fall or spring in California. For
quick stimulation during the growing season, ammonium sulfate may
be used. Its effects, however, are not so lasting as those of the organic
materials.

Weeds appear only occasionally in a healthy lawn and they should
be pulled or removed with a long-bladed knife, spud, or chisel. All roots
must be removed or they will sprout and grow again. Dandelions and
plantain have been known to send up shoots from a depth of 3 inches
or more.

Chemicals have been used against lawn weeds with varying success.
Spot treatments may be made with gasoline, kerosene, Diesel oil, carbon
disulfide, sulfuric acid, sodium arsenite, iron sulfate, ammonium sul-
fate, or any other toxic compound. The dosage must be adapted to the
weed in question and the type of herbicide. With the oils, about a tea-
spoonful per plant is sufficient. Somewhat less of the iron or ammonium
sulfate is required. A few drops only of the sodium arsenite or sulfuric
acid are sufficient. Usually a small area of the turf is killed but it soon
grows over. Where a large number of weeds are treated the whole area
may require fertilizing and reseeding.

60 California Agricultural Extension Service [Cir. 97

As a rule, the best procedure in treating an old lawn heavily infested
with Bermuda grass, dandelions, and other weeds is to spade or plow
it up, rake out the roots of weeds, fertilize well, and reestablish the
lawn. A small infestation of Bermuda grass, if detected soon enough
before it has seeded, should be dug up to a depth of 12 to 18 inches,
well beyond the limits of growth laterally, and the soil replaced with
new soil. Carbon disulfide has been used to eradicate Bermuda grass in
lawns and golf greens. This chemical may be applied by merely sprink-
ling the infested area with an ordinary garden sprinkling can, and im-
mediately covering it with wet canvas. Use about 1% gallons of carbon
disulfide to the square rod. To keep the vapors of carbon disulfide from
escaping, two individuals may work together, one applying the chemi-
cal, the other unrolling the wet canvas over the treated area.

Iron sulfate (copperas or green vitriol) has been successful in the
control of dandelions and chickweeds in Rocky Mountain and eastern
states. The spray solution is prepared by dissolving ly^ pounds of the
chemical in each gallon of water. It is best applied with a knapsack
sprayer of brass. As a fine mist, one gallon of the spray solution will
cover about 40 square yards of lawn surface. From four to six applica-
tions in one season are required to completely kill dandelions. The lawn
should not be watered until the weeds have been severely affected,
usually a period of 24 hours.

Although the grass may also be injured, it soon recovers with no per-
manent damage. The weeds, being broad-leaved, suffer more severely
than the grass. After several treatments, food reserves in the roots of
weeds are depleted and they are unable to survive the winter. Young
dandelions are destroyed with fewer sprayings, and broad-leaved an-
nuals are usually killed with the first application. Warning should be
given that iron sulfate is strongly corrosive and in addition will leave
rust stains on buildings, cement walks and curbs, and on clothing.

Oxalis is one of the most difficult weeds to control in California lawns.
In addition to having a means for throwing its seed a distance of several
feet as they mature, it forms small, underground tubers which enable
it to spread vegetatively. Every effort should be made to prevent it
from becoming established in a lawn by digging up each small plant that
appears. Once it spreads in a lawn, heavy fertilization should be prac-
ticed. If this fails, spraying in the fall with sodium chlorate at a rate of
3 pounds per square rod will give a temporary sterilization. If plants
appear again in the spring, respray. Then the lawn may be leached free
of chlorate, fertilized, and reseeded.

Crab grass, a creeping annual which roots freely at the nodes, re-
quires constant watching. It forms an abundance of seed which lays

1936] Weed Control 61

over and reinfests the areas during the following summer. It can be
eradicated by careful digging as the new plants appear. If allowed to
spread it can be killed by spraying with Diesel oil, a treatment from
which the perennial lawn grasses recover. However, this leaves an un-
sightly sod which requires reseeding. In severe infestations it is prob-
ably best to dig up the sod, work down and water frequently, with culti-
vations to kill the seedlings, and reseed after the soil has been freed of
the pest.

WEEDS IN DITCHES AND WATERWAYS

Weeds of ditch banks have been discussed to some extent under the head-
ing "Soil Sterilants." Along cement-lined ditches, in certain soils, chemi-
cal treatment has been economical and effective in controlling weeds.
Soil sterilants, however, are of no use below the water line of unlined
ditches in any soil, and in the heavier recent alluvial soils they are re-
quired in excessive dosages above the water line. In such cases mechani-
cal control seems best. Various mechanical methods are in use including
plowing, disking, dragging, and hand hoeing. Figure 19 illustrates a
piece of special equipment designed by Mr. W. F. Wooley, engineer for
the West Stanislaus Irrigation District, and successfully used in that
district for several years. The laterals in this district are of uniform
shape and size and all outlets are of the inverted siphon type. For this
reason this one piece of equipment can be used on all of the main laterals.
In passing the outlets the tractor draws the disk up to within a few
inches of the structure. Then a man on the disk (not shown in the pic-
ture) raises the gangs by means of the lifting devices, the tractor pulls
ahead, and the disks are lowered again as the structure is passed. In this
way only a few square feet of surface are left undisked at each structure,
reducing to a minimum the hand hoeing required. This equipment
covers many miles of ditch in a day and by going over the system several
times each year the ditches are kept open for a free flow of water and
devoid of weeds.

Ditches which carry water throughout the whole year usually harbor
plants which are rooted in the mud or float on the surface of the water.
Such weeds grow and spread rapidly, each year's growth accumulating
upon that of the preceding until the flow is seriously restricted and
dredging or other mechanical cleaning is necessary.

The only floating weed of any importance in California is the water
hyacinth, a most pernicious perennial which, fortunately, has gained
a foothold in only a few localities. The best control thus far found is
spraying with sodium arsenite. However, if arsenite is used there is the
possibility of poisoning animals which drink the water.

62

California Agricultural Extension Service

[Cm. 97

Common perennial weeds rooted in the bottoms of ditches are cattail,
tule, and yellow water weed. Control of these by chemical sprays has
seldom been attempted. Some ditches under observation have been

::k:^tM l.ifJ'^pM

ip^-

4

m^

*, 1.

P^^jfi

Fig. 19. — Three views of the special ditch-bank disk used by the
"West Stanislaus Irrigation District to control weeds on their laterals.
The upper two views show the machine in operation. The lower view
shows it passing an obstacle with the disk gangs raised.

Sprayed with oil, a method that deserves careful trial. These particular
ditches were sprayed three times the first year, twice the second, and
once or twice the third. Although oil does not penetrate roots and root-

1936]

Weed Control

63

stocks, it does destroy top growth ; thus, if applied persistently, it weak-
ens and may ultimately kill the underground parts. Weeds growing in
the water are apparently difficult to eradicate with herbicides ; but, even
so, such chemicals may be cheaper than dredging or pulling.

A drag-chain, with links made from 1 or 1%-inch material, has some-
times been effective in keeping down cattails and tules in ditches. This

Fig. 20. — Equipment used by the Turlock Irrigation District for
cleaning drainage ditches.

chain is dragged along the bottom of the ditch in one direction and then
back, thus bending over and breaking the stems of the weeds. The opera-
tion is repeated as often as is necessary to keep the cattails and tules
from clogging the ditch. The weeds, though not eradicated the first year,
are weakened so that the later treatments may be less frequent ; even-
tually the ditch is kept clean at a very low cost.

