Aspen tree guide: quaking aspen identification, clonal groves, roots and care

At a glance

What an aspen tree is

Aspens are the small-leaved, white-barked members of the poplar genus, Populus. The two North American aspens are quaking aspen (Populus tremuloides), also called trembling aspen, and bigtooth aspen (P. grandidentata). For how aspens fit alongside cottonwoods, balsam poplar and hybrid poplars, see the poplar tree guide.

The U.S. Fish and Wildlife Service calls quaking aspen the most widespread and numerous hardwood tree in North America, and it is unusual among trees for how it reproduces. A stand that looks like hundreds of separate trees is often a single organism: the trunks are shoots sent up from one shared root system. That clonal habit explains almost everything about aspen, from the way whole hillsides turn gold at once to the shoots that pop up across a lawn.

How to identify an aspen

Aspens are easiest to identify by leaf stalk, leaf margin and bark together.

• Leaves: alternate, simple, 1 to 3 inches long, heart-shaped to nearly round with a finely toothed edge, per Virginia Tech Dendrology. The leaf stalk is flattened, so the leaves tremble in the lightest breeze, which is where the name comes from.
• Bark: smooth and creamy yellowish-white to very light green at first, later developing thick furrows and turning dark, especially near the base, per Virginia Tech Dendrology. Dark, eye-shaped scars mark where lower branches dropped off.
• Buds and flowers: pointed buds; catkins in early spring before leaf-out. Aspen carries male and female flowers on separate trees, and because every stem in a clone shares the same genes, the USDA Forest Service lists sex among the traits that tell neighboring clones apart.
• Fall color: golden yellow. The USDA Forest Service notes that a clone may turn earlier or later, or show a different fall color, than its neighbors, so groves often show distinct patches of color that mark clone boundaries.

Sources for the comparison: Virginia Tech Dendrology fact sheets for both species, the University of Minnesota's bigtooth aspen profile and the Forest Service's Silvics of North America.

Aspen or birch?

White bark leads many people to call aspens birches, but they are in different families. Birch bark (especially paper birch) peels in papery curls and is marked with long horizontal lenticels; aspen bark does not peel and carries black, eye-shaped branch scars instead. Birch leaves are more oval with a double-toothed edge and a round leaf stalk, and they do not flutter the way aspen leaves do. The birch identification guide walks through paper, river and yellow birch.

Aspen or young cottonwood?

Both have flattened leaf stalks and pale young bark. Cottonwood leaves are larger (about 3 to 5 inches) and broadly triangular, its buds are resinous and sticky, and mature trunks are thick and deeply furrowed. Details are in the cottonwood tree guide.

How aspens grow: clones, groves and Pando

Aspens can reproduce from seed, but seedlings rarely survive; the USDA Forest Service points to the short time a seed stays viable, along with moisture, fungi and soil conditions. Most new aspen stems instead come from root suckers: shoots sent up from the shared root system that grow into trunks genetically identical to the original. Per the Forest Service, chemical signals from the trees to the roots govern when new sprouts start, and suckering generally happens as a stand deteriorates or is disturbed by fire or other events. The Colorado State Forest Service adds that aspen is often the first tree to recolonize burned, cleared or otherwise disturbed ground.

Because every stem in a clone shares the same genes, the trees in a clone have identical characteristics. Individual trunks are not long-lived: the Forest Service says aspens usually do not live more than 150 years, though some persist past 200, while the U.S. Fish and Wildlife Service notes that the root system beneath them may be hundreds or thousands of years old, continually replacing the trunks.

Are aspen trees all connected? Within one clone, yes; the stems share a root system. A large grove may hold several clones growing side by side, each with its own traits.

Pando

The best-known clone is Pando, a single male quaking aspen clone on the Fishlake National Forest in Utah, per the USDA Forest Service. Utah State University's Western Aspen Alliance puts it at about 106 acres (43 hectares) and an estimated 13 million pounds, and says its age cannot be accurately dated; the oldest individual stems are about 120 to 150 years old. Few young stems are replacing the dying ones. USU identifies browsing of young sprouts by elk, deer and cattle as probably the main problem, and the Forest Service is fencing much of the clone to protect them.

