UNIVERSITY OF CALIFORNIA

COLLEGE OF AGRICULTURE

AGRICULTURAL EXPERIMENT STATION

BERKELEY, CALIFORNIA

PRUNING BEARING DECIDUOUS
FRUIT TREES

BY
WARREN P. TUFTS

BULLETIN No. 386

April, 1925

UNIVERSITY OF CALIFORNIA PRINTING OFFICE
BERKELEY, CALIFORNIA
1925

PRUNING BEARING DECIDUOUS FRUIT TREES

By WAEREN P. TUFTS

Pruning consists of cutting, removing or repressing certain living
parts of the tree in order to modify and utilize its natural habits for
the purposes of the grower. By pruning, the form of the tree is
modified, its bearing regulated and thereby more and better fruit is
obtained at less cost and for a longer period than is possible without
pruning. The response of a tree to any kind of pruning is such, how-
ever, that it is practically impossible to modify its form without at
the same time profoundly influencing its functions. There must be
understood the various operations of pruning and how to apply these
operations to obtain the responses which are necessary.

Briefly stated, the chief purposes of pruning are :

1. To produce a vigorous, mechanically strong, healthy tree.

2. To secure a tree well shaped for convenience and economy in
orchard management.

3. To distribute the fruiting area well over the tree.

4. To secure fruit of good size and quality.

5. To regulate the annual succession of crops in order to obtain the
maximum average crop compatible with good fruit.

The principles and practices of pruning young deciduous fruit
trees have been discussed in a previous publication.1 The present
discussion will be limited largely to a consideration of the pruning of
bearing trees.

TERMS USED

Leaf-bud. — A leaf -bud contains the rudiments of a leafy shoot or
spur.

Fruit-bud. — A fruit-bud contains the rudiments or unexpanded
parts of a flower or flowers, with or without the rudiments of a leafy
shoot.

Adventitious buds. — All buds arising in unusual or abnormal
places, whether on parts below or above ground, are called "adventi-
tious buds. ' ' The usual position for all buds; of course, is in the leaf
axils (angles formed by leaf stalks and one-year wood) or at the tip
of the shoots.

i Tufts, W. P. Pruning young deciduous fruit trees. California Exp. Sta.
Bull. 313: 311-154, figs. 1-24. 1919.

4 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

Suckers and Water-Sprouts. — All growth arising from adventitious
buds on the parts below ground are called ' ' suckers, ' ' that arising from
such buds on above ground parts and from dormant axillary buds
" water-sprouts. "

Fig. l.-

-Fruit-bud of the peach. Note the two lateral fruit-buds with a
small leaf -bud between.

Spurs. — Many species of fruit trees produce short shoots which
bear flowers and fruit, and are called ' ' spurs. ' '

Current and One-Year Growth. — Any new shoot is referred to
during the growing season as "current" or "this year's" growth.
This same shoot, however, during the following dormant and growing
seasons, is referred to as " one-year ' ' wood. In the same way, reference
is made to "two-year-old" wood, etc.

Bulletin 386] pruning bearing deciduous fruit trees

FEUIT BEARING HABITS

In order to prune fruit trees intelligently, it is necessary to have a
thorough understanding of the various kinds of buds and of their
relation to the fruit bearing habits in the different species and varieties.
The following classification is based on the location and external
appearance of leaf- and fruit-buds.

Classification of Buds. — As to development, the kinds of buds are :

1. Wood- or leaf -buds, which develop into leafy shoots.

2. Fruit-buds, which are of two kinds :

(a) Flower-buds, which develop into a single flower (peaches,
apricots, almonds) or several flowers (cherries, plums).

(&) Mixed buds, which develop into a cluster of leaves and
flowers (apples, pears).
As to position, the kind of buds are :

1. Lateral buds, which are in the axils of leaves on the sides of the
shoots.

2. Terminal buds, which are on the ends of shoots.

3. Adventitious buds, which are produced at unusual points (not
leaf axils), on the roots, trunk, or branches.

Recognition of Fruit-Buds:

The positions of fruit-buds are :

1. On comparatively long shoots of the past season one-year wood.

(a) Lateral (fig. 1). In this position are found almost all the
fruit-buds of the grape, peach, quince, and olive, many of
the fruit-buds of currants and gooseberries, part of those of
apricots, almonds, plums, cherries, and sometimes pears,
apples, and walnuts ; and the fruit-buds for the first crop of
figs. Apricots, peaches, and Japanese plums often bear
three buds at a node, in which case the center one is usually
a leaf -bud, and the two outer ones fruit-buds.

(b) Terminal. This is the normal position of most of the fruit-
buds of the walnut. Some varieties of apples and pears also
frequently bear a considerable number of fruit-buds in this
way.

2. On comparatively short shoots (spurs) of the past season.
(These short shoots constitute either an extension of the spur system
already established or the beginning of a new spur.)

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

Fig. 2. — Apricot fruit-spurs. Scars where fruit has been produced at X.
Fruit-buds at Y. Leaf-buds at Z. Two-year-old spur at A. One-year-old
spur at B.

BULLETIN 38G] PRUNING BEARING DECIDUOUS FRUIT TREES

Fig. 3. — Fruit-spurs of plums. Left to right: Robe de Sargent (European);
Wickson (Japanese); Yellow Egg (European). Note small roundish fruit-buds
of the Wickson which are characteristic of Japanese plums. The Robe de
Sargent has a more compact spur system than most European plums.

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

(a) On short-lived spurs, which may live only 3 years or less

(% 2).
(1) Lateral buds. Most of the fruit-buds of apricots and
almonds are in this position. Peaches occasionally bear
a small part of their fruit-buds in this manner. All the
fruit-buds of currants and gooseberries on shoots older
than one year are borne in this way.

(b) On medium-lived spurs which may live 5 to 8 years (fig. 3).

Plums bear most of their fruit-buds laterally in this way.

Fig. 4. — Three-year-old portion of a pear spur. Note terminal fruit-bud at
A, and two lateral leaf -buds at B. Fruit was produced at C during season pre-
ceding that in which buds A and B were formed. Growth of "B" buds will
give rise to characteristic branching of older pear spurs.

Bulletin 386] pruning bearing deciduous fruit trees 9

(c) On long-lived spurs which live 10 to 20 years, but which
should be renewed (or at least invigorated) oftener.

(1) Lateral buds. Most fruit-buds of sweet cherries and
part of those of sour cherries are borne in this way.

(2) Terminal buds (fig. 4). Most of the fruit-buds of the
apple and pear are terminal.

The Appearance of Fruit-Buds.

The fruit-buds of the apple, pear, peach, apricot, and plum are
larger and plumper and are not so sharply pointed as the wood-buds.
Cherry fruit-buds are difficult to distinguish until they begin to swell.
The most certain way of determining fruit-buds is by sectioning the
bud lengthwise with a sharp knife or razor. The fruit-buds of the
apple, pear, cherry, and plum contain several roundish bodies, each of
which develops into a flower. The fruit-buds of the almond, peach,
and apricot each contains the rudiments of a single flower. A pocket
magnifying glass is of considerable help in this examination.

The following chart presents the above facts in a somewhat more
concise manner:

Positions of Fruit-Buds of Deciduous Fruits

Spurs (Short Shoots)

T, a,
o o

o

'c?

Terminal
Apple
Pear

T, a,
o o

o

Spurs and Long Shoots
Lateral
Almond
Apricot
Sour Cherry-
Japanese Plum
Currant
Gooseberry

Long Shoots
Lateral
Peach
Olive
Quince
Grape
Fig

Loganberry
Raspberry

Terminal
Walnut

Lateral

Sweet Cherry
European Plum
and Prune

Lateral

Peach (spurs
short-lived)

Lateral

Sweet Cherry
European Plum

and Prune
Quince
Walnut

Lateral and Terminal
Apple
Pear

10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

THE SOURCE OF MATERIALS FOR TREE AND CROP

In order to comprehend the responses of the plant to pruning,
certain facts should be known concerning the annual cycle of growth
and development of the tree. The first growth of the season (blossoms,
leaves, and new shoots) is made largely at the expense of plant food
reserves normally stored in the tree during the preceding summer and
fall. These stored foods are thus at least partially used up during the
early part of the summer. The soil solution containing mineral
nutrients taken up by the roots is transported to the leaves where, with
other substances built from the carbon dioxide of the air, the complex
plant foods are elaborated. These foods are then carried to other parts
of the tree for use or storage. After the active vegetative period of
early summer is past, the plant begins to store such elaborated foods
as are not used in the maturing of the current fruit crop, development
of leaves, the extension of the top and roots, and the development of
fruit-buds for the succeeding year's crop. Under favorable conditions,
the storage of reserves for the winter months and for the growth of the
following year becomes greater and greater as the season advances, at
least until near the time of leaf fall.