Ditches up to 10 feet in width can be dragged with one team, by using a
long stretcher and having one horse on each bank. On larger ditches one
team is required on each end of the chain. This is probably the most inex-
pensive method for controlling tules and cattails. In order to be effective,
however, the dragging must be done several times during the summer. A
single cleaning in the spring is beneficial but has no permanent effect
upon the roots of the plants.

The Turlock Irrigation District has used the equipment illustrated in
figure 20 for cleaning drainage ditches. The equipment now in operation
consists of a Diesel-powered Caterpillar 40 with a Willamette-Ersted
"Hyster" power towing winch and a 6-foot Killefer offset disk with

64 California Agricultural Extension Service [Cir. 97

22-inch blades. The disk has been reinforced throughout and an ac-
cessory hitch built on so that it can be pulled from either end. The
attachment shown in the picture acts like a buck-rake to gather the
plants which have been cut off by the disk and facilitates their removal
from the ditch.

By adjusting the length of cable and degree of offset of the hitch it
is possible, working from one side, to disk and remove the tules from
both sides and the bottom of the ditch.

The tractor and power winch are also used for pulling willows where
they occur. There is a hook on the end of the cable and when willows
are encountered the disk is dropped and the cable is wrapped around
the tree. The power winch is then used to pull the tree and drag it to
the bank. Having disposed of the tree, the tractor backs up, the disk is
picked up again, and the equipment proceeds with the cleaning opera-
tion. The Turlock District has found this equipment most valuable in the
maintenance of their drainage system.

WEEDS IN RICE FIELDS

Because of the conditions under which rice is grown, the weeds in rice
fields differ from those infesting other field crops. The most troublesome
ones grow, like rice, in standing water. After the rice has been seeded,
there is no practical means of combating the weeds ; and as the rice re-
quires a long growing season, most of the weeds mature and drop their
seeds before harvest time. Consequently, weeds in rice fields multiply
rapidly. Three years is, by common experience, about the maximum
time a field can be cropped continuously with rice before becoming so
weedy as to render the crop unprofitable.

The weeds most troublesome in the rice fields are varieties of water
grass, red stem, scale grass or sprangle-top, red rice, spike rush, cattail,
various species of umbrella plants or nut sedges, tule or bulrush, and
arrowhead.

Up to the present, water grasses have been fought more vigorously
than the other rice weeds. Yet the water grasses, though most prevalent,
are by no means the most serious. Such perennials as the nut sedge, cat-
tail, and spike rush, reproducing by underground parts as well as from
seed, are much more difficult to eradicate when once established.

Water grass is of two or more types. The common type fruits abun-
dantly, matures, and sheds its seed before the rice crop is ripe. It spreads
and infests the fields rapidly. Fortunately, common water grass and also
scale grass, or sprangle-top, can be controlled successfully by keeping
the fields flooded from the time the seed hed is prepared until the rice
crop is mature. Submerged under 4 to 6 inches of water, the seed of com-

1936] Weed Control 65

inon water grass will not germinate. When, however, rice is drilled or
broadcast and irrigated lightly for two to four weeks to bring it up
before it is permanently flooded, shallow submergence at 2 to 4 inches
apparently does not control the weed. The second type, the so-called
"white water grass," matures somewhat later, at about the same time as
the rice, so that when the crop is harvested most of the seed is removed
from the field. Although, in consequence, it spreads less rapidly than
common water grass, it cannot be controlled by flooding. Its seeds will
germinate and produce plants with any submergence that will permit
the growth of rice.

As none of the other important annuals can be controlled by flooding,
the only known method of checking them is the periodic fallow now com-
monly practiced.

The perennials offer a more difficult problem. Nut sedge, for example,
is spreading rapidly and, as it may reproduce either by seed, or vege-
tatively by the nut-like tubers, constitutes a real problem. It thrives
equally well on moderately moist and on wet soil. Its vegetative mode of
reproduction makes it extremely difficult to eradicate by cultivation.

Cattails, likewise, reproduce both vegetatively and by seed. In the
field they can be controlled by proper cultivation; but a fairly dense
stand, once developed, is not easily destroyed. Once cattails infest the
land, heavy seeding with rice apparently does not control them ; but a
heavy stand of rice does tend to prevent their entrance. Plowing in the
spring and thorough drying of the roots will destroy many plants. In
fact spring plowing, with a well-prepared seed bed, aids greatly in the
control of many rice weeds. Disked stubble does not, as a rule, provide
a satisfactory seed bed. Such roots as remain in the soil will renew
growth; and cultivation must usually be repeated two to three times
during the season to be fully effective. Most perennials can be controlled
by cultivation repeated often enough to prevent an appreciable develop-
ment of the aboveground organs. The number of cultivations necessary
and the time required will vary with the species. Judging from pre-
liminary tests, cattails and some other perennial weed pests may be con-
trolled with chlorate sprays ; but the conditions necessary have not yet
been fully worked out.

Destruction of weeds will not solve the rice-weed problem ; equally
important is prevention of their introduction. The weed seeds are in-
troduced in three ways : with the rice seed, with the irrigation water,
and by wind and other agencies, from plants growing on adjacent waste
or unused land.

New weeds are usually first introduced into a district with the rice
seed. Thus certain sedges now spreading in the rice area were brought

66 California Agricultural Extension Service [Cir. 97

in with rice seed from the Orient. E/ed rice is, as a rule, also distributed
with the seed. The use of clean seed is therefore important in preventing
infestations.

Since most weed seeds float or are readily transported by flowing
water, the seeds of weeds and grasses once allowed to mature on the
ditch banks soon find their way to the irrigated land. Unsuccessful at-
tempts to filter off such seeds from irrigation water by the use of screens
have shown that the numbers thus transported are enormous.

RANGE WEEDS

Although many of the introduced plants now common on California
ranges are weedy compared with the original vegetation, only those that
tend to supplant species of definite feed value will be considered. Among
the most common are Klamath weed, artichoke thistle, yellow and purple
star thistles, and milk thistle. As a serious menace to the foothill and
valley ranges of the state, these plants should be suppressed.

Although overgrazing has undoubtedly encouraged the spread of
these weeds, their effective means of seed dispersal, and the wide natural
variation in range cover due to the fluctuations in rainfall, render in-
vasions almost uncontrollable, regardless of the rangeman's policy.
These pernicious range weeds, all vigorous growers of little or no feed
value, have a strong competitive ability. Many are perennials with a dis-
tinct advantage over the common annuals. In these cases, deferred or
restricted grazing tends to favor the weed at the expense of the annual
range plants. Eradication of large areas usually costs more than the land
is worth.

Considering these points, the only feasible method in the lower ranges
of California is, apparently, to check the range weeds when they first
appear and to prevent their spread. Destruction of large areas of such
species as Klamath weed will at best be slow and costly.

The rangeman should learn to recognize the more noxious range weeds
and should make every effort to stamp out the first invaders. The unre-
stricted grazing of cattle from undetermined localities on public domain
is bound eventually to cause infestation, providing a constant source of
seed for further spread. The recognition and eradication of weed species
are therefore increasingly important to stockmen, especially to those
who use public ranges or buy stock that have used them. The chemical
methods already described should check new infestations.

KEEPING FIREBREAKS CLEAR

As fire is a constant menace to the forests and watersheds of California,
its prevention and control are all-important. The Forest Service has

1936] Weed Control 67

developed a system of firebreaks throughout the mountain regions, espe-
cially in the southern part of the state.

