Aspens: Not the Only Small
Tree in Town
By Robert Cox, Colorado State University, Horticulture
The best aspen is the aspen that grows in the mountains.
That's the consensus of horticulturists who see problems galore with
aspen that are grown in landscapes along the Front Range.
Most aspens used in landscapes are dug from groves in the mountains,
but the stressful digging process, along with poor soils along the Front Range, make it
rough for aspen to do well beyond their natural environment.
If you insist on aspens, choose "seed-grown" or
"nursery-grown" trees. Plant them in well-amended soil on north or east
exposures. Keep in mind that these nursery-grown aspens, while not as stressed as
"mountain dug," still are subject to a wide variety of insect pests and
diseases, such as oystershell scale, twig gall fly, aphids, rusts and leafspots.
If all this information makes aspen seem like a tree that's too much
trouble, what small trees can be used as alternatives?
The following list includes deciduous trees that will mature at
approximately the same size of aspens in metro landscapes: about 30 feet tall and with
about 15-foot width. Sizes given are approximate. Some listings may be difficult to find
Acer campestre, Hedge Maple, 30 x 25
feet: This maple is used in Salt Lake City landscapes and could be used
more here. It tolerates alkaline soil and some drought after it is established. Its
foliage turns golden in fall. The interesting bark becomes corky and striated with age.
Hedge maple gets its name from its use as a sheared hedge in Europe.
Acer grandidentatum, Wasatch or
Bigtooth Maple, 30 x 20 feet: This under-used maple native to the
southern Rockies and Four Corners area is slow growing and somewhat difficult to find in
nurseries. It grows as a small tree or large shrub, hardy to about 7500 feet elevation. It
tolerates dry soil after establishment, though it must have well-drained soil. This maple
develops orange-red and yellow fall foliage color.
Acer ginnala, Amur or Ginnala Maple, 20 x 15 feet:
This small maple often is used in Colorado landscapes. It is very
hardy and shows fall foliage colors that include red, orange and yellow. Amur maple is
grown as a small tree or large, multistem shrub. It may develop chlorosis (leaf yellowing)
in alkaline soils. Cultivars include Flame, Embers and Compactum.
Acer tataricum, Tatarian Maple,
25 x 20 feet: This small maple should be used more in Colorado. It
is similar to Amur maple above, but tolerates alkaline soils better. The samaras (seed
pods) that develop in summer are red for a few weeks, then turn brown. They are attractive
contrast with the green foliage. Yellow and red-orange fall colors are seen; colors may
Alnus tenuifolia, Thinleaf Alder, 20 x 15 feet:
This native shrub commonly is found along mountain streams. While
usually shrubby, it can be pruned to form a singlestem tree. Alders form interesting
cone-like fruits that persist through winter. These are sometimes collected, spray painted
gold, and sold as miniature "pine cone" jewelry. Alders prefer moist sites.
Amelanchier species, Serviceberry, variable, 20 x 15
feet: There are several Serviceberries in the nursery trade. Some are more
shrubby, others are small trees. All have attractive white flowers in April-May, edible
blue fruit, and attractive fall foliage color, ranging from yellows to scarlet. Cultivars
include Autumn Brilliance, Cole, Cumulus, Regent, Robin Hill and Princess Diana.
Betula nigra 'Heritage', Heritage River Birch, 35 x 25
feet: This birch is resistant to bronze birch borer damage, a
frequent problem on some other birches. It prefers acidic to neutral soil and tolerates
wet, poorly drained soil. Leaves turn yellow in fall. The interesting tan-cream color bark
peels away from the trunk with age.
Betula occidentalis, Western Water Birch, 20 x 15
feet: This native birch often is found along streams in association
with Thinleaf Alder. Usually a multistem shrub, it can be pruned to a singlestem tree. It
is resistant to bronze birch borer. Leaves become yellow in fall. Bark is an attractive
cinnamon-copper color; this is especially effective in winter when leaves are not present.
Betula papyrifera, Paper Birch, 35 x 25 feet:
This tree resembles aspen in appearance, yet the two trees are not
related. Plant in well amended soil on north or east exposures. Water during dry periods,
summer or winter. White bark contrasts with copper colored branches. Yellow fall color.
Bark peels off in thin "papyrus" sheets with age.
Cornus mas, Corneliancherry Dogwood, 20 x 15 feet:
This small tree or large shrub has yellow flowers in early spring.
