Aspens

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 locally.

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 vary yearly.
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 fireblight.
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 plums.
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.