Although these strips, cleared of small trees and other vegetation,
may sometimes stop the spread of fires, they serve principally as a means
of access to remote areas and as starting places for backfires. For this
reason they are located along ridges wherever possible.

Experience during the present season has shown that in years of high
rainfall the growth of annual species on these cleared areas not only
reduces the efficiency of firebreaks but, unless they are promptly cleaned
during the early summer, makes them more hazardous than the original
trees and brush. Clearing by hoe labor, which has prevailed in the past,
is excessively expensive under these conditions and, according to experi-
ments, may be superseded by chemical soil sterilization.

In the absence of stock and game, sodium arsenite is the most practical
chemical ; applied at the rate of 4 pounds per square rod to a strip 6 or
8 feet wide through the middle of the break, it should prove of great
value in backfiring. Where animals are grazing, a mixture of Colemanite
and sodium chlorate may be used. Considering the high cost of hand
labor, any chemical treatment good for several years and applied at a
nominal cost should be of value.

Another problem on firebreaks and in cleared pasture areas is the
killing of stumps. Here, again, chemical treatments are indicated. Ac-
cording to experiments by the California Forest Experiment Station,
most of our sprouting species may be easily controlled by girdling at or
below the ground level and treating the girdle with a solution of sodium
arsenite containing 20 per cent AsoOs by weight. In the presence of
livestock, the girdle may be made below the level of the soil and covered
with soil after the arsenic is applied. If this is not practicable, a 20 per
cent sodium chlorate solution or Diesel oil can be substituted for the
arsenic. In either instance, observe all the precautions listed on pages
32, 35, and 41.

Stump treatments can be made at any season except spring. Sprouts
appearing after treatment should be gone over again.

SPECIAL WEEDS

Often the peculiar environmental conditions under which certain weeds
grow in California create special situations not encountered in other
localities where studies on weeds have been made. For this reason there
have been developed in this state methods not commonly used in weed
control elsewhere. A few of these special situations are considered here
under the names of the specific weeds. In all cases, however, the reader

68 California Agricultural Extension Service [Cir. 97

should familiarize himself with the section "Principles of Weed Con-
trol" before considering the particular subject presented.

PUNCTURE VINE^"

Puncture vine has been known in California since 1903 but was prob-
ably introduced some years earlier. Having appeared first along the
railroads, it spread rapidly with the increase of automobile travel. Al-
though tires have been the principal means of dissemination, it is also
carried by animals, by various crops and products, and by almost any
other object with which it comes in contact.

Beginning about 1925, California counties made a more or less con-
certed effort at control, except in a few counties considered too badly
infested. For several years the total amount thus expended averaged
about $160,000 annually, mostly for spraying with chemicals, in addition
to expenditures by the State Division of Highways, the railroads, and in-
dividuals. Since 1932, appropriations have been considerably reduced.

The damage done by puncture vine consists principally of mechanical
injuries to persons and animals, increased operating costs in the pro-
duction of crops, diminished value of land and of crop and livestock
products, and limitation of markets for infested products. Though au-
thentic cases of direct injury to livestock from the feeding of infested
hay are rare, cows are said to fall off in milk production when first given
hay containing large quantities of puncture-vine burs. A disease of
sheep in South Africa is reported as caused by grazing on this weed.
Contrary to a rather general belief, puncture vine injures very few
tires except those light in construction or worn thin.

Germination of Puncture-Vine Seed. — The longevity of puncture-
vine seed is not known. A plot on which no seed matured for eight years
continued to sprout seed each season in undiminished quantities. When
more than one seed is present in a bur, the uppermost usually germinates
first; but given favorable moisture and temperature the others may
germinate the same season. The seasonal growth is apparently cor-
related with soil moisture and temperature. Under nonirrigated condi-
tions 90 per cent of the total number of plants for each season sprouted
by July 15 on one plot observed. The total number sprouted was highest
in those seasons when May and June were warmest, those being the only
months when sufficiently high temperatures accompanied favorable soil-
moisture conditions. Whereas summer rains increase the rate of germina-

^^ This section was prepared by Mr. Ethelbert Johnson, Deputy Agricultural Com-
missioner, Orange County.

" Johnson, Ethelbert. The puncture vine in California. California Agr. Exp. Sta.
Bui. 528:1-42. 1932.

1936] Weed Control 69

tion, fall rains, after the mean temperature has dropped, have no
apparent influence; nor is the rate affected by the amount of winter
rainfall.

The relation between temperature and germination was further estab-
lished by laboratory tests. Burs incubated at relatively high tempera-
tures showed a higher rate of germination than burs incubated at lower
temperatures.

During the season of rapid growth, the period from blossoming to
maturity of seed is very brief. Burs picked less than 10 days from the
time of blossoming failed to germinate, but a large percentage germi-
nated when permitted to remain on the plant 10 days or more after
blossoming.

Apparently puncture-vine seed requires a rest period after maturity
before germination will take place at a normal rate ; but after the rest pe-
riod germination is rapid. Burs of the current season, therefore, should
be held at least six months before being tested for germination. When
tested one or more seasons after maturity, more than 90 per cent of the
total viable burs germinated within the first 3 days of incubation, and
practically all germinated within 10 days. In tests made the same season
the burs matured, germination could be induced only with difficulty in
the first 60 or 90 days and was, in most cases, not completed for 200 days
or more. If the plant is killed or the burs are picked before the end of
the natural ripening, germination is accelerated, apparently by inter-
ruption of the physiological processes responsible for the rest period.

The approximate viability of puncture-vine seed may be estimated
by inspection. If the burs are soaked overnight in water and cut open,
the number containing apparently sound seed checks rather closely with
a laboratory germination test of the same sample.

The treatment of products infested with puncture vine to make them
acceptable in districts where the pest is under control has presented a
difficult problem, especially with reference to bulky materials. Alfalfa
meal from infested hay, milled by the ordinary process, contains viable
seed ; but if in the milling the meal is passed through a mesh not greater
than y^2 inch, no viable seed remains.

Composting of infested manure is not a sure method of destroying
puncture-vine seed. A test was made of the effect of decomposition of
straw and other organic refuse in the making of artificial manure.
Though seeds in the interior of the compost stack were destroyed, the
exterior was not sufficiently decomposed to kill the seeds.

Control of Puncture Vine. — To control growing puncture vine, culti-
vation and cutting are satisfactory if done often enough to prevent seed
formation. Heat from a weed torch or from live steam may be used, but

70 California Agricultural Extension Service [Cm. 97

burning of green plants without previous treatment is considered too
slow and too expensive. Although several chemicals will destroy punc-
ture-vine plants, only certain petroleum products are known to kill the
seeds in all stages.

Diesel oil is satisfactory and widely used for the purpose because of
its availability, its low cost, its high proportion of aromatic and cracked
oils, and its ability to emulsify satisfactorily with water when the
asphalt content lies within the range of 2 to 5 per cent, which makes
possible the use of a spray containing 50 to 75 per cent water in the oil.
The effectiveness of oil in destroying seeds depends on the completeness
of coverage and on the quantity deposited on the burs.

Soil may be sterilized with chemicals to prevent the growth of punc-
ture vine.

The following methods are recommended for controlling this pest :
First, treat with oil to kill the seeds, if present, without disturbing the
vines. Whenever young plants appear, and before the burs form, cut
off just below the crown ; or spray with oil or any cheap chemical weed
killer. During the summer, by frequent irrigation encourage the sprout-
ing of seeds remaining in the soil.

