THE Q ENTER FOR URBAN HORTICULTURE
University of Washington

Volume 3, Number 1

January 1985

THE DOZEN TOUGHEST BROADLEAVED TREES FOR
CITY CONDITIONS . Selection by Marvin
Black, Seattle City Arborist.

The following is condensed from a presen-
tation by Mr. Black at a joint meeting of
the American Society for Horticultural
Science and the Canadian Society for
Horticultural Science, Vancouver, B.C.,
August 1984.

1 .

Ailanthus altissima — Tree of Heaven.
"Grows better under city conditions in
this country than any other tree,
native or exotic." (Wyman). Tolerates *'
a wide variety of soil conditions; not
particular about pH. No serious
disease or insect pests. Can with-
stand root submergence in salt water.

2. Platanus x acerifolia — London Plane.
Survives really bad city conditions
well, except overly poor drainage.

Some susceptibility to auto-exhaust
pollution, mildew in some areas, and
anthracnose fungus where springs are
cold and wet. Husky root system is no
problem in fertile soil, but can badly
damage pavement on shallow soils.

3 . Populus / Salix complex. Poplars and
Willows. Tolerant of extremely poor
soils and often of dry soils. These
trees also grow readily in moist or
soggy soils. Some willows will
tolerate standing water part of the
year .

4

Acer. Maples. Toughest probably
Norway (A. platanoides) , Bigleaf
A. macrophyllum) , and Red (A. rubrum) .
Red Maple tolerates poorly drained
soils .

5. Tilia . Lindens. Littleleaf Linden
(T. cordata) , Crimean Linden

(T. x euchlora) , and Silver Linden
(T. tomentosa) probably the best.

Avoid Silver Linden in dusty or sooty
areas, as hairy leaves collect dust.
Very tough, durable trees. Subject to
summer attacks of leaf-feeding
insects, which are not fatal but
annoying .

6. Aesculus . Horsechestnuts . Subject to
disease in some sections, but less so
in Northwest. Heavy limbs said to
break in storms, though little of this
observed in our area. Durable in poor
city soils.

7. Ginkgo biloba. Maidenhair Tree. Here
is a long-lived tree that needs no
spraying, very little pruning, has
handsome leaves that color well in the
fall, and has a notable pollution
tolerance; yet the tree is not much
planted .

8. Robinia pseudoacacia (Black Locust) and
Gleditsla triacanthos (Honeylocust) .
Both are quick-growing, with fine,
feathery foliage. Both tolerate poor
soils including alkaline soils (winter
road salts) and some coastal salt
spray. They can be weedy and may seed
and sucker freely. Both have several
insect and disease pests which may need
controlling. Honeylocust in particular
has experienced dieback and quite a few
other in-city problems over the past
decade; it is probably overused. In
the Pacific Northwest, both locusts are
intolerant of poor drainage.

9. Ouercus . Oaks. Particularly good spe-
cies include Pin Oak (Q. palustris) and
Willow Oak (Q. phellos) , which tolerate
poorly drained sites, and two non-swamp
species, Red Oak (Q. rubra) and Texas
Red Oak (Q. shumardii) . Willow Oak
grows particularly quickly, to two feet
annually.

10. Carpinus betulus. European Hornbeam.
Common form is the cultivar

' Fastigiata' , which grows egg-shaped to
30 feet. A neat and formal tree
tolerant of heavy clay soils; dislikes
gravelly soil. Needs little pruning,
but can be sheared to hedge. Very few
pests. Intolerant of really poor
drainage .

11. Ulmus parvifolla. Lacebark or Chinese
Elm. The true Chinese elm — the name
is in disrepute because nurserymen have
often erroneously offered the inferior
Siberian Elm (U. pumila) as the Chinese
Elm. U. parvifolla is somewhat
resistant to Dutch elm disease and also
to elm beetle. Tolerates dry, poor
soil and a wide range of pH. Though
rarely seen, a tough city tree.

12. Magnolia. M. heptapetala (formerly
M. denudata) and M. x soulangeana are
especially good. Both are showy in
flower. Tolerate most city conditions,
including pollution, well. Fairly good
drainage is required. They perform
best in good soil. These two magnolias
are better city flowering trees than
the cherries, crabapples, and plums
more often used.