A fruit tree with the crops it produces represent an accumulation
of materials drawn from the soil by the roots and from the air by the
leaves. When the materials from either source can no longer be
obtained, it ceases to exist as a living thing. Since this is so, any plan
of developing and managing an orchard from the time it is planted to
the time it is no longer profitable, must take into consideration the
treatment of the soil, and the treatment of the top, especially pruning.
Trees develop or produce very differently in different locations, and
respond rather readily to cultural practices. This being true, it is
necessary to know what influence a change in the quantities and kinds
of material available in the soil may have on the appearance and
behavior of the top, especially in relation to materials already in the
top, or to those being brought in by the leaves. Or from the other
point of view — just what influence pruning, spraying, insect infesta-
tion, or any other factor may exert in changing the material income
of the top, and its relation to the materials derived from the soil, and
the subsequent development of the top ? Only by such a consideration
of orchard problems can they finally be worked out with certainty.
The full so" ill 'on of this problem will require time and labor, but some
progress has b^en made, and it seems possible to give the orchardist a
working insight into the problem at the present time, and to offer
suggestions, as to the character of pruning and of the supplementary

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 11

soil treatments that should be applied under varying conditions of
growth and bearing. There is much observational evidence and no
small amount of direct experimentation to show that plants respond
very differently in growth and fruit production, according to the
amounts of nitrogen available, the amounts of carbohydrates that can
be manufactured, and the manner in which these are combined within
the plant. Very great differences likewise may be brought about by
variations in temperature and the length of the light period to which
plants are exposed. Plants growing out-of-doors, supplied with suffi-
cient moisture and subjected to the usual range of conditions will vary
in development and productivity according to the amounts of nitro-
genous materials available in the soil and to the amounts of available
carbohydrates manufactured by the leaves or stored in the plants.

It is possible to distinguish four general classes or states of vegeta-
tiveness and fruitfulness of the plant in this respect.

1. Mineral nutrients including nitrates abundant but no available
carbohydrate supply. Result : vegetativeness weak and plants non-
fruitful.

2. Mineral nutrients including nitrates abundant and a carbo-
hydrate supply available. Result : vegetativeness strong but little or
no fruit.

3. Mineral nutrients including nitrates relatively less than in class
2. This reduces vegetativeness and leads to an accumulation of carbo-
hydrates. Result : fruitfulness.

4. Mineral nutrients including nitrites even relatively less than in
class 3. Result : this leads to a marked accumulation of carbohydrates
and a suppression of both vegetativeness and fruitfulness.2

Naturally these classes grade into one another, but it is possible to
place trees in one or another of them with a considerable degree of
accuracy, by observing the appearance and performance of the trees
and knowing the general cultural conditions which have been employed.
For the fruit grower, it is obviously impracticable to attempt an actual
analysis of the trees in an orchard to determine their chemical content
and therefore the treatment as to pruning, irrigation, fertilization,
and other cultural practices, which should be given. But, as indicated
above, the crops produced and the new wood growth made, generally
furnish an excellent basis for judgment and a fairly accurate decision
as to what is happening in the plant. The author has attempted to
cite below some points to be kept in mind in connection with the fore-
going suggestions and instances in which fruit trees apparently fall
in one or another of the above outlined classes.

2 Kraus and Kraybill. Vegetation and reproduction with special reference
to the tomato. Ore. Exp. Sta. Bull. 149: 1-90, tigs. 1-22. 1918.

12 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

It has been demonstrated many times that heavy cutting, whether
done on young trees or old, generally results in rank vegetative growth,
and, if the trees are of bearing age, in reduction of f ruitfulness. How-
ever, if in addition to the pruning, the trees are heavily fertilized and
irrigated, the new growth will be still ranker and more succulent, and
little or no fruit will be produced. On the other hand, no pruning,
insufficient soil moisture and few available mineral nutrients will
result in scanty new wood growth and, in bearing trees, a tendency
towards over-production. If this latter treatment is continued trees
soon reach a condition where there is neither wood nor fruit produced.
Between these extremes may be found all gradations of vegetativeness
and reproductiveness, depending upon the environment under which
the plants are growing.

Fruit growers will readily call to mind orchards exhibiting varying
degrees of vegetative vigor and fruitfulness. Unfortunately, many
attempts in the past have been made to influence the wood growth and
the productiveness of the tree by pruning alone. This is a mistake,
since pruning is only one of the factors which modify plant growth
and productiveness.

Cultivation, irrigation, and application of fertilizers are some of
the other factors which must be considered along with pruning in
attempting to regulate the vegetative growth and fruitfulness of an
orchard.

Trees just planted have relatively little carbohydrate, but enough
to start them into growth. As they develop, leaves are formed which
manufacture a new supply of carbohydrates; and as the leaf area
becomes greater this supply is increased, the roots are extended further
into the soil, and the tree becomes vigorously vegetative, that is, it
belongs to class 2.

If, however, such young trees, or even older ones, are severely
summer pruned so that the leaf area is seriously reduced, the food
stored in the branches will be consequently removed, and the tree as a
whole will fail to make as much growth as one less severely pruned,
other conditions being the same. This would represent a class 1
condition.

On the other hand, if the young vigorous trees are not so severely
cut back, the consequent larger leaf area will permit of a greater
manufacture of carbohydrates, and provided the nitrogen supply is
not markedly increased, there will be a tendency toward a decrease of
vegetative activities, an increase in carbohydrate accumulation and
the trees will become fruitful. Such plants are in class 3. It is the
ultimate aim of every commercial orchardist, to maintain his trees in

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 13

this class and his main efforts in the way of cultivation, irrigation,
pruning, and other cultural practices should be performed with the
idea of securing a proper balance between vegetative growth and
fruitfulness. The production of a large number of fruits is not
profitable, unless it can be continued and for this a constant supply
of new growth must be maintained as well. By knowing some of the
materials concerned in the maintenance of this condition, and some of
the means for their regulation, the fruit grower has a direct and fairly
accurate method for securing the type of tree he desires.

The conditions of class 4 are commonly observed in old prune and
pear orchards in California, which make little or no new growth and,
although there is present a great number of fruit-spurs, little or no
fruit is produced. In these cases the difficulty is frequently one of
lack of nitrogen, because there is an accumulation of large quantities
of carbohydrates stored in the top. Such trees as these when nitrogen
and moisture are supplied, or if there is available nitrogen already in
the soil, when moisture alone is applied, frequently are unusually
productive for a year or two. Likewise, a pruning which removes
much of the old wood and permits a relatively greater nitrogen supply
to the branches and buds remaining results in a greatly increased
vegetativeness of the remaining portions, often accompanied by abun-
dant fruit production as well. But to attempt to regulate such trees
by pruning alone would be futile, because to balance the top with the
available nitrogen or moisture, would mean reducing the top to a size
where it could hardly produce a commercial crop. It would be far
more profitable to give the proper attention to soil requirements, as
well as to a more rational pruning method.

CORRELATION BETWEEN PRUNING AND OTHER CULTURAL

PRACTICES

The above considerations emphasize the fact that pruning alone
cannot be relied upon to regulate the growth and productivity of the
orchard. It is doubtless one of the most important factors in deter-
mining the balance between carbohydrate and nitrogen supply, but a
thorough understanding of the principles involved must be obtained
before a rational pruning system can be adopted for any particular
set of conditions.

There may be an adequate supply of available nitrogen, but if soil
moisture is reduced to such an extent that the tree cannot utilize this
food material, then the pruning must be such that a proper balance
will be maintained. Pruning under such circumstances should prob-

14

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

ably be considerably heavier in order to reduce the amount of water
lost through evaporation, and to maintain a relatively higher propor-
tion of nitrogen to the carbohydrates present or being manufactured,
than if there were available an adequate supply of soil moisture and
nitrogen.

In the same way, cultivation, irrigation, and fertilizers affect the
pruning practice by limiting or increasing the amounts of soil moisture
and available minerals, especially nitrogenous compounds.