MORNING-GLORY

The common or wild morning-glory, undoubtedly the most serious and
most pernicious weed in California today, infests every county where
agriculture is extensive. It invades our choicest land and, so far, has
defied all efforts to depose it. Its presence may not necessarily be, there-
fore, an indication of poor farming.

Characteristics of Morning-Glory. — Morning-glory is a perennial that
renews its growth from year to year from its underground stems and
roots (fig. 2), and through the spread of these subterranean structures
rapidly widens its area of infestation. Ordinarily it starts in a field from
seed. Its rapid spread in the early years of farming in California was
probably caused by the sowing of uncleaned wheat and barley seed. It
is almost impossible to remove morning-glory seed from these cereals
because of the similarity in seed size. Grain farming affords this pest an
excellent opportunity to produce seed and thus to become established.

The root of the plant will, under favorable conditions, penetrate the
soil to a distance of 20 feet or more. From the main root and shallow
laterals, new shoots come to the surface, so that the plants spread vege-
tatively in an everwidening circle. The seed, being produced in abun-
dance, rapidly increases the infestation and is in fact the most important
means of dissemination. From a small area the pest can quickly be
spread over a field by the tillage implements, which drag vines bearing

1936] Weed Control 71

mature seed. The underground parts torn loose by the implements and
carried for considerable distances may, if embedded in moist soil, also
start a new infestation ; but the spread in this way is small compared
with the spread by seed.

Morning-glory seed probably retains viability in the soil for many
years. Though data on wild morning-glory (Convolvulus arvensis) are
not available, in trials with a closely related species, seed buried at depths
of 8, 22, and 42 inches showed germinations of 27, 41, and 43 per cent,
respectively, after 20 years. "Where morning-glory has become estab-
lished, therefore, one must not only destroy the established plants but
also prevent recurrence from seed that has lain dormant in the soil.

Control of Morning -Glory. — Although morning-glory has received at-
tention for many years, no completely satisfactory or economical method
of control has yet been devised. As experiments show, the pest can be
checked or almost completely exterminated, but only with a considerable
expenditure of time, labor, and money.

Morning-glory has been attacked in several ways. Cultivation, smoth-
ering (either with a smother crop or with nonliving material), crop-
ping methods, and chemical herbicides have been used with varied
results. All these methods are useful, and often two or three may be
combined. Pasturing and flooding have also been tried, with indifferent
success; but such methods are, at best, applicable only under special
conditions.

Control hy Cultivation. — The numerous attempts to eradicate morn-
ing-glory by cultivation have met with some success and many failures.
Poor results can be attributed to careless or incomplete treatment.
The underlying principle in eradicating perennials by cultivation is
that the development of the green leaves must be prevented. A second
rule is to continue cultivation until the reserve food material is used
up in new growth or until underground parts are starved to death. Poor
results are usually attributable to failure to prevent leaves from de-
veloping, for even a small leaf surface will soon manufacture and store
in the root sufficient food material to undo the effect of several culti-
vations. The best procedure is to cultivate at definite intervals, often
enough so that no new shoots reach the light and to do the work faith-
fully. Just how often the field must be weed-cut depends on the depth
of cultivation, the season, the amount of reserve food in the roots, and
perhaps certain local conditions. With the ordinary depth of cultiva-
tion of 4 to 6 inches, the intervals cannot be more than one week ; and
during the summer heat, when growth is most rapid, cultivations must
be more frequent.

During July and August, according to observations at Davis, shoots

72 California Agricultural Extension Service [Cir. 97

from plants cut off at a depth of 16 inches will reach the surface in 10 to
12 days. Cultivation must begin in the spring as soon as growth starts
and must continue until growth definitely stops in the late fall or early
winter. On shallow soils or soils with a high water table, where all root
development is near the surface, less time is required to eradicate morn-

Fig. 21. — One-horse weed knife, made by blacksmith.
(From Cir. 256.)

ing-glory than on soils that permit deep root penetration. Under the for-
mer condition, some farmers have reported eradication in a single season.
At Davis, however, on a Yolo sandy loam soil, plants were still thriving
and producing vigorous shoots after three years of clean cultivation.
On the latter type of soil, morning-glory grows most luxuriantly, and
eradication by cultivating is obviously slow.

The implement most suitable for cultivating morning-glory is the
straight-blade weeder (figs. 21, 22, 23), kept sharp and in good cutting
condition. The field should be plowed rather deep in the early spring,
and the soil worked down thoroughly. If a deep layer of loose soil is thus
provided at the start, the weed cutter can be run at the necessary depth
more easily than if the plowing had not been done. When weed-cutting
the field, the weeder must be lapped at least 18 inches to 2 feet to insure
the cutting of all the shoots. Care and eternal vigilance are the price of
success in controlling morning-glory by cultivation.

1936]

Weed Control

73

Fig. 22. — Three-horse weeder, used for morning-glory control.
Made by a blacksmith. (From Cir. 256.)

Fig. 23. — Two-horse weeder used for morning-glory control. The blade can
be raised from the seat. (From Cir. 256.)

74 California Agricultural Extension Service [Cir. 97

Control hy Smothering. — ^Nonliving materials, such as tar paper and
straw manure, when used to control mornmg-glory, by smothering have
generally failed. Though tar paper, properly lapped, has proved effec-
tive for small areas, one must watch carefully to prevent the vines from
growing out between the layers. The principle involved in smothering
the plant with such materials is the same as in cultivation — namely, to
prevent the formation of green leaves, thus forcing the plant to use up
its reserve material in developing new shoots. Efforts to smother morn-
ing-glory with organic substances, such as straw and manure, have uni-
formly failed.

Cropping. — Of the crops used, a vigorous stand of alfalfa, well
watered, is best and will greatly reduce, though not completely eradi-
cate, the morning-glory. Experience shows that if the field is kept in
alfalfa four or five years, morning-glory will be so reduced in vigor that
other crops can be grown for several years before the weed renders them
unprofitable. The use of alfalfa, followed by cultivation or by chemical
treatment, will in the long run probably prove the most effective means
of control.

Flooding. — In morning-glory control, flooding has been used with
variable results. Where water is plentiful and cheap, it would warrant
trial. The infestations should be diked and submerged for 60 to 90 days
in midsummer. "Where seeds are abundant in the soil, the area may be
allowed to dry until germination starts and then reflooded for two or
three weeks. Best results from flooding are apparently obtained on light
or sandy soils.

Control with Chemicals. — The attempts to eradicate morning-glory
with chemical herbicides have met with varying success because of in-
sufficient knowledge as to the method of absorption of the chemical by
the plant, and of the proper physiological and environmental conditions
for absorption. Even yet, knowledge on some of these points is meager ;
but certain conditions of growth and environment are known to be neces-
sary for success, as explained in a previous section (pages 27 to 30) .

The chemicals most widely used and most likely to succeed on morn-
ing-glory are carbon disulfide, arsenicals, and chlorates. One arsenical
is marketed under the trade name of "K. M. G." The nature and methods
of application of these herbicides are given on pages 27 to 38.

