Aspen Trees

Joyce Starr
Aspen trees

If you're searching for a native, medium-sized tree that grows fast and brings fall color to your landscape, aspen might be the tree for you. Typically found in mountainous forests, they are a suitable choice to naturalize your landscape.

Basic Characteristics and Common Types

Quacking aspen (Populus tremuloides) and bigtooth aspen (Populus grandidentata), both common names, are the most likely types of the trees you will find in the wild and planted in home landscapes. Both are very similar in looks and preferences for growth. They are hardy, growing throughout USDA zones 1 through 6, withstanding the coldest temperatures winter throws at them.

Size, Shape, and Longevity

Both types reach a mature height of approximately 50 to 60 feet, though they can grow as tall as 100 feet, and 30 feet wide, with an open canopy. On average, each tree has a lifespan of 50 years due to pests and disease problems.

Bark and Foliage

The light-colored, whitish bark looks similar to that of a birch, making it an eye-catching landscape addition. Trees have one main, straight trunk. As the tree matures, the bark develops warty ridges. Its wood is weak, due to the fast growth, so it's best planted in a site where potential fallen branches don't damage structures or power lines.

When temperatures are warm and the trees are actively growing, the foliage is green and changes to a yellowish color in autumn before it drops for winter. The 3-inch foliage is oval-shaped with pointy tips and toothed edges. It is easy to identify the two trees by inspecting their foliage. The bigtooth has heavier toothed leaves than that of the quaking. Other than the differences in the foliage, the two trees look alike.

Aspen bark
Aspen bark
Aspen leaves
Aspen leaves

Flowers, Seeds and Reproduction

Aspen flower with seeds

The trees are dioecious, meaning that separate trees produce male and female flowers or catkins. The 2-inch long, yellowish catkins bloom before the foliage sprouts in early springtime. Over several months, the catkins ripen producing cottony seeds, with female trees producing more of a cottony mass than male trees. Seeds do not require stratification to germinate and are ready for planting as soon as the catkin splits showing the ripe seed. The tree reaches a seed-bearing age when it reaches around 10-years-old.

All reproduce through seeds and root sprouts, with root sprouts the most common way of reproduction. Gardeners will notice an aspen in their landscape sending up multiple sprouts around the tree.

Purchase Considerations and Sellers

You can probably locate specimens for sale in local nurseries or native plant sellers throughout the tree's growing range. If purchasing a tree, make sure its foliage is healthy and free of spots or other signs of diseases or pests.

Inspect the tree's container, making sure the tree hasn't outgrown it with roots growing out the drain holes. Living too long in containers that are too small can permanently affect the tree's growth stunting the tree.

If you cannot locate one in your local area, a variety of online plant nurseries sell them. Trees are usually around one year old or younger and shipped in their deciduous stage before they leaf-out in springtime. Online nurseries include Cold Stream Farm and the Arbor Day Foundation.

Uses in the Landscape and Considerations

Fall Colors

Aspens make handsome trees utilized in the proper area of the landscape where its aggressive root system does not create problems. Do not plant the tree close to house foundations, sidewalks or driveways, as the roots will lift the structures. Since the tree's roots seek out moisture, you do not want to plant the tree close to septic systems, sewers or drains, as the root system will do damage.

The trees make terrific additions to native and wildlife gardens. Many types of birds utilize the seeds as a food source and woodpeckers use the tree as a nesting site. They also offer filtered shade and their medium size won't overpower smaller yards.

Preferred Conditions for Growth

For aspens to grow properly, it's necessary to plant and grow the tree in its preferred conditions.

Preferences for Light

For the best growth, plant in a location that receives full sun throughout the day.

Preferences for Soil

In the wild, the trees grow in moist locations. When selecting a site to plant the tree, make sure to choose a location that is rich in organic materials, drains well but is moist. The tree does not perform well in alkaline soils that have a tendency to be dry and will decrease its lifespan.

Water Requirements

They require regular applications of water to grow properly. Water the tree deeply weekly, especially when conditions are dry and throughout the spring into early fall. During the dormant season through winter, the tree only requires a monthly application. Water the tree right after planting and continue applying water several times weekly for several months while the root system establishes itself into the landscape.

Planting Tips

After selecting the appropriate location in the landscape to plant your aspen, it's time to prepare the area for introducing the tree. Clear all weeds and grasses growing in a planting site that is at least 3 feet in diameter. The unwanted growth robs the tree of moisture and nutrients and harbors pests and diseases. Keeping the area under the tree clean makes it less likely lawn equipment will bump into the trunk damaging it, which leads to diseases.

Digging the Hole

Loosen the soil in the planting site to make it easier for the tree's root system to spread by digging a hole that is twice as deep and wide as the root ball. Backfill enough soil into the hole so the tree is sitting as deep as it was growing in its container. You don't want to put undue stress upon the tree by planting it too deep.

Plant Roots

Gently tease the roots apart and place the root ball into the hole, backfilling halfway with soil. Firm the area around the roots and saturate with water to release any pockets of air and settle the soil. Fill the remainder of the hole with soil and saturate the area again with water.