NEW WOOD NECESSARY FOR FUTURE FRUITFULNESS

As previously suggested, the amount and character of the new wood
produced during any season may be taken as an index of the condi-
tions existing within the tree. The amount of new vegetation neces-
sary to maintain the correct balance apparently differs with the species
in question. In general, it may be said that those fruits, such as the
peach, which bear the bulk of their crop on one-year wood, associate
fruitfulness and a relatively greater amount of new growth than seems
necessary with spur producers. In the case of spur producers, those
species which have the longest lived spurs apparently need the least
amount of new wood. This seems reasonable, since at least a portion
of such new growth must be. used for replacement of fruit-wood which
has outlived its usefulness. The amount of new growth necessary for
continued fruitfulness may vary from a few inches, in the case of
such fruits as the apple and pear, to from two to four feet in the case
of the peach.

TABLE 1

Desirable Amounts of New Growth for Bearing Trees

Fruits

Peaches

Apricots

Plums (excepting prunes) and Quinces

Almonds, prunes, apples, pears, cherries

Under 10 years of age

Over 10 years of age

20 to 40 inches

12 to 30 inches

10 to 24 inches

9 to 18 inches

12 to 30 inches

10 to 24 inches

9 to 18 inches

6 to 10 inches

BULK VERSUS FINE PRUNING

The question may well be asked whether or not pruning should
consist in a few large or many small cuts distributed well over the
tree. Horticulturists are generally of the opinion that the immediate
influence of a pruning cut does not extend far from the place it is
made and that, therefore, it is desirable to distribute the pruning

Bulletin 386] pruning BEARING DECIDUOUS FRUIT TREES 15

over as large a portion of the tree as is economically possible. (Of
course the total amount of cutting done does eventually produce an
influence throughout the tree as a whole.) Whether the cuts should
average one-fourth inch, one-half, or one inch in diameter, very largely
depends upon the species under consideration. In the case of the
peach, where the bulk of the fruit wood must be renewed annually,
many cuts must be made in order to secure a large number of new
shoots well distributed. With those species producing on spurs, how-
ever, a comparatively few cuts may often suffice.

Bulk pruning by the removal of relatively large limbs commonly
gives rise to numerous water-sprouts. These may at times be utilized
in rebuilding the framework of the tree and a few of them, under
favorable conditions, may be converted into a part of the fruit-spur
system. Generally speaking, however, these growths are of little value
and should be removed entirely as soon as they appear.

SPUR PRUNING

Spur pruning is essentially "fine" pruning and is often utilized
in the rejuvenation of orchards which, for one reason or another, have
fallen into class 4 (page 11). It consists in the removal of whole
spurs or of portions of spurs, and tends to stimulate vegetative growth.
On account of the labor cost involved and the fact that most trees are
very "brushy," comparable results are usually secured by a somewhat
"coarser" method of cutting.

TRANSITION FROM NON-BEARING TO BEARING

The lighter the pruning of young trees, consistent with the securing
of a proper framework of scaffold branches, the greater will be their
development and the sooner profitable crops will be produced. The
more severely young trees are pruned, the longer they remain unfruit-
ful. Continued heavy cutting not only reduces the chances for a
fruit crop, especially with spur producers, and very materially reduces
wood development (fig. 5 and fig. 6), but it also tends to move such
trees from class 2 into class 1.

It is a matter of common experience that with fruit trees which
are just beginning good production, it is possible by injudicious heavy
cutting during one season so to disturb the delicate balance between
wood growth and fruiting that profitable cropping is delayed as much
as three or four years. The best judgment must always be exercised
when determining the pruning treatment to be given any particular
orchard; this is especially true when the trees are just approaching
the bearing age.

16

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

As the trees become older and bearing is fully established, many
instances will doubtless arise in which heavier pruning will be neces-
sary in order to continue regular bearing and to maintain the necessary
wood growth at the same time. To this end supplementary fertilization
may be advisable or even necessary.

Fig. 5. — Five-year-old Nonpareil almond tree after pruning. This tree has
been lightly pruned each year. Compare figure 6 of an adjoining tree which
has been severely pruned annually. Note relative sizes of trees and branches.
Photographed at same distance as tree shown in fig. 6. With the exception of
the pruning, these trees have received identical treatment. This tree is a
representative specimen of a large number of trees similarly treated.

PEOMOTION OF FEUITFULNESS

Distribution of Fruiting Wood. — Pruning is of value in distribut-
ing fruiting wood uniformly jthroughout the tree, thus securing greater
tonnage of better quality. By this means, the trees are kept within
bounds and the costs of maintenance and harvesting are reduced.

Fruit-Bud Formation. — Sufficient light in all fruit producing parts
of the tree is essential in order to secure a uniform distribution of

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 17

fruit-buds. Pruning is practically the only means of securing favor-
able light distribution. (See also page 18.)

Although fruit-buds for any crop begin their development the
previous summer (June to August, under practically all California
conditions), nevertheless, the pruning of bearing trees at that time is
of doubtful value as a means of securing greater fruitfulness. This
point will be discussed later in more detail (page 18).

Fig. 6. — Five-year-old Nonpareil almond tree after pruning. This tree has
been heavily pruned each year. Compare fig. 5 of an adjoining tree lightly
pruned. Note relative sizes of trees and branches. Photographed at same
distance as tree shown in fig. 5. This tree is a representative specimen of a
large number of trees similarly treated.

Regular Pruning. — Optimum results are obtained only when the
pruning treatment is consistently and regularly carried out over a
period of years. It is possible with some species to allow the trees to
remain unpruned for several years, but the net returns from , such
treatment are smaller than those secured from annual cutting which
insures a constant renewal of fruiting wood. Pruning should be part
of each season's operations even though the amount of wood removed
in some instances may be comparatively small.

New Wood and Size of Fruit. — There is apparently a direct cor-
relation between new vegetative growth and size of fruit. Trees which
make a satisfactory wood growth usually bring the fruit crop through
with larger sizes than do those which are unsatisfactory in this respect.
It is the common experience of fruit growers that relatively young,
vigorously growing trees nearly always produce fruit of large size.

18 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

EEGULATION OF SIZE AND QUALITY OF THE FEUIT CEOP

Fruit trees normally tend to produce more fruit than can be suc-
cessfully carried through to maturity, hence, it has become an estab-
lished orchard practice to reduce this burden somewhat at the annual
dormant pruning. On account of certain fruiting habits, danger from
spring frosts, liability to the "June drop," and other factors which
may decrease the final crop, it is not feasible to attempt to do all the
thinning of fruit with the pruning shears. In order to reduce by any
pruning system the number of fruits set so that there will exist no
necessity for hand-thinning, it is necessary to limit the crop pro-
duced to less than that which the trees can successfully mature.
Such severe pruning as this requires, will moreover be inimical to
future productivity.

Color of fruit is one of those characteristics difficult to define, which
with size, finish, and flavor, constitute "quality." Light is of prime
importance in securing color, but if the tree is rightly "opened up"
by pruning, little difficulty will be experienced in this connection. In
attempting to secure good color by allowing light to penetrate to the
interior of the tree, care must be exercised that the scaffold branches
are not exposed to the direct rays of the sun, as this would result in
sunburn and in subsequent decay of the wood. Likewise, the new
growth must be sufficient to shade the fruit and protect it from
sunburn, especially in the dry, hot interior valleys.

WHEN TO PRUNE

The old axioms of "prune when the knife is sharp," and "summer
prune for fruit and winter prune for wood," are an unsafe guide
and the fruit grower should have a more reasonable basis for his
pruning practice.

The general statement may be made that for all deciduous orchards
in California, pruning gives the best results when done during the
dormant season, after leaf-fall and before new growth starts the
following spring.

It has frequently been argued that to increase fruitfulness, trees
should be pruned in the summer just before fruit-bud formation in
order that the buds which remain may have available all the nourish-
ment which otherwise would be more or less evenly distributed over
the entire top.

Such a line of reasoning is fallacious because the leaf area is just
as important in the nutrition of fruit-buds as is the root system, and

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES

19

to remove a large portion of this manufacturing area just as the tree
is beginning to store food (page 10) robs the plant to that extent, and
at the same time may start new growth which will also take additional
food materials.

In order to determine the influence of summer pruning at the
time of active fruit-bud development, forty-eight Muir peach trees —
three rows of sixteen trees each — planted in February, 1912, and rather
severely thinned and headed in February, 1913, 1914, 1915, and 1916,
were pruned as follows between August, 1916, and August, 1920 :

Row 1. — Thinned and headed moderately in February of each
year — dormant season. (No summer pruning.)

Row 2. — Thinned moderately in early August each summer.
Headed moderately in February each winter, thus rendering trees in
this row similar in height to trees in rows 1 and 3. (Moderate summer
pruning.)

Row 3. — Thinned and headed moderately in early August each
summer and when necessary the older wood thinned out in winter.
(Heavy summer pruning.)