Though the use of herbicides offers a promising means of eradicating
morning-glory, as stated before good results cannot be obtained without
due attention to the stage of growth, the condition of the plants, the
nature of the soil, and the environmental conditions at the time the
spray is applied. No single application of any chemical is likely to eradi-
cate morning-glory completely. Chemicals now available, however, are

1936] Weed Control 75

effective on small infestations ; and their use, though expensive, is often
justified in preventing the spread of the weed. At present the most satis-
factory chemical methods are too costly for large areas. Until more re-
liable and cheaper ones are found, cultivation and cropping are more
feasible for large-scale control.

JOHNSON GRASS

Johnson grass, first introduced as a forage plant, has become one of our
worst weed pests. It is a prolific seeder and spreads vegetatively by un-
derground stems as well. The first step toward control is to prevent
spread by seed. To do this, cultivation, mowing, hand-cutting, or oil may
be employed. Cultivation, if practiced, should not extend beyond in-
fested areas, for portions of the roots may be carried to noninfested
parts and thus spread the pest instead of controlling it. On rather in-
accessible areas, such as fence rows and ditch banks, the use of "stove-
top" or Diesel oil is probably cheaper, for the area can thus be covered
in less time and with much better results than with any other method.
In a few cases where Johnson grass was sprayed with oil during hot sum-
mer days, the oil penetrated the underground stem as deep as 8 inches
and thinned heavy infestations more than 50 per cent. The number of
sprayings depends largely upon the growing conditions. With abundant
moisture and good soil, Johnson grass is more difficult to kill than under
dry, unfavorable soil conditions.

Many different herbicides have been tried on Johnson grass. At pres-
ent, the chlorates appear most promising. In California "Atlacide" (cal-
cium chlorate) has possibly been used more widely than sodium chlorate.
A solution containing 1 pound of chlorate to 1 gallon of water may be
applied as a fine spray to plants in full bloom or in the soft-seed stage.
Sodium chlorate may be used in solution, or dry as a soil treatment in
the early winter. The dosage depends upon soil type and rainfall. Three
pounds per square rod is a good average treatment. Plants treated with
chlorates should not be mowed or burned immediately, but should be
allowed to stand in the dry condition until the following spring, when
they may be burned. On small areas Johnson grass is effectively treated
with carbon disulfide. Careful grubbing, particularly in loose or sandy
soil, is also a practical method if all rootstocks are removed and destroyed.

Johnson is often confused with Sudan grass, which it frequently in-
fests. In fact, the State Seed Laboratory finds that many samples of
Sudan grass seed examined contain this impurity. Johnson grass is
readily distinguished from Sudan grass by the presence of rootstocks,
which differ from roots in having joints.

76 California Agricultural Extension Service [Cir. 97

CAMEL THORN

Camel thorn, a comparatively new pest in California, is confined to cer-
tain areas in Imperial, Riverside, Kern, Kings, Merced, Madera, San
Diego, Tulare, Los Angeles, and Fresno counties. Being a pernicious
weed, it has been strenuously fought in these limited areas. As the plant
is browsed by animals, the seed has been spread great distances by the
movement of stock.

This thorny, semiwoody plant, in addition to producing innumerable
seeds, develops roots that may go to depths of 10 or 12 feet and spread
laterally 25 to 40 feet in a single year, sending up new shoots the next
spring.

In the ten counties where camel thorn has been found, nearly all areas
have been treated either with carbon disulfide, with arsenic by the jar
method, or by flooding. The carbon disulfide and flooding methods did
not prove wholly satisfactory, because of excessive costs and lack of
water. The jar method, which employs a 1 per cent solution of arsenic
trioxide, was first tried in an attempt to kill plants growing in the ballast
of railway roadbeds. On these areas carbon disulfide or flooding could
not be used. Evidently the plants growing in the ballast originated from
roots growing through the railroad grade. When several of these plants
were treated with arsenic solution, not only shoots growing in the ballast
were killed, but also shoots on the opposite side of the grade, a fact show-
ing definitely that all these plants were connected by underground roots
to the one treated.

In view of these results, 10,000 jars were obtained, and some 70 acres
of rather solidly infested land were treated in Kern County. Plants
were selected at random and placed in the jars. After 36 to 48 hours the
jars were moved to plants that had not been affected. A season's experi-
ence has proved that best results can be expected if the plants are mature
when treated.

The cost of this method proved to be much less than that of either the
carbon disulfide or flooding. Rather large areas were treated in Merced
and Kern counties at an average cost of $25 per acre. Carbon disulfide
treatment under similar conditions averaged approximately $250 per
acre.

RUSSIAN KNAPWEED

Russian knapweed was introduced into California about 1912 in impure
sugar-beet seed or Turkestan alfalfa seed. Since then it has spread
widely and now occurs in isolated areas throughout the state. A per-
nicious perennial, very similar in its habits to morning-glory, Johnson
grass, and hoary cress, it spreads rapidly, especially where seeds and

1936] Weed Control 77

portions of the underground parts are carried by cultivating tools. The
spread by seed is somewhat less rapid than with many other weeds be-
cause the seeds are born in cup-shaped heads that do not open readily at
maturity ; ready dispersal by wind is thus prevented. The seed is readily
collected by harvesting machinery, however, and is a common contami-
nation in cereals and seed crops.

Kussian knapweed is one pest that deserves the serious attention of
Californians, for invariably it works great crop injury. The methods of
control mentioned under perennials apply here. Acid arsenical spray
(pages 27 and 32) and sodium chlorate have given promising results.
Apparently Russian knapweed is more susceptible to chemicals than
either wild morning-glory or hoary cress.

HOARY CRESS

Hoary cress, which appeared in the Pajaro Valley in California about
1907, seems to have come originally as an impurity in alfalfa and lawn-
grass seed. Importations of impure alfalfa seed during the World War
started infestations in many parts of the state. At present this weed is
spreading rapidly and threatens to be a serious pest.

Hoary cress is a perennial that first appears early in the spring and
often matures by the last of May. Its underground stems are even more
persistent than those of morning-glory. Although it often fails to set
viable seed, a good crop if produced will germinate well and survive
even under adverse conditions. Hoary cress has been known to displace
alfalfa. It has no feed value whatever.

Thus far, carbon disulfide has proved reasonably satisfactory, al-
though it has sometimes failed. Sodium chlorate, under certain condi-
tions, has also brought fair results. Applications of 4 pounds or more
per square rod on adobe-clay soils have given promise when made in
the early winter. As explained on page 33, the amount of rainfall follow-
ing the application of the chlorate determines the depth of penetration.
In some localities hoary cress has proved very susceptible to control by
clean cultivation and can be held in check by this method.

WILLOWS

The banks of ditches, levees, and drainage canals often become over-
grown with willows. Gradually encroachment of the plants will greatly
reduce the carrying capacity of a waterway. Much money is spent in
controlling willows, especially by hand cutting, which usually causes
a thicker stand the next year and must be repeated annually.

Chemicals have been tried to some extent. Common salt is effective
when applied in sufficient quantities — approximately 10 pounds to an

78 California Agricultural Extension Service [Cir. 97

average-sized clump of willows. Usually the willows are so thick that a
solid ground cover of the salt is necessary — about 250 pounds to each
square rod, or 20 tons to the acre. In the few instances where the chlorates
have been tried as a foliage spray, a root penetration on some of the
plants was distinctly noticeable, and in some cases complete eradication
has been obtained. The observations were made on plants treated with
1 pound of chlorate to a gallon of water. In one area, willows were
largely eradicated by chlorates applied to the cut stumps. The willow
growth was rather thick, and the individual plants varied in size. Two
to 4 pounds to the square rod, applied dry in January, was effective.
In this case, however, as in others, follow-up applications were necessary.

