Ask a horticulturist about the use of quaking aspens (Populus tremuloides) in the home landscape and the advice you likely will hear is "OK, but...."
Forty years ago, aspen was seldom used in home landscapes. In the interim, however, the use of this tree has increased dramatically in Colorado's urban landscapes.
Aspen grows fast, lending the landscape an early finished look, and it provides an informal touch of Colorado to the suburban home or urban landscape. Aspen is a small to medium-sized tree that won't overwhelm smaller yards so typical of today's urban subdivisions. It has attractive bark, leaves tremble in the slightest breeze and the tree can develop good fall foliage color.
That's the good news. Now for a reality check: Aspen is affected by numerous insects, diseases and cultural problems. While there are plenty of good-looking aspen around the region, it also is the most common problem tree discussed in calls or samples brought to Colorado State University Cooperative Extension's Plant Diagnostic Clinic.
Ecologically, aspen serves as a "succession" tree, quickly seeding in where other vegetation was lost because of erosion, fire, logging, insects or disease. It provides cover for seedlings of pine, fir and spruce, and as these become larger, the "nurse crop" of aspen may die out.
Aspen reproduce not only by seed but also by extensive suckering. An aspen grove starts as suckers shoot off the roots of a mother tree, which arrived at the site by seed. This suckering habit can be a nuisance in the urban landscape, coming up in lawns and gardens.
Aspens are short-lived trees, as expected from their role in forest ecology. In the urban landscape, even properly cared-for aspen may not reach 20 years. Life spans can be shortened further by one or more of several insects or diseases that attack aspen. Fungal diseases, such as Cytospora or other cankers which attack the trunk, are common, as are diseases of the foliage such as rusts, or leaf spots. Of the many insects that attack urban plantings of aspen, oystershell scale, aphids and aspen twiggall fly are most prevalent.
Aspen prefers the moist but well-drained, slightly acidic soil found at higher elevations. Much of the soil in the Front Range of Colorado is compacted, poorly drained alkaline clay. Aspens transplanted to such soils are at a disadvantage, especially considering that much of the original root system was lost in the digging process.
Aspens transplanted to landscapes are collected primarily from the mountains. A few nurseries offer nursery-grown aspens,which are grown to salable sizes in pots or in the field. While these should not experience the stress that mountain-collected aspens do, later insect and disease problems still are possible.
Horticulturists and plant pathologists are hesitant to recommend aspen as a landscape tree for the Front Range. For those who insist, aspens should be planted on north or east slopes, or on north or east sides of the house, in soil well amended with organic materials and mulched after planting.
Conditions often become extremely hot and dry on south or west exposures; this would further stress aspen. Should an aspen trunk become severely affected by Cytospora or oystershell scale, the gardener should be willing to cut down that trunk and allow other sucker shoots to develop into new trees.
Another complaint about aspens in Front Range landscapes is that they do not develop as brilliant a yellow fall color as those in the mountains. Differences in soil chemistry and texture, soil moisture; day/night temperatures and sunlight intensity between the Front Range and the mountain areas all contribute to this.
Should you plant aspen? Probably not, if you have space only on south or west exposures, or if the planting site is small and narrow. Yes, if you are willing to amend soils before planting a nursery-grown aspen and will monitor the aspen for problems. Remember that your nearby nursery or garden center carries many other tree choices for the landscape.
For alternatives to planting aspen see: Aspens: Not the Only Small Tree in Town
Cankers on Western Quaking Aspen, USDA Forest Service bulletin (off-site)
Photo: Judy Sedbrook
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010