Considering the average yield to the tree of the sixteen trees in
row 1 as 100, the production for row 2 and row 3 was that indicated
in Table 2.3

TABLE 2
Average Yield of Muir Peach Trees With and Without Summer Pruning

Year

Rowl
Winter Thinned
Winter Headed

Row 2

Summer Thinned

Winter Headed

Row 3
Summer Thinned
Summer Headed

1917

100— ( 51 lbs.)
100— ( 43 lbs.)
100— (203 lbs.)
100— (116 lbs.)

133— ( 78 lbs.)
158— ( 68 lbs.)
100— (204 lbs.)
115— (134 lbs.)

98— ( 50 lbs.)

1918

128— ( 55 lbs.)

1919

67— (136 lbs.)

1920 ,

90— (104 lbs.)

Total

100— (413 lbs.)

117— (484 lbs.)

83— (345 lbs.)

During the season of 1919 the crop from row 2 did not appreciably
exceed that from the trees receiving only a moderate winter pruning.
The increased yields in row 2 during the other years were due perhaps
to admission of sunlight to the interior and lower portions of the

3 The difference recorded in Tables 2 and 3 are perhaps not statistically
significant, but since the tendencies shown are exactly in accord with the results
of other experiments and with observations extended over many years, it seems
desirable to include these data. The reader should realize, however, that any
figures presented in connection with experiments with plants merely indicate
general tendencies rather than exact responses because of the great variability
within the plants themselves and of the conditions under which they grow.

20

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

summer pruned trees with consequently more economical manufacture
and storage of carbohydrates throughout a larger portion of the fruit-
ing area. Row 3 was materially weakened by the rather heavy summer
pruning.

Development of Trees. — That summer pruning of the Muir peach
as practiced in these experiments is devitalizing is shown not only by
the trees themselves, but also by measurements of the trunks, which
seem a fair indication of both root and top development.

It has been shown4 that trunk circumference measurements may
be confidently depended upon as a reliable index of the total develop-
ment of the young tree, both root and top. It will be seen from the
figures presented below that summer pruning may lessen the chance
for future crops by limiting tree growth.

TABLE 3

Average Trunk Circumference of Trees

Year

Rowl
Winter Thinned
Winter Headed

Row 2

Summer Thinned

Winter Headed

Row 3
Summer Thinned
Summer Headed

Nov. 1919

50.8 cm.
54.5 cm.
56.7 cm.

50.1 cm.

51.2 cm.
53.8 cm.

43.9 cm.

Nov. 1920

46.5 cm.

Nov. 1921

49.3 cm.

PEUNING SYSTEMS

In recent years there has been much discussion in California regard-
ing the relative merits of the so-called "long" and "short" pruning
of deciduous fruit trees. It is rather difficult to draw a definite line
of demarcation between these two systems because with their modi-
fications they merge into each other.

Briefly, it may be said that the system of "short" pruning consists
essentially in a more or less severe thinning out, accompanied by a
heavy heading of the new growth ; while the system of "long" pruning
consists essentially in a moderate thinning out and in keeping the tree
within bounds by cutting to laterals which are left untouched. The
latter system removes somewhat less wood, but the conclusion of many
growers that it really amounts to little or no pruning is incorrect.

There has been a tendency throughout the state towards less severe
pruning so that it is now becoming increasingly difficult to distinguish

4 Tufts, W. P. Influence of heavy and light pruning upon the growth of
deciduous fruit trees. California State Dept. Agr. Mo. Bull. 8: 529-535,
figs. 139-140. 1919.

Bulletin 386] pruning bearing deciduous fruit trees 21

between orchards, which are "short" and those which are "long"
pruned. Thoughtful and observing orchardists are beginning to
realize that the same principles of plant growth and response apply
with equal force to all systems of pruning and as their idea of the
relation of vegetative growth, tonnage, quality, and economy of
orchard management become clearer they will realize that the different
systems at present in use merge into each other, and that the best
system for any particular case will lie at some point between the
extremes of "short" and "long" pruning.

Fig. 7. — Winters System .of apricot pruning. Note low spreading top and
severity with which new shoot growth has been headed.

Mention should be made of several modifications of the so-called
short and long systems of pruning at present in vogue in certain
sections of California. Undoubtedly many growers who are following
in all essential details one of these methods may differ as to the
classification here made.

Figure 7 shows an apricot tree trained by the so-called "Winters
System" (sometimes aptly referred to locally as the "Wolf skill
Hack"). This flattened expanse of the fruiting area is designed to
give better exposure to the sun with the object of ripening the fruit
earlier. It does not bring about the result desired, however, because
the heavy cutting gives rise to succulent vegetative growth early in
the season, the shade from which more than offsets the advantage of

22

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

a flat head. In a test5 conducted in the Winters district, in 1919, trees
pruned by thinning produced larger and earlier crops than comparable
trees pruned by the standard "Winters System." This method of
training is also objectionable in that the upper sides of the main
branches are very often badly sunburned which results in wood decay,
even though the vegetative growth is sufficient to shade the fruit and
retard its development. Moreover, restricting the height of the tree

Fig. 8. — Mature Blenheim apricot tree pruned by the so-called ' ' short ' '
system which consists essentially in a severe thinning out and heading back of
new wood growth. Note lack of fruit-spurs in lower parts of tree due to
shading out. Too many main scaffold branches.

so that all work is done from a six-foot ladder materially reduces the
fruiting area and the possible tonnage of fruit of high quality.

Figures 8 and 9 show another system by which deciduous tree
fruits are pruned in California. This method is very generally applied
and consists essentially in a heavy thinning, accompanied by the
removal of all but three or four buds on the shoots which are headed —
an example of the so-called "short pruning.''

s Tufts, W. P. Why prune bearing apricot trees heavily? California State
Dept. of Agriculture Mo. Bull. 8: 15-22, figs. 10-17. 1919.

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES

23

Certain growers have worked out for their own conditions quite
satisfactory systems of training' and pruning which are distinct depar-
tures from the ordinary methods.

Fig. 9. — Fifteen-year-old Bartlett pear tree typically " short" pruned. Note
severe thinning and heading back. Only three or four buds have been left
on each one-year shoot

Figure 10 shows a mature peach tree pruned by the "Sims
System. ' ' The relatively few, upright main scaffold branches clothed
with long, slender fruiting twigs are characteristic of this system.

24

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

The few upright scaffolds are able to hold a heavy load of fruit with
the minimum of bracing. The rather narrow spread of trees thus
pruned is perhaps an unwarranted restriction of the fruiting area.

One of the most interesting departures from standard pruning
practice is the so-called "Caldewell System" of pear pruning (fig. 11).

Fig. 10. — Mature peach tree pruned by the "Sims' System." Note heavy
framework branches. Fruit is produced on long pendant "hangers. " By this
unusual method much fruit is produced on spurs. Trees after reaching a certain
height (12 to 16 feet) are never allowed to grow taller.

Briefly stated, this system consists in the tying down of the upright
one-year-old shoots at an angle somewhat below the horizontal. The
resultant new growth arises just below and behind the highest point
of the bend. Practically all growth beyond this point is reproductive
in character and rapidly develops a good fruit-spur system. In prin-

Bulletin 386] pruning bearing deciduous fruit trees

25

ciple the Caldwell plan is very similar to the espalier method of train-
ing used in Europe. In both cases the bending of the branches seems
to induce fruitfulness.

At each dormant season all the new shoots are tied down in such
a way that there is as little shading as possible of the lower branches.
The later tying is done to any convenient point on either trunk or
branch. At ten years of age few pear orchards attain the size and
productiveness of Mr. Caldwell's, whose trees, with the exception of
the first winter, have not been touched with the pruning shears.

\ \
\ \

Fig. 11. — Caldwell System of pear pruning. Six-year-old Bartlett pear tree
which has been trained by tying down new shoots each year. Note new
branches which arise at bend, these will in turn be tied down to a position a
little below the horizontal.

Briefly, the advantages claimed for the method are : large trees of
good mechanical strength, combined with a large fruiting area close
to the ground, are secured in a comparatively short period of years;
they come into bearing from two to four years earlier than trees which
are trained by severe cutting back; they can be trained to a more
shapely form in a windy section or where prevailing winds hinder
symmetrical development; and finally, they produce larger crops of
equal quality.