On small areas carbon disulfide is very effective. One to two cups- in-
jected around the roots of individual clumps will kill the root system.

In some districts tractor pulling of willows has proved effective and
economical. The jar method, using a 1 per cent arsenic solution, has
given fairly satisfactory results. Large containers are required so that
several branches can be included.

BERMUDA GRASS

Bermuda grass, widely used as a pasture and lawn grass in the south,
has now spread from these areas and rather generally infests the central
and southern parts of California. Its ability to withstand heat and dry
summer conditions makes it one of our worst pests.

Many chemicals have been unsuccessfully applied in attempts to con-
trol this weed. Though occasional treatments have been encouraging,
most have failed. Heavy applications of chlorates in early winter have
proved promising, especially on sandy soils. Wherever Bermuda grass
infests ditch banks, roadsides, or noncultivated soil, sterilization of the
soil, as described in a preceding section (page 38), is the best method.
Crop rotation and clean cultivation have sometimes proved fairly suc-
cessful. As this grass cannot withstand dense shading, a rotation system
that includes a crop such as Sudan grass or cowpeas, or an intertilled
crop, followed by grain and then a heavy seeding of alfalfa, is recom-
mended. In some sandy soils dry-plowing in the summer, followed by
harrowing and frequent cultivation is used.

KLAMATH WEED

The mode of introduction of Klamath weed into the northwestern part
of California is obscure ; but within 50 years after its first recorded ap-
pearance it has spread over approximately 100,000 acres. A common
roadside plant of Europe, it is now present in several eastern states and
in Washington, Oregon, and Idaho. In California it occurs in nearly

1936] Weed Control 79

every county north of San Francisco Bay and in Santa Cruz County.
Although its further spread southward will probably be limited by its
requirement of rather moist conditions for optimum growth, there is still
much territory available for increase in the north coast counties. Control
methods, therefore, are highly desirable.

Klamath weed is a relatively shallow-rooted perennial, growing vigor-
ously to a height of 1 to 3 feet during the summer. It tends to form an
interconnected root system and grows so densely as to crowd out most
of the common annuals of our ranges. It not only lacks forage value, but
is toxic to certain breeds of sheep and cattle.

Noting the seriousness of this pest, the California Agricultural Ex-
periment Station established in 1934 a field project to study its control
and eradication. No definite report can yet be given upon this experi-
mental work, which so far has taken the form of a comprehensive test of
herbicides on the weed. A few significant points, however, should be
helpful to those interested in chemical control.

Sodium chlorate, the chemical most commonly used on Klamath weed,
is best applied in the spring when it will receive from 2 to 5 inches
of rainfall after the treatment. Applied in this way, 1 pound of chlorate
per square rod has killed all established plants so that the plots were
covered with a heavy stand of grass by the following spring. New seed-
lings, however, were present ; and, judging from experience with chlor-
ates in the past, the areas will probably be reinfested within two more
years. The effects of chlorate are not lasting enough to insure eradication.

If applied earlier in the season, so that more rainfall occurs afterward,
up to 3 pounds per square rod is required for comparable results ; and
when subject to the total seasonal precipitation even this amount has
been leached away without killing all the plants.

Borax, which is fixed more firmly than chlorate (page 42), will prob-
ably prove valuable in controlling Klamath weed. Applications of 4 to 8
pounds per square rod in February, 1935, gave excellent results. More
tests, however, will be required before this material can be generally
recommended.

Sodium arsenite has produced the most lasting results of any chemical
tested and should be very useful. Applications equivalent to 4 pounds
or more of AsoOg per square rod have given complete sterilization for
two years. In this time all seeds in the surface soil should have germi-
nated ; and as grass returns these areas may remain clean.

All any chemical can do is to kill the established plants and prevent
recurrence from seed previously deposited in the soil. Reinfestation
from seed disseminated by grazing animals is an ever present problem.
All movements of stock from infested to clean ranges should be avoided,

80 California Agricultural Extension Service [Cir. 97

and a control program once inaugurated on a given range should be
followed out rigidly until all infestations are eradicated. More work
must be done before any suggestions can be made as to control on large
infested areas. In such cases, obviously, chemicals are out of the ques-
tion, for they would cost many times the value of the land.

Fig. 24. — Eemoving the seed stalks of artichoke thistle.

POISON OAK

Small infestations of poison oak can be grubbed out by an individual
who is immune. In some cases fair results have followed spraying with
Diesel oil. One spraying usually kills the top growth ; and although
there may be some regrowth later in the season, as a rule a second spray-
ing eradicates most of the plants. If regrowth follows, a third spraying
with oil may be necessary. Oil applied to the stumps of cut plants is also
effective.

A little chlorate solution (see page 32) applied around the roots of
each plant will usually give a satisfactory kill. Roots of other plants
interlaced with the poison oak will be killed also. If poison-oak plants
are burned, care should be taken to keep out of the smoke, which carries
the volatile oil that causes injury.

ARTICHOKE THISTLE

This weed has infested approximately 70,000 acres of grazing land in
Solano County. Scattered areas occur in Contra Costa, Alameda, Napa,
and Orange counties. Particularly in the Bay region, environmental
conditions favor its rapid spread. A tall perennial from a rather shallow
crown, it produces an abundance of seeds. Although oils, chlorates, and

1936] Weed Control 81

sulfuric acid have been tried as herbicides, the most practical method of
eradication is grubbing. Individual, scattered plants may be removed
with a grub-hoe (fig. 24) ; and heavy infestations are plowed out with a
special tractor-drawn implement equipped with a heavy blade (fig. 25).
The cost of this latter operation has averaged from $8 to $10 per acre,

•fc^^^^^^*i^«&*.^*^--« . .^. *^

fl^^'-:..^^^^^^^^^^

3^^Ei

^^^^^^^]

L^^^IE^

rr^^o

^^~^^^^M

^

^

2""^ ^.y-"'

-M

* isTM^s^

■'/ ' • " "' *v<i?<f - -^

"^ ■•ill!;

tate^^^^w^SH

fij-llllppSS^- ''■

H

i^P

imB

^^fc

%i!!m

^^P

^^^

^^^Sl

^^^^^^S-4 -^

^^^^^

^^E^^BH

Fig. 25. — Equipment used in plowing out artichoke thistle.

according to the roughness of the area. If the weeds are growing on
slopes too steep to plow, treatment with oil is the cheapest control
method. Just before the growth of the seed stalk, about % pii^t of Diesel
oil is applied to the crown of each plant. The cost of this treatment varies
greatly with the terrain and the degree of infestation.

REGULATORY PHASES OF WEED CONTROL"

When, despite general recognition that a particular agricultural pest
needs control or eradication, a few persons neglect their responsibility
and do not take the advisable steps, there arises a demand for laws and
regulations to compel the negligent to cooperate.

Laws for weed control when necessary should meet three needs : (1)
they should prevent introduction of any new and seriously noxious weed
pests; (2) they should authorize responsible official agencies to eradi-
cate any new or limited infestations; and (3) they should provide for
the abatement of any noxious weeds against which there is continual

^* This section was prepared by W. C. Jacobsen, Supervisor of Kodent and Weed
Control, State Department of Agriculture, Sacramento.

82 California Agricultural Extension Service [Cir. 97

or active warfare and against which recalcitrants do not take the neces-
sary steps.