Where aspens grow (and where they struggle)

The Forest Service's Silvics of North America calls quaking aspen the most widely distributed tree in North America. Its range runs from Newfoundland and Labrador west across Canada to northwestern Alaska; in the western United States it grows mostly in the mountains from Washington to California, southern Arizona and Trans-Pecos Texas; and from Iowa and eastern Missouri it reaches east to West Virginia, Pennsylvania and New Jersey, with outliers in the mountains of Mexico. In Colorado, the Colorado State Forest Service reports aspen from about 6,500 to 11,500 feet, covering roughly 20 percent of the state's forested land. Bigtooth aspen is an eastern tree, native from the Northeast into the Midwest, west to Minnesota and south to Tennessee and West Virginia, per the University of Minnesota.

Are there aspen trees in Massachusetts? Yes. Bigtooth aspen's native range covers the Northeast, and quaking aspen's range spans eastern Canada and the northeastern states, so both turn up in New England, especially on disturbed ground, where Silvics notes aspen is quick to pioneer. For other native trees suited to the region, see New York native plants.

The pattern matters for planting. In the wild, aspen is a sun-loving pioneer of cool, often high-elevation sites. In towns, Colorado State University Extension notes that aspens prefer light, acidic, well-drained soils rarely found in urban areas, do best on the north and east sides of buildings, and have a life expectancy of only about 25 years in home landscapes. Arbor Day Foundation lists quaking aspen for USDA zones 1 to 7, but within those zones, site conditions matter more than the zone number.

Should you plant an aspen in your yard?

Colorado State University Extension is blunt about aspens in towns: they have a life expectancy of only about 25 years in the home landscape, partly because nearly all aspens sold are collected from the wild with limited root systems. They prefer light, acidic, well-drained soils that are rarely found in urban areas, do best on the north and east sides of buildings, and come with insects and diseases, some of which have no controls. They can work well when the site matches their native habitat.

If your goal is white bark and golden fall color in a warm or small yard, compare options in trees for small yards and trees and shrubs for Northeast fall color.

How to plant aspen trees

1. Buy nursery-grown container or balled-and-burlapped stock. Growing aspen from seed is impractical for home gardeners because the seed is viable for only a short time.
2. Pick the coolest suitable spot. Full sun on the crown; near buildings, CSU Extension says aspens do best on the north and east sides. On heavy clay, CSU recommends a raised bed of sandy loam.
3. Plant in groups. Clumps of three or more look natural and mirror how aspens grow; give the group room to widen as suckers appear.
4. Keep lawn away from the trunks. A wide mulched bed avoids mower and trimmer wounds and reduces competition. Follow the planting depth and backfill steps in how to plant a tree.
5. Keep the trees vigorous. Water during dry spells; CSU Extension notes that cankers usually occur on weakened trees.
6. Prune minimally. Remove dead or cankered branches in dry weather during dormancy; avoid unnecessary wounds to the thin bark.

Aspen roots and suckers

Aspen roots are shallow and spread well beyond the trunks, and each root can sprout new stems. In a lawn, shoots appear along the root paths, often many feet from the nearest tree, and anything that damages roots (trenching, edging, aerating, cutting a trunk down) increases suckering.

How to get rid of aspen shoots in a lawn

• Mow or clip them regularly. Repeatedly cutting suckers at ground level gradually exhausts them without disturbing the parent roots.
• Do not spray suckers on a tree you want to keep. The shoots are connected to the same root system as the trunks, so a herbicide applied to them can move into and injure the parent trees.
• Reduce root stress. Avoid cutting roots and keep the trees watered; stressed and damaged roots sucker more.
• Removing the tree does not stop suckers. Cutting or killing the trunks is exactly the kind of disturbance that triggers suckering, so expect a flush of shoots for several seasons afterward.

General sucker-removal technique, including cuts and growth-regulator products, is covered in removing tree suckers.

Pests and diseases

Colorado State University Extension warns that aspen growers should expect insects and diseases, some with no controls, and that problems hit weakened trees hardest.

• Marssonina leaf spot: dark brown to black, bull's-eye spots on leaves from July until frost, favored by a warm, rainy spring. CSU Extension recommends raking up fallen leaves in autumn and thinning trees for airflow; it is usually not severe enough to need fungicide.
• Cytospora canker: sunken, discolored areas on bark that girdle stems; CSU notes cankers usually occur on weakened trees, so prevention (water, no wounds) is the main control. See how to identify tree cankers.
• Oystershell scale: named by CSU Extension as one of the common aspen pests. Trees with lots of deadwood and a thin crown are best removed.