The chief disadvantage is that this system to be successful must be
part of a comprehensive and intensive orcharding programme. Since

26

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

all wood is conserved, special care must be exercised to see that ade-
quate moisture and fertility are present in order to secure an abun-
dance of new wood and at the same time carry large crops through to
maturity and to supply the enormous leaf area which is being increased
from year to year. In most soils some renewal pruning will un-
doubtly be necessary as soon as the trees begin bearing heavily in order

Fig. 12. — A well made pruning cut. Note callus formation which in the
course of a few years will completely cover the large wound. Compare size of
cut with the quarter-dollar.

to replace the fruiting wood which has outlived its usefulness and to
maintain a proper condition of vegetative vigor. Success depends
very largely upon the skill and judgment of the individual. The
method is probably applicable only to pears and should not be
adopted without a full understanding of its requirements and
limitations.

Bulletin 38G] pruning bearing deciduous fruit trees 27

In adopting any pruning system, the first and most important
question to be asked is : " Will this method insure the annual produc-
tion of a good tonnage of fruit of high quality?" The growth of
new wood is probably the only index by which the grower can judge
whether or not future yields are being sacrificed for the sake of one
or two heavy crops. On the other hand, the production of too great
an amount of vegetative growth will reduce the yields far below the
maximum which can profitably be maintained.

Fig. 13. — Improperly made pruning cuts. Such stubs will never heal over
and are a constant menace as affording an entrance to wood rotting organisms.

PROTECTION OF PRUNING WOUNDS

If a tree is properly pruned from the start, the removal of large
limbs can for the most part be avoided.

All cuts should be made close to the branch from which the limb
is taken (fig. 12), leaving no stub (fig. 13). This will greatly facilitate
the healing of the wounds.

The treatment of pruning wounds depends largely upon local
conditions. In any section, like the Watsonville apple district, where
the foggy climate favors the germination and growth of the fungi
which cause wood decay, it is best to disinfect the pruning cuts as

28 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

soon as the wood has dried a little, but before any cracks have formed.
Cyanide of mercury, one part to 1000 parts of water by weight, is a
good disinfectant. The solution should not be put into a metal con-
tainer. Containers made of wood, glass, or enamelware should be
used. Cyanide of mercury is a stomach poison and must be handled
with care.

In localities where the air is relatively dry, it will ordinarily be
unnecessary to disinfect pruning wounds, but in any case large prun-
ing wounds should be covered with some protective substance. This
is not to aid the healing, but to prevent the entrance of rot-causing
fungi. All wounds an inch and a half, or more, in diameter should
be covered.

The following are three of the best materials for covering wounds :

White lead paint mixed with raw linseed oil.

Asphaltum, Grade D, which must be heated before applying.

Oronite, which is similar to asphaltum, but which can be applied
cold. When too thick, a special non-injurious "oronite thinner" can
be added.

PEACTICAL SUGGESTIONS

1. The total amount of new growth which the tree makes, taking
into consideration the crop produced and the conditions under which
it is grown, is the best criterion by which to judge whether or not the
pruning has been of the proper severity (page 14).

(a) If the resultant new growth is longer than seems desirable for
a tree of the kind and conditions under which it is grown, then the
previous pruning was too severe (page 12).

(&) If the new growth has been inadequate the cutting was too
light and a heavier pruning programme should be adopted, except in
those cases where sparse growth has resulted from non-availability of
carbohydrates.

2. The pruning should include adequate thinning out of the fruit-
ing shoots, and this should be followed, where necessary, by a thorough
thinning of the fruit (page 18).

3. The tree should be thinned out sufficiently to admit an adequate
light supply (page 18).

4. All interfering branches, dead wood, and diseased parts should
be removed.

5. The larger limbs should be spaced far enough apart throughout
their length so that they will have ample room to develop the desired
fruiting branches and spurs.

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 29

6. The trees may be kept from becoming too high by regularly
cutting back the tallest branches to strong outward growing laterals.

7. All branches should be cut off close to the limb from which they
arise, leaving no stubs (page 27).

8. It should be emphasized at this point that it is impossible to
secure an ideal shape, height, and distribution of fruiting wood unless
the trees are spaced far enough apart when planted to enable them
to develop normally. There has been a decided tendency among
California fruit growers to plant their trees too close together for
normal development.

TEEATMENT FOR THE DIFFERENT FRUITS

Almonds. — The almond produces its fruit largely on short spurs,
although some nuts are found on one-year wood, especially in certain
varieties such as the Jordan and California. The fruiting branch of
the almond remains economically productive for about five years.

For the sake of economy, the pruning of bearing almonds is usually
restricted to the removal of limbs one-half inch to an inch and a half
in diameter. Little or no time is spent in thinning out one-year shoots,
except for the removal of water-sprouts which are frequently abundant.

This practice is probably satisfactory when the pruning is done
annually and conditions are such that it results in sufficient new
growth to cause a continual renewal of the fruit-spurs. Approxi-
mately one-fifth of the fruiting wood should be replaced each season.
Trees under ten or twelve years of age can be expected to make from
nine to eighteen inches of new growth over the tree as a whole, while
older trees should make from six to ten inches. Many old almond
orchards do not make six inches of new growth each year, but it is
very probable that the average production would be increased if the
pruning were sufficiently heavy to induce this amount. The cutting
of trees lacking in vigor should consist of a rather heavy pruning of
the type mentioned above together with some cutting back of the tops
to large lateral limbs and some thinning of the smaller shoots and
fruiting branches.

It may be said that in general the almond tree does not need to
be thinned out quite so much as other trees because of the fact that its
foliage is rather sparse, and hence light readily filters through the
branches. Furthermore, the almond is seldom, if ever, pruned with
the idea in mind of preventing overbearing, since it does not produce
a crop heavy enough to break any of the limbs and the yield of the tree
is of more importance than the size of the nuts.

30 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

On account of the fact that almonds are not picked by hand it is
not essential to keep the tree as low as with most fruits. It is well,
however, to keep them low enough so that all the nuts can be reached
from the ground with a pole not longer than sixteen feet.

On very rich soils with a plentiful supply of moisture, the trees
may make an excessive wood growth at the age when they should be
coming into bearing. Since pruning, except under conditions where
carbohydrates are limiting, is one of the factors which tend to stimu-
late vegetative growth, the rational procedure in such a case seems to
be to stop pruning for a year or two. This will tend to throw the trees
into bearing and curb the heavy growth. After the bearing habit is
once thoroughly established, pruning may be resumed.

Apples. — Most varieties of apples generally produce fruits termin-
ally on spurs which are found on wood that is two years old or older.
The life of an individual apple spur is frequently as much as fifteen
or twenty years, but in most orchards the period over which a spur
bears sufficiently often to be an important factor in production is not
over eight or ten years. Frequently these terminal producing spurs
fruit in alternate years. Unfavorable weather conditions may prevent
most of the spurs from bearing in a particular year, and as a conse-
quence, a very heavy crop will be produced the next. These trees will
very likely then be thrown into the habit of alternate bearing. There
are, however, certain notorious alternate bearing varieties such as the
Oldenburg, Baldwin and Wealthy, which may firmly establish the
alternating habit regardless of weather conditions. In California it
very often happens that a spur elongates and sets a terminal fruit-bud
during the same summer that it is maturing fruit, in which case the
tendency towards alternate bearing is broken. Possibly one of the
best methods of circumventing the alternate bearing habit is to keep
the trees in such a state of vegetative vigor that only half or less than
half of their spurs bloom in any one year.

While the major part of the apple crop is commonly borne on spurs,
a smaller portion is produced on one-year wood. This is particularly
true of young trees of certain varieties, such as the Gravenstein, Ben
Davis, Jonathan and Winesap. Varieties differ as to the part of the
shoot on which the fruit is developed. Normally, the fruit-buds are
borne either terminally or laterally near the apex. Varieties that
produce an abundance of fruit-buds on the one-year wood usually
come into bearing at an earlier age and are more likely to bear annually
than those that fruit entirely on spurs.

A thinning out of the branches to admit sufficient light to penetrate
to all parts of the tree is a prime consideration in causing new spurs
to develop and in aiding those that already exist to bear regularly.

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 31

Many varieties of apples tend to over-bear, especially in the
alternate years, and as a result the fruit requires a great deal of hand-
thinning. The type of pruning that will encourage annual bearing
and reduce the amount of hand-thinning that is necessary is that which
tends to reduce the crop somewhat and to invigorate the trees in the
alternate years when they have a heavy set of fruit-buds. This may
be done not only by thinning the shoots and branches, but also by
thinning out the spurs, and renewing the long, much branched ones by
removing the older parts of the spurs and saving the newer, i.e., by
"spur pruning.'7 The pruning particularly in reference to the fruit-
buds and spurs, should be much lighter the following year, when the
tree has comparatively few fruit-buds.