Certainly plant quarantine as a preventive measure should constitute
a primary phase of weed suppression. Federal recognition of this need
should precede that of the state or county. So far, dodder seems to be
the only weed pest that has claimed federal attention. If noxious weeds
are to be ignored in seed shipments from other nations, and if the appar-
ent policy of allowing a tolerance of weed seeds in such importations is
not modified, American agriculture is faced with a continuous hazard,
and we shall ever have camel thorn and Russian knapweed cases with
which to cope. Another very important means of acquiring undesirable
plant immigrants is through new plant introductions. A plant or shrub
seemingly of value for ornamental or even forage purposes may develop
noxious tendencies in a new environment.

Our immediate concern, however, is this : What has California accu-
mulated in the way of regulatory measures affecting noxious weeds?
History shows that a new country must look to outside sources for seeds
and planting stock. Of course, weeds are sure to be stowaways and will
land unless detailed inspection is provided. A country with wide diver-
gences in climate and soil offers a fertile field for establishment of weeds.
When large land holdings were the order, weeds were usually taken as
necessary evils. Should one field become abnormally weedy, it was not
difficult to transfer operations to another.

Apparently the first law affecting weed control was passed by the
legislature in 1903 and was specifically directed against Johnson grass.
Since then more specific authorities have been added.

Obviously, not every weed could be considered noxious, for some were
reasonably desirable as covercrops. If we ignore the role of such cover-
crop weeds in harboring injurious insect pests and plant diseases detri-
mental to commercial plantings, we might say they were beneficial.
Hence, one of the original and practical necessities was a sound definition
of a noxious weed. This point apparently took years of study ; the 1929
legislature first enacted the authority to consider as "noxious" any
plants or seeds detrimental to agriculture. With the enactment of the
Agricultural Code in 1933, the definition of a weed was broadened to
include "any form of vegetable life" that is, or is apt to be, dangerous,
or detrimental, to the agricultural industry of the state ; and as such is
included in the definition of a "pest." (Sec. 100, Agricultural Code."^^)
Undoubtedly, the court must ultimately decide whether or not a par-
ticular weed belongs in the category of pests which the Code declares

^^ Published in pamphlet form by the California State Department of Agriculture,
Sacramento.

1936] Weed Control 83

to be public nuisances. Those generally concerned with the problem of
weed control or eradication, however, understand what this term really
means, and regulatory officers will be able to substantiate their deter-
minations by evidence in the event that a weed ''pest" is involved in
court proceedings.

There are actually available now in California laws, mainly in the
Agricultural Code, the following authorities and provisions relative to
weed control which are apparently operative. They are segregated to
show how they may be responsive to the three needs previously noted.

To Prevent the Introduction and Spread of Noxious Weeds. — (a) The
quarantine authority available to the Department is applicable to pre-
vent the introduction of weed pests just as it is against injurious insects,
plant diseases or pest animals involved in interstate shipments. However,
if the consignee can eliminate weed seeds by cleaning or crushing, then
the shipment may be released. (Sections 105, 106, 115, and 116, Agricul-
tural Code.) The use of this authority is limited since in most instances
weed infestation is capable of determination upon inspection.

( h ) Section 124 of the Agricultural Code reflects the same authority
for the rejection of weed-pest-infested shipments moving intrastate as
pertains to other agricultural pests. The alternative procedures in this
section will permit release after cleaning.

(c) Section 125 of the Agricultural Code provides that if anything
originating in another county or locality of the state or in another state
or foreign country is found upon inspection to be infested with the seed
of any pest not of common occurrence at destination, the Director or the
agricultural commissioner shall notify the owner or bailee of the ship-
ment to return it within 48 hours, provided that if the seeds can be
destroyed by a treatment the shipment may be released.

{d) Section 128 of the Agricultural Code gives authority to hold, after
written notice, any commodity found to be actually weed infested on the
premises where the commodity is encountered until it is cleaned of the
"pest" to the satisfaction of the county agricultural commissioner or
the Director of Agriculture.

{e) The weed-free area provisions of the Agricultural Code make it
unlawful to transport, into the district proclaimed by the Director, the
seeds of any specific weeds for which the weed-free area has been de-
clared.

Eradication of Limited Infestations. — {a) The general pest-abate-
ment provisions of the Agricultural Code are available for eradication
procedure but are advisable for eradication only on limited infestations
of the more serious kinds or for newly introduced noxious weeds (Sec-
tions 129-138, Agricultural Code.)

84 California Agricultural Extension Service [Cm. 97

(h) The weed-free area provisions, sections 155 to 158 inclusive, make
it unlawful to permit any weed named in a weed-free-area proclamation
to continue to propagate itself or mature seed.

(c) A section of the Code which is in effect a special law is directed to
the eradication of camel thorn. (Sec. 159, Agricultural Code.)

(d) Austrian field cress is the subject of special eradication authority.
(Sec. 159a, Agricultural Code.)

(e) The Director of Agriculture may enter into cooperative agree-
ments with boards of supervisors and other official, state, and federal
agencies for the purpose of eradicating pests dangerous to the agricul-
tural industry of California. (Sec. 34, Agricultural Code.)

(/) The board of supervisors may authorize county agricultural com-
missioners to enter into contracts with landowners for the eradication
of '^pests." (Sec. 149, Agricultural Code.)

(g) An area of limited infestation of a particularly serious and in-
jurious weed pest can, if necessary, be circumscribed by quarantine lines
to aid in eradication or prevention of spread. (Sec. 108, Agricultural
Code.)

General Abatement Procedure Against Weed Pests. — (a) The gen-
eral abatement provisions of the Agricultural Code, sections 129 to 138
inclusive, provide for the service of abatement notices wherein the owner
of infested property is notified to eradicate, control or destroy the ''pest"
within a specified time. If not done by him, then the county agricultural
commissioner has authority to proceed, the cost of such abatement, if
not paid by the owner, becoming a lien on the property. This is applica-
ble where general weed-control programs as to particular weeds are in
effect and where there are negligent property owners or recalcitrants
who must be required to clean up. This phase of abatement is also ap-
plicable to publicly owned or controlled areas subject to the jurisdiction
of the governing board of a political subdivision of the state.

( J) ) The governing board of a county may declare a particular weed
growing upon streets or sidewalks or even on private property a public
nuisance. Under this authority the county agricultural commissioner or
the County Board of Forestry, or some other officer, board, or commis-
sion designated by the supervisors is authorized to post notices requiring
abatement. If the owner fails to act, abatement may be proceeded with
by the officer posting the notice and the costs included with the annual
tax bill. (Chapter 457, Statutes 1929.)

(c) Under the general permanent powers of boards of supervisors,
authority is given to provide for the destruction of noxious weeds. (Sub-
section 3, section 4041.11, Political Code.) This authority is generally
exercised through the passage of county ordinances.

1936] Weed Control 85

(d) "Maintenance" as defined in the new Streets and Highways Code
includes "weed control" as one of the general utility services thereunder.
(Sec. 27, Streets and Highways Code.)

(e) There are general law provisions applicable against public nui-
sances.

Incidental to the above specific references, there is authority in the
pest-abatement sections of the Agricultural Code that materially assists
in the case of canals and ditches which are not subject to the jurisdiction
of any political subdivision, special provision having been made for the
service of abatement notice upon the users of water from such ditches
and making them responsible for weed cleanup. Lacking response from
them, the commissioner is allowed to proceed, the users being jointly
responsible for the costs.