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RESERVES A FACTOR. Excerpts from "Winter
of ’83 in Retrospect," by James R. Feucht,
Metro Horticulture, Cooperative Extension
Service, Colorado State University,
November 1984 .

Until recently, the role that carbohydrate
reserves (starches, fats and sugars) play
in winter survival did not receive much
attention .

Research in the last ten years or so has
shown that a major factor in winter sur-
vival of woody plants is their ability to
store reserve "food energy” (carbo-
hydrates ) .

Paul J. Kramer and Theodore T. Kozlowski,
Plant Physiologists, discuss at great
length the value of carbohydrates in plant
survival in their book, Physiology of
Woody Plants (Academic Press, 1979). They
indicate that carbohydrate distribution in
plants varies from species to species and
even changes as a plant gets older. Young
trees may have more reserves in their
roots, but as a tree matures, more is
found in the trunk and branches. There
are also seasonal variations of car-
bohydrate reserves.

Plants under stress from lack of water,
nutritional deficiencies, over-watering
and other problems will fail to build up
carbohydrate reserves.

INTEGRATED PEST MANAGEMENT NEWSLETTER
Sharon Collman, King County Extension
Agent, publishes a pest management
newsletter entitled OF BUGS & BLIGHTS. It
is available free of charge to
professional horticulturists. To receive
OF BUGS & BLIGHTS send the name of your
company, address, and zip code to Sharon
>1 Lman , King County Cooperative Exten-
sion, 312 Smith Tower, Seattle, WA 98104.

0L0RFUL WINTER LANDSCAPES
Host gardeners consider winter the least
colorful season of the year. Take a
.Poser look though, and you will find many
landscape plants to brighten up the dismal
pray of winter. Cornus mas , Jasminum
nudiflorum, and Rhododendron mucronulatum
all bloom in mid-winter; Hamemelis mollis
and Sarcococca spp. even have fragrant
flowers. The bark and twigs of Acer
palmatun ’Sango Kaku’, and Betula spp. add
.ntertiL to the winter lar' ape.

Pyracantha spp. and Cotoneaster spp. pro-
vide ornamental fruits. Blue- and golden-
foliage conifers can also brighten up the
landscape. These are just a few examples
of the more common winter-colorful plants.

To further explore this subject, you can
attend guided tours at the Washington Park
Arboretum or attend the February 20 semi-
nar — "Winter Color in the Landscape"

(see URBAN HORTICULTURE PRESENTS) . A free
bulletin, "Winter Flowering Shrubs",
//KC-103, is available from King County
Extension (344-2686).

EDUCATIONAL OPPORTUNITIES’

LANDSCAPE MAINTENANCE SEMINARS
Special notices for these programs are no
longer being sent. Instead, the Landscape
Maintenance Seminars will be listed in
URBAN HORTICULTURE PRESENTS..., the Center
for Urban Horticulture’s new continuing
education newsletter. Those who are on
the PRO HORT mailing list will also
receive URBAN HORTICULTURE PRESENTS.

EDMONDS COMMUNITY COLLEGE will offer the
following horticulture courses winter
quarter: Plant I.D., Soils, Pruning,

Greenhouse Studies, Landscape Studies,
Plant Propagation, Landscape Design, House
Plants, Grafting, Wholesale Nursery
Practices, Urban Tree Management,

Landscape Business, Small Engine Repair,
Japanese Gardens, Rock Gardens, Wood
Construction Projects. Call 771-1545.

SOUTH SEATTLE COMMUNITY COLLEGE will offer
the following horticulture courses winter
quarter: Nursery Operations, Winter Plant

I.D., Landscape Design, Pruning, Pesticide
Application & Management (preparation for
state licenses), Irrigation System Design,
Landscape Construction, Arboretum
Planting. Call 764-5336.

PESTICIDE TRAINING SH0RTC0URSES
W.S.U. Cooperative Extension will again
offer shortcourses for those planning to
take the State’s pesticide licensing
examinations. In western Washington the
locations and dates are:

Mount Vernon .... .January 28,29 & 30

Tacoma January 29,30 & 31

Sequim. February 12,13 & 14

Vancouver February 26,27 & 28

Bellevue ........ .Feb. 27,28 and Mar. 1

Call your county extension office for
registration information.