In general, the apple does not require heavy pruning. On account
of its habit of bearing largely on long-lived spurs, approximately only
10 per cent of which must be replaced each year, it requires somewhat
less new wood growth than some other fruits. The young bearing
apple tree should ordinarily make from ten to twenty inches of new
wood growth each year, while with the older trees six to ten inches is
sufficient. This amount of new wood growth can ordinarily be secured
by cutting back to laterals, leaving a few of the one-year-old "shoots
intact. However, if the wood growth shows a tendency to drop below
the above mentioned minimum amounts, heading back the laterals
which are left after thinning may be necessary to secure the proper
vigor.

A thorough thinning out of the tree is of special importance in the
case of varieties having red fruit, as highly colored apples are the
result of abundant light supply throughout the tree. While it has
been stated that in general summer pruning does not seem desirable
for bearing trees, it is possible that a light summer thinning may be
beneficial in securing a better color of the fruit on trees of red varieties,
having foliage which is rather thick and dense.

The apple powdery mildew (Sphaerotkeca leucotricha (Ell. & Ev.)
Salm.) which has done considerable damage to the apple orchards in
California, particularly in the humid coastal regions, can be largely
controlled by proper pruning in conjunction with spraying. The
pruning treatment in this respect consists in removing the diseased
shoots and spurs at the regular dormant pruning, and thus eliminating
the sources of hold-over infection. Mildewed shoots are easily recog-
nized by their gray or silvery appearance.

The undesirability of removing large limbs has already been
pointed out (page 27), but it must be emphasized again that it is
especially dangerous to cut off main branches in damp coastal dis-

32

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

tricts, such as the Pajaro Valley. When limbs more than an inch and
a half in diameter are removed it is very probable that a serious wood-
and-bark disease, known locally as "sappy bark," will develop. The
serious nature of the malady will be apparent from the fact that it
often travels down the limb from a large wound and eventually reaches
the trunk and kills the tree. Disinfecting and painting the wounds
has not as yet proved effective in preventing the disease.

Apricots. — The apricot produces the bulk of its fruit laterally on
spurs which generally do not live more than three years. A relatively
small portion of the crop is produced laterally on one-year shoots.

Since the fruit-spur system of the apricot is short-lived, the pruning
treatment must be such as to cause its continual renewal. Young
bearing apricot trees should make from fifteen to thirty inches of new
growth each year. On older trees the desirable length of new growth
ranges from ten to twenty-four inches. The peach and Japanese
plums are probably the only fruits that ordinarily require a heavier
pruning than the apricot in order to produce sufficient new growth
for continued profitable production.

TABLE 4
Yield of Six-Year-Old Eoyal Apricots, 1922

Pruning treatment

Average yield per tree

Average size of fruit

Thinned and headed severely

23.9 lbs.
200.6 lbs.

8.3 fruits per lb.
13.4 fruits per IK

Thinned out and cut to laterals

Particular pains should be taken not to allow the branches to
become too long, willowy and unbranched. This condition can best
be avoided by cutting back to laterals. As the tree becomes older and
only a short growth follows a thinning system of pruning, it is quite
possible that a more severe type of cutting will have to be used to
maintain the vigor of the tree.

The apricot is one of the fruits which has in the past in California
been very commonly subjected to heavy pruning regardless of the age
and vigor of the trees. The yield record of a six-year-old Royal apricot
orchard given in Table 4 is interesting in this connection. The orchard
was planted at the University Farm at Davis in February, 1916. It
is growing on a sandy loam of the Yolo series and all trees have
received the same good, uniform, commercial culture except as regards
pruning treatment. In each case the figures are the average of sixteen
trees in a single block receiving the same pruning treatment.

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES

33

These figures bring out very strikingly the influence of heavy and
light pruning on young bearing apricot trees.

As the tree becomes older, the continuance of a comparatively light
system of pruning, especially in the absence of adequate soil moisture,
will usually result in a slowing down of wood growth and a consequent
falling off in yield.

This fact is rather clearly demonstrated in Table 5, which shows
the yield records over a series of years for a block of fifteen Tilton
apricot trees, five trees to a treatment.6 The trees in question were
all planted in the University orchards at Davis in March, 1908, and
were thinned out and headed severely from 1909-1915 inclusive, the
pruning experiment starting the next year. With the exception of the
pruning, these trees have received identical cultural treatment.

TABLE 5
Average Yield of Tilton Apricot in Pounds per Tree

Pruning treatment

Thinned and headed se

verely

Thinned and headed light

ly

Thinned out only

1916

1917

1918

1919

1920

1922

1923

23

59

50

88

30

53

58

20

119

107

169

252

241

74

77

171

146

137

172

214

107

Average

52

140
146

It will be observed that the lighter pruning resulted during the
very first year in a marked increase in yield over that obtained from
the trees headed back. The greater yield of the thinned trees was due
to the production of fruit on the one-year wood, which was removed
from the trees which received a heavy or light heading. The com-
paratively low yield of the trees during this year is very largely
accounted for by the fact that these trees had been accorded a very
heavy cutting from the beginning with consequent shading out of all
fruit spurs by the denseness of the vegetative growth thus secured.

For the first three years the yields were approximately inversely
proportional to the severity of the pruning. By 1919 the results of
the lighter pruning and heavier fruiting were evidenced by the larger
yield of trees which were lightly headed, and for the following two
seasons the trees receiving this treatment retained this lead. This
was naturally to be expected in view of the decreased wood growth on

6 It is realized that the number of trees included in each plot is too small to
give the data statistical reliability; nevertheless the figures are presented since
they bear out the author's observations in hundreds of California orchards. It
is not intended that the yields and sizes in any of the tables should be taken
as an exact measure of the results obtained, but merely as showing tendencies.

34 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

the very lightly pruned trees which had no way of renewing their
fruit-spur systems. There was no crop in 1921 on account of killing
frosts.

The heavily headed trees matured their crop from a week to ten
days later than those which received only a light heading or simply a
thinning treatment. This fact is of significant importance in those
sections where the fruit is primarily intended for eastern shipment.

Although the fruit was slightly larger on the severely pruned trees,
the size has been entirely satisfactory in the other treatments as may
be seen from the following average for a six-year period. However,
the fact should be recognized that in some sections of the state and
especially with older trees very light pruning may result in fruit of
inferior size.

TABLE 6
Average Size of Fruit per Pound — Tilton Apricot, 1916-1922

Pruning treatment

Average size of fruit

Thinned out and headed severely

8.9 fruits per lb.

Thinned out and headed lightly

10.7 fruits per lb.

Thinned out only

10.8 fruits per lb.

These data lend further weight to the argument that it is necessary
to maintain the vegetative activity of the bearing fruit tree if con-
tinued profitable production is to be secured. Under conditions exist-
ing in this part of the University orchards, where no irrigation facili-
ties were available, it is quite evident that it was impossible to main-
tain a proper balance between fruit production and wood growth by a
reliance on a thinning out system of pruning unaccompanied by head-
ing. In other words, lack of irrigation brought about a depletion of
soil moisture which either killed a certain number of roots or rendered
them for the time being incapable of functioning thus reducing the
feeding area and destroying the balance between carbohydrates and
available mineral nutrients, which is necessary for the best production
of fruit.

In sections which are subject to the ravages of the brown rot
(Sclerotinia cinerea (Bon) Schrot.), efforts should be made to remove
all infected spurs and shoots, as these continue to produce spores of
the fungus throughout the winter and spring and make control by
spraying more difficult.

The pruning of trees infected with bacterial gummosis (Bacterium
cerasi Griffin) warrants special mention. This disease is characterized
by gumming cankers on the trunks and larger limbs during the winter

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 35

and spring months, and if not carefully and promptly treated may
girdle the affected parts and cause their death. The disease is caused
by bacteria and may be readily spread from one tree to another by
pruning tools. In case this malady is known to exist in an orchard,
it is a wise precaution to disinfect the shears and saw used in pruning
after finishing each tree. A solution of bichloride of mercury, one
part by weight to 500 parts of water, is a satisfactory disinfectant.
If the solution is to be used for treatment of the cankers as well as
for a disinfectant for the tools, it should contain also cyanide of
mercury one part by weight to 500 parts of water.

Sweet Cherries. — Sweet cherries are borne laterally on long-lived
spurs which are economically productive for ten or twelve years. On
account of this persistence of the fruit-spur system, the sweet cherry
needs less renewal wood than practically any other deciduous fruit.
If the trees have been properly trained and brought into bearing, but
little pruning is necessary to maintain them in a productive condition.
That pruning which is necessary, however, should be done annually
and should be of sufficient severity to renew approximately ten per
cent of the fruit bearing area each year.