Specific authority is to be found in the Irrigation District Improve-
ment Act, which authorizes the organization of improvement associa-
tions for weed-control purposes.

The seed inspection provisions of the Agricultural Code are directed
primarily to the prevention of introduction of certain primary and sec-
ondary weeds through the planting of impure seed. (Sections 911 to 920
inclusive. Agricultural Code.)

There are still a few important needs in connection with regulatory
measures affecting weed control ; namely, improved federal regulations
to prevent the introduction of seeds of injurious weeds in plant ship-
ments and in imported commercial seed from foreign countries; an
improvement in seed inspection laws, both federal and state, tending
toward the elimination of a permissible tolerance, since even one viable
noxious weed seed may start an infestation ; and, further, for the guid-
ance of enforcement officers, basic research inaugurated or intensified
by agencies such as the United States Department of Agriculture, coop-
erating with state experiment stations, in some such manner that en-
forcement officials can devote a full share of their time to eradication or
control work and not be burdened with investigation.

Above all, the dissemination of sound information to aid and encour-
age the recognition of the importance of weed control is vital to the
proper application of regulatory measures.

No matter how well conceived and elaborately stated our regulatory
measures, they cannot he fully and effectively administered unless public
sentiment is in thorough accord with their application and enforcement.

86

California Agricultural Extension Service

[CiR. 97

SENDING WEEDS FOR IDENTIFICATION

As this circular does not deal with classification of weeds, the individual
is encouraged, when in doubt, to send in weed specimens for identifica-
tion. One should know the specific weed in order to plan the best program
for its control. Very often weed specimens sent in by growers are so
framentary, so incomplete, or so poorly packed, that they cannot be
identified with certainty. The leaves, flowers, and fruits, and a portion
of the root system should always, if possible, be included in a specimen.
In certain families of plants, such as the mustards and carrots, the
mature fruits are frequently needed for identification. In other families,
such as grasses, the underground structures may be necessary. In any
event, include flowers as well as leaves, for of all plant structures the
flowers are the most useful in making an identification. Also helpful is
a brief description of the growth habit of the weed, including its height,
and the conditions under which it grows.

Specimens should be wrapped in moist paper, moist cotton or cloth,
or moist moss, and enclosed in a box or heavy paper wrapping. Or they
may be pressed between several thicknesses of newspaper, or between
folds of blotter paper or soft corrugated cardboard, and mailed in a
thoroughly dry condition. Material simply placed dry in an envelope
or box, usually arrives in such poor condition that it cannot be used for
determination.

Send specimens for identification to any of the following agencies :
County Agricultural Commissioner ; County Farm Advisor ; State De-
partment of Agriculture, Bureau of Rodent and Weed Control, Sacra-
mento ; Department of Botany, University of California, Berkeley ; or
Division of Botany, University Farm, Davis.

COMMON AND SCIENTIFIC NAMES OF WEEDS REFERRED
TO IN THIS CIRCULAR

Alkali mallow Sida hederacea

Arrowhead Sagittaria latifolia

Artichoke thistle . . . .Cynara scolymus
Austrian field cress . . Boripa austriaca
Barnyard grass . . Echinochloa crusgalli

Beggar-ticks Bidens frondosa

Bermuda grass Cynodon dactylon

Black bindweed Polygonum convolvulus

Blessed thistle Cnicus henedictus

Broad-leaf plantain . . . Plantago major

Brome grass Bromns spp.

Buckthorn AmsincMa spp.

Bulrush Scirpus occidentalis

Bull thistle Cirsium lanceolatum

Bunch grass Sporoholus airoides

Bur clover Medicago hispida

Burdock Arctium minus

Camel thorn Alhagi camelorum

Canada thistle Cirsium arvense

Cattail Typlia latifolia

Cat's-ear Hypochaeris spp.

Cheat Bromus secalinus

Chess Bromus secalinus

Chicory Cichorium inty'bus

Chickweed Stellaria spp.

Cocklebur XantJiium canadense

1936]

Weed Control

Common spikeweed . Hemizonia irmigens

Corn cockle Agrostemma gitJiago

Crab grass. . .Syntherisma sanguinalu
Creeping buttercup . Banmiculus repens

Creeping malloAv Sida hederacea

Curled dock Rumex crispus

Dandelion Taraxacum officinale

Death camas . . . .Zygadenus venenosns

Dock Bumex spp.

Dodder Cuscuta spp.

Doorweed Polygonum aviculare

Dyer's woad Isatis t'mctoria

Fiddleneck AmsincMa spp.

Field madder Sherardia arvensis

Foxtail Hordeum spp.

Hoary cress Lepidium draha

Jimson weed Datura stramonium

Johnson grass Sorghum halepe7ise

Klamath weed . Hypericum perforatum

Knotweed Polygonum aviculare

Lamb's-quarters . . Chenopodium album

Larkspur Delphinium spp.

Lupine Lupinus spp.

Marlahan mustard .... Isatis tinctoria

Mayweed Anthemis cotula

Milk thistle Silyhum marianum

Morning-glory Convolvulus spp.

Napa thistle Centaurea melitensis

Nightshade Solanum spp.

Nut grass Cyperus rotundiis

Nut sedge Cyperus rotundus

Oxalis Oxalis spp.

Pigeon grass .... Cliaetochloa lutescens

Pigweeds Chenopodium spp.

Poison oak Ehus diversiloha

Prickly lettuce Lactuca scariola

Prickly pigweed A maranthus retroflexus
Frostrsite-pigweed Amaranthus hlitoides

Puncture vine Trihulus terrestris

Purple star thistle . Centaurea calcitropa

Purslane Portulaca oleracea

Ragweed Ambrosia psilostachya

Eed rice Oryza rufipogon

Red stem Ammayinia coccinea

Russian knapweed . . . Centaurea repens

Russian thistle

Salsola Tcali var. tenuifolia

Sandbur Cenchrus pauciflorvs

Scale grass. . . .Leptochloa fascicularis

Seaside heliotrope

Heliotr opium ciirassavicu n i

vSedge Cyperus spp.

Shepherd's purse

Capsella bursa- past oris

Sour clover Trifolium fucatum

Sow thistle Sonchus spp.

Spike rush Eleocharis palustris

Speedwell Veronica spp.

Spiny sow thistle Sonchus asper

Spotted spurge . . . Euphorbia maculata
Sprangle-top . .Leptochloa fascicularis
St. Johnswort. .Bypericum perforatum

Sunflower Helianthus spp.

Tall marsh elder Iva xanthifolia

Tall pigweed. .Amaranthus retroflexus

Tocalote Centaurea melitensis

Tule Scirpus spp.

Tumble weed. . .Amaranthus graecizans

Umbrella plant Cyperus spp.

Water grass

Echinochloa crusgalli (and varieties)

Water hemlock Cicuta spp.

Water hyacinth . . . Eichornia crassipes

White horse nettle

Solanum elaeagnifolium
Whorled milkweed. J scZepias mexicana

Wild aster Aster spp.

Wild blackberry Eubus spp.

Wild garlic Allium vineale

Wild hemlock Cicuta spp.

Wild mustard Brassica spp.

Wild lettuce Lactuca pulchella

Wild oats Avena fatua

Wild onion Allium spp.

Wild radish Eaphanus sativus

Willow Salix spp.

Wood sorrel Oxalis spp.

Yellow star thistle

Centaurea solstitialis
Yellow water weed. Jus.^iaea calif ornica

22m-3,'36