These are followed by small, cherry-like fruits that can be made into syrup or jam. Leaves
are attractive, glossy green, but no fall color develops. Like many small trees that also
are grown as multistemmed shrubs, it may sucker extensively.
Cornus racemosa, Gray Dogwood, 15 x 12 feet:
This small tree/large shrub may sucker extensively. It can tolerate
wet, poorly drained alkaline soils. Small white flowers are followed by small white fruits
that are eaten quickly by birds. The fruit stems left behind are red and quite attractive.
Leaves are reddish purple in fall.
Corylus colurna, Turkish Filbert, 35 x 20 feet:
This is an uncommon tree with the ability to tolerate alkaline soil,
but it should be well drained. The Turkish filbert has few insect pests or diseases. It is
mildly drought tolerant when established. While this tree has attractive foliage, it
develops little fall color. It is a good small shade tree to use in a lawn area.
Crataegus species, Hawthorns,
variable, 12 - 30 feet tall: It is possible to grow several hawthorns
here. Consider the thorns before planting in certain spots, such as children's play areas.
In lawns, they may receive too much fertilizer and develop fireblight. Most hawthorns have
white or pink flowers, attractive red fruits, good orange-to-red fall color and attractive
bark. One called Thornless Cockspur Hawthorn, as the name implies, is thornless.
Euonymus europaea, Spindletree, 15 x 15 feet:
This small tree develops reddish purple fall color. While the
flowers are not easily seen, the capsule fruits are pink and have an interesting shape.
When they split open, the orange seed coats (called arils) are exposed. One cultivar is
called Redcap, which refers to its red capsule fruits.
Koelreuteria paniculata, Goldenraintree,
25 x 25 feet: This tree usually is pest-free. It works well as a
small shade tree in lawns or to shade a patio. Goldenraintree is tolerant of alkaline soil
and some drought. It is valuable for its mid-summer yellow flowers; fruits are 2-inch long
capsules that look like "Japanese lanterns." Seeds inside are black and about
the size of peas.
Malus 'Centurion', Centurion
Crabapple, 25 x 15 feet: This crabapple is upright when young,
becoming more oval with maturity. Flowers are rosy-red, small fruits are an attractive
glossy red. In Colorado State University trials, Centurion has shown good resistance to
Prunus armeniaca or Prunus mandshurica, 25 x 25
feet: Apricot or Manchurian apricot often are overlooked as landscape
trees. Both have pinkish-white flowers in April, attractive leaves, and yellow-to-reddish
(Manchurian) fall color. Apricot fruits are not dependably produced in our area. Colorado
State University research suggests these adapt well to the Front Range.
Prunus cerasifera, `Newport' or `Mt. St. Helens', 20 x
20 feet: These similar flowering plums have purplish leaves, although
by late summer the leaves may be more greenish purple. Flowers, in April-May, are light
pink to white. In some years these trees will develop edible one-inch diameter purple
Ptelea angustifolia, Hoptree,
15 x 15 feet: Native to Colorado, this small tree/large shrub
tolerates shade and dry conditions after it is established. Its flowers are inconspicuous
but fruits are seeds in a papery "wafer," that resembles hops or a "fried
egg." These fruits are light green at first and contrast with the dark green leaves.
Oddly, this tree is in the citrus family - crushed leaves smell faintly of citrus.
Pyrus calleryana, Callery
Flowering Pear, variable: White April flowers, glossy green
foliage and good fall color from red to reddish purple typifies the Callery Flowering
Pears. Fruits are sparsely produced and are about the size of the thumbnail. Those
cultivars with good hardiness and an approximate 30 x 15 foot size include Chanticleer,
Cleveland Select and Stone Hill.
Sorbus aucuparia, European Mountainash,
variable: White flowers and clusters of orange or red
"berries" make mountainash a popular landscape tree. It also develops bronze
fall foliage color. The cultivars with an approximate 30 x 15 foot size are Black Hawk
(orange berries) and Cardinal Royal (red berries). Some other cultivars are selected for
pink, yellow or white berries.
Syringa pekinensis, Pekin Lilac (Chinese Tree Lilac),
20 X 15 feet: When younger, this small tree has reddish-brown bark,
becoming brown and peeling off in flakes or sheets. Flowers are creamy white.
Syringa reticulata, Japanese
Tree Lilac, 20 x 15 feet: Bark is reddish-brown. Flowers are
creamy white, with a fragrance different from common Lilac. Cultivars include Ivory Silk
and Summer Snow.
Photograph courtesy of Judy Sedbrook.
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