It is characteristic of the sweet cherry tree to produce most of its
new growth from terminal buds. There is, therefore, a decided
tendency to develop long pole-like branches if no heading is done. It
is usually considered necessary to cut back cherry trees moderately the
first four or five years, or until such time as the framework is com-
pleted. After the first few years, the further heading will cause
delay in fruiting, and the desired form can generally be attained
without resorting to this practice. The cherry is slow in coming into
bearing and just as long as new wood growth is encouraged by rather
heavy cutting, the tree will make only shoot growth and fail to develop
fruit-spurs.

As the sweet cherry tree is naturally upright in growth, care should
be taken to give it as much spread as possible by retaining the outward
growing shoots and removing the upright ones. This is of special
importance with such upright growing varieties as the Lambert and
Black Tartarian. The Napoleon (Royal Ann) naturally assumes a
spreading habit with age. Certain cherry growers have had consider-
able success in keeping their trees low by very little if any pruning
during their earlier years, thus forcing them into rather precocious
fruiting and allowing the weight of the crops to bring about the desired
spread. Careful judgment, however, must be used in adopting such
a plan.

36 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

The cherry tree is very particular as to the conditions under which
it will thrive. It is particularly susceptible to drought and excessive
soil moisture. The reinvigoration of weak trees is a more complicated
problem than the mere adoption of a rational pruning method. In
order to keep a bearing cherry orchard in a good state of vigor, par-
ticular emphasis must be laid on proper soil conditions. When it is
found necessary to cut back cherry trees into older wood, either for
the purpose of lowering the trees or of stimulating the wood growth,
care should be taken to cut to a lateral, wherever possible. Trees
showing much die-back of the leading branches can sometimes be
rejuvenated by rather severe pruning. However, it should be men-
tioned again that soil conditions must at the. same time be improved.

The sweet cherry, like the apricot, is susceptible to bacterial
gummosis. Suggestions regarding the handling of apricot trees suffer-
ing from this disease apply with equal force to cherries.

Peaches and Nectarines. — The fruiting habit of the nectarine is
practically identical with that of the peach.

There is probably no other fruit which responds to proper pruning
and which declines with neglect so readily as the peach. This behavior
is largely explained by the fact that the peach bears its crop almost
entirely on one-year-old wood. Only occasionally is fruit found on
spurs, and these are very short-lived. For this reason it is necessary
to prune peach trees more heavily than most other fruits in order to
produce sufficient fruit-bearing wood each year. Twenty to forty
inches of growth over the tree as a whole is not excessive for young
bearing trees. Older trees may be expected to produce each season an
abundance of shoots of from twelve to thirty inches in length.

It is characteristic of the peach that its bearing area is carried
higher and higher each year, leaving long stretches of non-fruiting
branches that are subject to sunburn and decay and that serve only
as a connection between the fruiting periphery of the tree and its
sources of water and mineral nutrients — the roots. The denseness of
the foliage of the peach makes it difficult to maintain a satisfactory
distribution of the fruiting wood in the interior of the tree. Care
must be taken to see that the trees are thinned out in order that ade-
quate light may reach all parts and thus insure the production of
fruiting branches on the lower limbs. If careful attention is not given
to this detail then the characteristic " leggy" peach tree with sun-
burned branches and practically all new growth confined to the
periphery is the result.

Hanging shoots and branches, sometimes spoken of as ' ' hangers, ' '
are particularly desirable for fruit bearing on peach trees and should

Bulletin 386] pruning bearing deciduous fruit trees 37

be encouraged. Care should be exercised, however, to see that these
do not become too long by the addition of a certain amount of growth
each year. Such shoots can frequently be shortened and renewed by
cutting back to a new lateral. Fruit on these hangers is borne on
relatively short annual growths and sometimes on spurs.

In California it is seldom that a peach tree fails to set enough
fruit-buds for a heavy crop. In fact, one of the most perplexing
problems in the pruning of bearing peach trees is to determine what
amount of fruiting wood should be left to blossom and mature fruit.
After the trees have been thinned of the superfluous larger branches
and the desired form has been secured, the question arises as to how
best to reduce the fruit-buds to such a number that the crop which is
likely to set will be well distributed and at the same time will not
impose too heavy a task at the thinning season. Generally speaking,
it is more economical to reduce the crop as much as possible with the
pruning shears during the dormant season than entirely by hand
thinning after the fruit is set. However, to attempt to thin with the
pruning shears so that no hand-thinning will be necessary to secure
the desired size and quality is likely to reduce the total tonnage to such
a degree that the practice will not be economical.

This reduction of the fruiting wood may be accomplished either by
a thinning out and heading back or a combination of these two
methods. Care must be taken not to head the new shoots on varieties
which bear only at the tip or on the outer half of the shoot, especially,
when the set of fruit-buds is light. This method of fruiting is char-
acteristic of such varieties as the Tuscan and Phillips Cling peaches.
On the other hand, the Elberta, Muir, Alexander, Early Crawford,
and Lemon Cling are typical of the varieties that bear rather uni-
formly throughout the entire length of the shoot. With the latter
varieties, heading of the fruiting twigs may be entirely feasible and
in fact desirable.

In most instances the proper amount of fruiting wood should be
obtained by :

(a) Thinning out the unbranched one-year-old fruiting shoots.

(b) Cutting back lightly or moderately to laterals the fruiting

shoots that are branched.

(c) Thinning out the remaining laterals on these branched shoots.

(d) Heading back the fruiting shoots which are thus left, where

experience has shown this to be a desirable practice, either
from the standpoint of:

1. Thinning the crop ; or

2. Securing new wood growth for the crop of the following year.

38

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

Where possible, it is well to cut to the brownish colored, stocky
laterals rather than to shoots that are slender and reddish in color, as
the former are better adapted to continue the framework of the tree
and generally produce fruit of somewhat higher quality.

In cutting back to a lateral that is likely to bear fruit the following
season, it is advisable to leave a short stub just above the lateral — a
quarter or one-half inch long — to prevent the lateral from being
broken off by the weight of the fruit.

Fig. 14. — Seven-year-old Elberta peach tree before pruning. Note vigorous
new wood growth which, however, is not excessive. Compare with fig. 15 after
pruning by thinning. This tree has been well pruned during preceding seasons.

Figures 14 and 15 show a young bearing peach tree before and
after pruning. This tree has responded well to a thinning treatment —
so-called "long pruning."

Pears. — The fruit-bearing habit of the pear is almost identical with
that of the apple, and hence suggestions regarding the pruning of the
apple apply with equal force to the pear. The pear of course has
varietal characteristics just as the apple has. Some varieties tend to
come into bearing earlier than others. Likewise, some varieties such
as the Bartlett, Winter Nelis, and Angouleme, tend to produce a con-
siderable portion of their crop, especially during the early bearing

BULLETIN 38G] PRUNING BEARING DECIDUOUS FRUIT TREES

39

years, at or near the end of one-year wood. The tendency towards
alternate bearing is not so pronounced in the pear as in the apple, the
fruit spurs are generally a little shorter, and the tree is more upright
in habit of growth.

Figure 16 shows an ideal shaped Glout Morceau pear tree. Figure
17 illustrates the pruning of a nine-year-old Bartlett. Under favorable
soil and moisture conditions, neglected pear trees sometimes make an

Fig. 15. — A well pruned seven-year-old Elberta peach tree pruned ' by
"thinning out." Compare with fig. 14 before pruning. Note large amount of
growth removed. This tree has been pruned for four consecutive years by
thinning — no cutting to stubs.

exceptional development (fig. 18). Little or no pruning is not to be
generally recommended, but may occasionally be used to advantage on
very rich and moist soils, increasing yields and rendering the trees less
subject to the ravages of pear blight (Bacillus amylovorous) .

Plums and Prunes. — There are two general classes of plums com-
mercially cultivated in California — the Japanese and the European.
The former includes only shipping varieties, while the latter includes
both shipping plums and prunes. Since the fruiting habit and the
pruning of these two species are somewhat different it will be well to
divide the discussion into two parts.

40

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

Japanese Plums. — Japanese plums are, in a large part, borne
laterally upon short, thick spurs. The spurs are found mostly on
wood of from two to six or eight years of age. The life of a spur varies
from five to eight years. A minor portion of the crop is borne laterally

Fig. 16. — A well pruned ten-year-old Glout Morgeau pear tree. Note size
of the few scaffold branches. An ideal framework capable of supporting
immense crops without breakage. Compare fig. 9.

on one-year wood, the fruit being produced in general at the basal part
of the shoot.