Mulch

To help reduce weeds and grasses, apply a 3- to 4-inch layer of organic mulch over the planting site. Pull back any mulch that is butting against the trunk and keep it several inches away.

Continued Care

Aspin Path

Annual applications of fertilizer and trimming keep the tree healthy.

Applying Fertilizer

Fertilize in springtime with an all-purpose blend specifically formulated for trees, such as a 19-8-10. Use approximately 1/2 pound of fertilizer for each 1-inch diameter of the trunk. Spread the fertilizer evenly under the tree's canopy and water in well.

When to Prune

The trees have a tendency to bleed whenever they are pruned, but the bleeding is normal and doesn't harm the tree. However, it's best to do any heavy pruning in winter. Trim off any crossing, broken, damaged or diseased branches back to a main branch. It is fine to prune off damaged or diseased branches anytime throughout the year.

Aspens are notorious for sending up root suckers that grow a good distance from the tree and pop up in the lawn. Cut these off at ground level by using a lawn mower or string trimmer. If left uncut, the suckers eventually grow into clones of the mother tree.

Since they are susceptible to a host of diseases, use sterilized pruning tools when pruning. Sterilizing your tool's blades is as easy as wiping them off with a damp cloth saturated in alcohol.

Common Aspen Diseases and Pests

Aspens are a bit needy when it comes to pests and diseases as the tree is susceptible to a variety of problems. As with any tree, providing it with the proper growing conditions and cleaning up fallen debris under the tree goes a long way in helping the tree remain healthy.

Problematic Pests

Depending on the culprit, some pests only create cosmetic damage and treatment isn't necessary. However, other more damaging pests may warrant a treatment of insecticides to control the problem.

  • Aphids: Aphids are a common insect found on the tree. The small pear-shaped insects feed on the plant's juices and cause distortion in the foliage, discoloration and their secretions cause the black fungus sooty mold to cover the leaves. Rarely are aphids life-threatening, but if outbreaks are severe, you can treat the tree with an insecticidal soap.
  • Oystershell scale: This is the most damaging sapsucker. The scale is dark gray to brown and attach along the tree's branches sucking out the juices. When infestations are heavy, the scale can weaken or kill an otherwise healthy tree. If the problem is small, you can scrape the insects off the affected branch. In large infestations, spray the entire tree with horticultural oil in springtime just as the tree is beginning to leaf out.
  • Tent-making caterpillars: Tent caterpillars create a fine mass of white webbing generally in the crotches of trees making the pest easy to identify. The caterpillars feed on the foliage and if the outbreak is severe can defoliate the tree. If the problem is severe, the caterpillar weakens and kills trees. Pick small infestations off and drop into a bucket of soapy water, but if the outbreak is large, spray the tree with Bacillus thuringiensis or spinosad as soon as you notice the problem will help in controlling the caterpillar damage.
  • Bark borers: They are susceptible to several bark borers with the poplar borer being the most common pest affecting the tree. The beetle lays its egg on the tree's bark, which then tunnels inside eating at the inside and weakening the tree and creating holes in the bark. Trees that are unhealthy and stressed are most susceptible to a borer attack. Insecticidal treatment is only successful when the pest is active and laying eggs on the outside of the bark, usually in late spring through summer. Saturating the bark and trunk with a product containing carbaryl should control the pest problem.

Fungi That Cause Cankers

Aspens are susceptible to several fungi causing deep cankers in the bark and trunk. The cankers are most damaging to younger trees, though some are lethal to older and established trees. Sooty bark canker can kill mature specimens and the fungus enters the tree through wounds and affects the heart of the tree. The canker creates a sunken area in the bark, which eventually strips off leaving a black ringlike appearance lining the trunk. There is no treatment for the problem and not wounding the bark is the only preventative measure.

Black canker is a slowly developing problem that usually isn't life-threatening and causes trunk deformity. Large, black conks form around the affected area and cause sunken areas in the trunk. Affected areas eventually die and can become susceptible to borer problems. Prevent the problem by keeping the tree healthy and the bark free of wounds.

Foliage Diseases

Several foliage diseases affect aspen trees and are preventable by cleaning up fallen debris under the tree where the disease overwinters. Ink spot fungus is most prevalent when springtime weather is rainy. Dark brown spots appear on the foliage in summer and increase in size eventually forming shot holes. In severe outbreaks, the tree can suffer leaf drop, which can affect the health of the tree. Spraying the tree with a copper fungicide in winter before the foliage sprouts helps control the problem.

Marssonina leaf spot fungi overwinter in fallen debris under the tree. Outbreaks are most common when springtime weather is warm and rainy. The fungus creates small dark brown spots on the leaves with a yellow circle. When the problem is severe, foliage drops from the tree. The best prevention is keeping the area under the tree clean and in the event of a severe outbreak, spraying the tree with a copper fungicide helps control the problem.

A Tree Worth the Attention

Due to their tendency to be a high-maintenance tree, aspens might not be everyone's cup of tea. However, when properly cared for, their tall and striking form makes them worth the extra care they require.

Aspen Trees