Wildlife value

Aspen is one of the most important wildlife trees in North America. The Colorado State Forest Service ranks aspen forests just behind riparian areas for biodiversity and notes that their open canopy produces a lusher understory than neighboring conifer forest.

• Browsers: the U.S. Fish and Wildlife Service calls aspen a staple for elk, moose, deer and beaver, and the Arbor Day Foundation adds snowshoe hare. Do elk eat aspen trees? Yes; elk browse the leaves and young sprouts, and Utah State University identifies browsing by elk, deer and cattle as the main reason Pando is not regenerating.
• Beavers rely on aspen for food and building material, per the Arbor Day Foundation.
• Birds: grouse eat aspen buds in winter (Arbor Day Foundation), and woodpeckers such as flickers and sapsuckers prefer to nest in aspen (U.S. Fish and Wildlife Service).

Common mistakes

Frequently asked

What does an aspen tree look like?

Aspens are slender trees with smooth, creamy-white to very light green bark marked by dark, eye-shaped branch scars. Their leaves are 1 to 3 inches long, nearly round and finely toothed on flattened stalks that make them shimmer in any breeze. In fall they turn golden yellow, often in patches that reflect separate clones.

How fast do aspen trees grow?

Quaking aspen is a fast-growing tree, per the Arbor Day Foundation. Growth slows on heavy, poorly drained or stressful urban sites, which is where landscape aspens tend to struggle.

How tall do aspen trees grow?

The Arbor Day Foundation lists quaking aspen at 40 to 50 feet tall and 20 to 30 feet wide, while Virginia Tech Dendrology describes it as a small, 30 to 40 foot tree that often grows in thickets. The crown stays fairly narrow, but a clone widens over time as suckers fill in around it.

How long do aspen trees live?

Individual aspen trunks usually live no more than about 150 years, though some persist past 200, according to the USDA Forest Service, while a clone's root system can be hundreds or thousands of years old. In home landscapes, Colorado State University Extension puts aspen life expectancy at only about 25 years.

Where do aspen trees grow?

Quaking aspen is the most widely distributed tree in North America, per the Forest Service: from Newfoundland across Canada to Alaska, through the mountains of the western US, and from Iowa and Missouri east to Pennsylvania and New Jersey. It is quick to pioneer sunny, disturbed sites. Bigtooth aspen grows from the Northeast into the Midwest and south to Tennessee and West Virginia.

How do aspen trees reproduce?

Mostly by root suckers: shoots that sprout from the shared root system and grow into new trunks genetically identical to the parent. They also produce cottony seed, but the seed is viable only briefly and seedlings rarely survive. Fire or other disturbance that kills trunks triggers heavy suckering.

Are aspen trees one organism?

A clone is. All the trunks that grow from one root system share the same genes and behave as one organism, which is why Pando in Utah, about 106 acres and an estimated 13 million pounds, is counted among the largest living things. A single grove can also contain several different clones.

Are aspen trees birch trees?

No. Aspens are poplars in the willow family; birches are in the birch family. Birch bark peels in papery strips and has horizontal lenticels, while aspen bark stays smooth with black, eye-shaped scars, and aspen leaf stalks are flattened rather than round.

Sources

1. How Aspens Grow, USDA Forest Service
2. Quaking Aspen (Silvics of North America), USDA Forest Service
3. Quaking aspen fact sheet, Virginia Tech Dendrology
4. Bigtooth aspen fact sheet, Virginia Tech Dendrology
5. Bigtooth Aspen, Populus grandidentata, University of Minnesota
6. Aspen Trees (PlantTalk 1701), Colorado State University Extension
7. Aspen Leaf Spot (PlantTalk 1403), Colorado State University Extension
8. Pando: Frequently Asked Questions, Utah State University Western Aspen Alliance
9. Know Your Trees: Aspen, U.S. Fish and Wildlife Service
10. Hardwood: Primarily Aspen, Colorado State Forest Service
11. Quaking Aspen tree guide, Arbor Day Foundation

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