The fruit-bearing habit of the Japanese plum is quite similar to
that of the apricot, the principal difference being that the spurs on

Bulletin 386] PRUNING BEARING DECIDUOUS FRUIT TREES

41

the former are somewhat longer lived. As a natural consequence the
pruning of these two fruits is much alike. The fact that the spurs
of the Japanese plum are longer lived than those of the apricot would
lead one to the conclusion that these plums should be pruned somewhat
less severely than the apricot, there being less necessity of replacing
the fruiting wood with new shoots. The Japanese plum, however, is
most prolific and as only large fruits are commercially valuable, the
trees as a consequence must be pruned rather heavily, in order to

Fig. 17. — A nine-year-old Bartlett pear tree before and after pruning. This
tree has been consistently pruned rather lightly, however. Compare fig. 18 of
an adjoining tree which has not been pruned for five years. The photographs
were taken at same distance from each tree.

reduce the fruit crop and at the same time induce sufficient new wood
growth. As a final result, therefore, the severity of the pruning of
these two fruits should be very similar. The plum should be pruned
in such a way that some ten to twenty-four inches of new wood growth
annually is secured on young bearing trees and ten to twelve inches
on older ones.

There is considerable difference in the habit of growth of the
different varieties. Some varieties such as the Burbank, are rather

42

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

low-spreading growers and attention must be paid even after the trees
come into bearing to divert the growth into the more upright branches.
On the other hand, the Wickson, Kelsey, and Santa Rosa are examples
of varieties that make a very narrow, upright growth and require
careful pruning to secure the proper spread.

Figures 19 and 20 show a young bearing Climax plum tree before
and after pruning.

Fig. 18. — A nine-year-old Bartlett pear tree before and after pruning.
Deliberately left unpruned for the five years preceding. Note splendid develop-
ment of fruiting wood and framework branches. Compare fig. 17 of an adjoin-
ing tree which has been pruned lightly. In addition to increasing enormously
in size this tree has produced from six to eight times as much fruit as the
lightly pruned tree. This illustration is introduced not in the way of recom-
mendation but merely to emphasize the greater development secured by little or
no pruning so long as the tree makes sufficient new wood growth.

European Plums. — The fruit-spurs of the European plums are
quite different from those of the Japanese. They are longer, more
slender, and frequently branched. Old spurs, or, as they are some-
times called ' ' fruiting brush, ' ' may reach a length of two or three feet,
while the spurs of Japanese plums are rarely longer than two or
three inches.

The pruning of European shipping plums is very similar to that
of Japanese plums, but it is for two reasons a less severe type of

Bulletin 386] pruning BEARING DECIDUOUS FRUIT TREES

43

10.

/

/

/

\

Fig. 19. — A young bearing Climax (Japanese) plum tree before pruning.
Note vigorous new wood growth produced by thinning method of pruning. This
tree has produced several good crops. Compare with fig. 20.

44

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

Fig. 20. — Young bearing Climax (Japanese) plum tree after pruning. Com-
pare fig. 19 before pruning. This tree might perhaps have been somewhat better
handled by removing two or three of the heavier branches in addition to the
thinning of the smaller shoots. A mistake was made in the earlier years in
not reducing the number of scaffold limbs.

BULLETIN 386] PRUNING BEARING DECIDUOUS FRUIT TREES 45

pruning. European plums do not tend to over-produce as do the
Japanese, hence, a light pruning is sometimes necessary to favor fruit-
bud formation in the former, rather than a severe pruning to reduce
the quantity of fruit-buds as with the latter species. Furthermore, a
relatively larger proportion of the fruit of the European plum is
borne on spurs. Keeping these facts in mind, the grower may handle
the European shipping plums by much the same methods as are
described for the Japanese plum and apricot.

The fruiting habit of prunes is identical with that of European
shipping plums. The only difference between the two kinds is that
in the case of the prunes the mature fruit contains sufficient sugar to
dry without fermenting at the pit, while the latter does not. Since,
with the exception of the Sugar variety, it is not customary to thin
the fruit nor to pick it from the tree, no great attempt has been made
on the part of the prune growers to keep their trees within bounds.
However, the relation of the operations of pruning and spraying to
the height of the tree should not be overlooked.

The pruning commonly given prune trees in this state consists in
a rather severe thinning and heading back of both the fruiting brush
and the larger limbs. This treatment is often given only at irregular
and infrequent intervals. Vigorous new shoots arise near the cuts,
necessitating a severe pruning the following season. It often follows
that two or three years may elapse before the trees that have been thus
heavily pruned again settle down to good production. It seems that
one of the most important facts to be emphasized in regard to the
handling of prune trees is that the pruning should be done regularly
and in most cases should be merely a light thinning out of the shoots,
old spurs, and smaller branches (fig. 21). The hanging branches are
often cut back to upward and outward pointing limbs in order to keep
the tree well shaped. From an economical point of view it is fortunate
that few cuts less than a quarter of an inch in diameter are necessary
in order to keep sufficient new wood growth coming. An average
yearly extension of the leading branches of from nine to eighteen
inches is adequate for young bearing prune trees. Older trees should
be expected to make at least six inches of growth each summer.

It is a matter of common observation in the older prune orchards
that have been given little pruning that the bearing area is located as
a thin shell on the periphery of the tree. A branch eight feet long, for
example, often has all its fruit-spurs on two or three feet at the
terminal end of the branch while the rest of the limb is bare and
unproductive. This concentration of the bearing area at the ends of
the branches is generally due to an insufficient amount of thinning,

46

UNIVERSITY OF CALIFORNIA EXPERIMENT STATION

resulting in a shading out of the interior and lower fruiting wood.
Where the fruit-spurs have not been killed they are often long, slender,
and unproductive in contrast with the shorter, stockier, fruit-produc-
ing spurs to be found on the outside of the tree. Such shaded, willowy
spurs produce small fruit and are short-lived. It is obvious that the
corrective treatment for such a condition lies in an adequate thinning
out of the smaller branches, where the tree has been properly trained,

&aam

Fig. 21. — Thirty-year-old French prune tree pruned by the removal of a few
of the larger branches and a thorough thinning of the i( fruiting brush."

and even the removal of a few larger branches, in case the framework
is crowded. A word of caution is necessary at this point. Since the
prune tree is very susceptible to sunburn, the best of judgment must
be exercised in the thinning out of bearing trees as recommended above.
The Sugar prune is a variety which behaves somewhat differently
from the other sorts commonly grown in California. It has a pro-
nounced tendency to make the most of its new growth on the tips of
the previous season's shoots, in the same manner as does the sweet
cherry. It has proved to be an alternate bearer in a number of
different localities, and since the wood is brittle, the branches fre-
quently break during the year of heavy production. In the alternate

Bulletin 3SCJ PRUNING BEARING DECIDUOUS FRUIT TREES 47

years, then, when they have a heavy set of plump fruit-buds, the trees
should be pruned heavily, new shoots cut back especially to prevent
the formation of long, pole-like branches, and the fruiting wood
thinned out. Thinning of the fruit itself may possibly tend to correct
this alternating habit. In the off-year, when the set of fruit-buds is
rather light, comparatively little pruning, especially in the way of
limiting fruit-buds, should be done.

Quinces. — The fruit of the quince is borne terminally on com-
paratively long current season's shoots, arising from lateral and
terminal buds on one-year wood. Since fruit-buds are produced only
on one-year wood, attention must be given to the stimulation of such
amounts of vegetative growth that the fruiting wood will be renewed
annually. Experience has shown, however, that heavy production can
generally be secured without a severe pruning. In fact, quinces some-
times continue to bear good crops of large-sized fruit when very little
pruning is practiced. It seems best, nevertheless, to give a pruning
which will stimulate from 10 to 24 inches of new growth each year on
young, and from 9 to 18 inches on older, quince trees. The pruning
should consist mainly of a thinning out and heading back to laterals.
Occasionally, however, it may be necessary to prune more severely in
order to induce sufficient new growth.

ACKNOWLEDGMENTS

The writer wishes to express his appreciation to the following :

Drs. E. J. Kraus, W. L. Howard, and W. H. Chandler for many
helpful suggestions and criticisms in the preparation of the manuscript.

Professor G. L. Philp and Mr. C. L. Austin for help in the field
experiments.

Miss Edna Russ for her help in the preparation of the illustrations.