Weigh advantages/disadvantages when choosing aspen

Depending upon who's talking, the idea of growing aspen along the Front Range may give them the quakes. Populus tremuloides (quaking aspen) some say, should stay in the mountains where they belong. But, despite the naysayers, aspen in home landscapes enjoy wide popularity.

Aspen grows fast, providing a finished look earlier and giving the suburban home or urban landscape a "mountain woodland" feel. 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, its leaves "tremble" in the slightest breeze and the tree can develop good fall foliage color.

Despite these attractive characteristics, however, aspen is affected by a variety of insect and disease problems. It is by far the most common problem tree discussed in calls received or samples brought in to Extension offices around the metro area.

Ecologically, aspen serve as "succession" trees, quickly seeding in where other vegetation is lost because of erosion, fire, logging, insects or disease. They provide cover for seedlings of pine, fir and spruce as they grow; as these become larger, the "nurse crop" of aspen dies out.

Aspens reproduce not only by seed but also by extensive suckering. A grove of aspens in our mountains may have started as suckers off the roots of an original, "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 aspens may not reach 20 years. Lifespans may be further shortened by one or more of the several insects or diseases that attack aspen.

Fungal diseases, such as Cytospora or other cankers that attack the trunk, are common, as are diseases of the foliage such as rusts or leafspots. Of the many insects that attack urban plantings of aspen, oystershell scale, aphids and aspen twiggall fly are most prevalent. Spraying aspen foliage to control leaf diseases or insects often results in injury to the foliage (phytotoxicity). Aspens are notoriously prone to phytotoxicity from insecticides or fungicides.

The thin bark of aspens is damaged easily by lawnmowers or weed eaters (stringtrimmers). This bark injury allows pathogens and certain insects "easy entry."

Aspen prefers the moist but well-drained, slightly acidic soil found at higher elevations. Conversely, much of the soil of Front Range Colorado is compacted, poorly-drained alkaline clay. Aspens transplanted to such soils are at a disadvantage, especially considering that a significant portion of the original root system is lost in the digging process.

Aspens transplanted to landscapes usually are collected from the mountains. Some correlation can be established between how well an aspen transplant adapts to Front Range soil/climate and where it was obtained in the mountains (elevation, soil type, slope). Aspen transplants from some mountain groves seem to "take" better than from others.

A few nurseries may offer nursery-grown aspen - the nursery propagates aspen from root cuttings or from seed and allows them to grow to saleable size in pots or in the field. While these should not experience the stress that mountain-collected aspens would, later insect and disease problems still are likely.

Horticulturists and plant pathologists are hesitant to recommend aspen as a landscape plant for the Front Range. If you insist upon them, plant 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. It often becomes extremely hot and dry on south or west sides of the home or on south or west slopes; this would further stress aspen.

Homeowners often complain that aspens planted in yards 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.

Extension recommends leaving aspens in the mountains, driving there to see them turn brilliant gold in September and depending on your local nursery to supply landscape trees better adapted to Front Range soils.

For more information on Gardening, contact your local Colorado State University Extension office.