Aspen Trees: Common Problems & What To Do

Aspens are the prevailing tree in our part of Colorado and are a favorite among many residents in Evergreen and the surrounding areas. Unfortunately, aspen trees are also susceptible to a wide range of problems, like injuries from deer or elk rubbing or an onslaught of pests and diseases.

But that doesn’t mean aspens are a poor choice for your landscape. It just means they might need a little extra care to ensure they thrive. Here are some of the most common problems our aspens face among the Front Range and what you can do about them.

Key Takeaways:

1. Aspen trees in Colorado face numerous challenges, including pests, diseases, and environmental stressors.
2. Soil quality is crucial for aspen tree health, especially in residential landscapes.
3. Common issues include insect infestations, fungal diseases, and animal damage.
4. Proper care, including irrigation, pruning, and fertilization, can help maintain aspen tree health.
5. In some cases, alternative tree species may be better suited for certain Colorado landscapes.

All About Aspen Trees

The aspen (Populus tremuloides) is an iconic keystone tree species, and its mixed understory supports a diverse ecosystem throughout the higher elevations of the Rocky Mountains.

Aspen trees naturally grow in groves, rather than as individual trees. An aspen grove will have trees at all stages of life, from sprouting shoots to mature specimens, all connected underground by a large system of interconnected roots. In fact, one of the largest known living organisms in the world is the Pando aspen grove in the Fishlake National Forest.

The Problem with Aspens in Your Landscape

In an ideal setting, aspens remain healthy and resistant to damage while spreading roots and sprouting new trees from them. But in residential or commercial settings (such as the landscape surrounding your home or business), problems can severely affect aspen tree health and appearance.

For example, the aspen is commonly affected by:

• Insect pests
• Fungal infections
• Animal browsing and injury
• Drought and heat
• Fire suppression and conifer growth

However, the biggest problem aspens face in an ornamental landscape is the soil it’s planted in.

Aspen Issue #1: Poor Soil Quality

Many aspens are planted in the landscape surrounding a home (often within 10′-15′ of the foundation). The problem is that there’s no good soil in that area.

When homes are built, we excavate to put the foundation in. The area around it is then backfilled with the mix of material that was dug out of the hole to put in the foundation. This mixture isn’t topsoil and it’s certainly not what aspens need (it’s nothing like what’s found in the drainage areas of mountain climates where aspen trees thrive).

Aspen Issue #2: The Need to Multiply

But soil problems are just the beginning. Aspens need to clone themselves to stay healthy (that’s why they naturally grow in groves). They do this by sending out tons of “suckers,” many of which end up in spots where homeowners don’t want them.

So, we cut them out or let the elk and deer eat them to the ground. Either way, the aspen can’t grow to put out more leaves that would help them combat stressors, such as insects and disease.

As a result, aspens have the most problems of all our local plants. In fact, they seem to be perpetually afflicted by a new problem, no matter how many or how often treatments are applied. And it all starts with how and where they’ve been planted. When something starts out in a bad way, rarely will it improve.

Can I improve the soil / environment for my aspens?

Assuming your aspen has already been planted, there are two things you can do to improve its chance of success.

1. Have a tree care professional apply a soil injection that includes a blend of fertilizers and nutrients to improve soil health, as well as a systemic product to get rid of insects and mites.
2. Protect the young shoots so the aspen can help itself. Sometimes it’s as simple as not cutting them out but you can also put up a physical barrier to prevent deer or elk browsing.

Aspen Issue #3: Tree Insects

Several insects damage aspens, causing various problems for these trees. Common aspen tree pests include:

• Aspen leafminer: Causes leaf rolling and eating of foliage, primarily affecting the tree’s appearance and photosynthetic capacity.
• Caterpillars: Various types that feed on leaves, potentially causing significant defoliation if populations are high.
• Aphids: Secrete honeydew, leading to black sooty mold on leaves and beneath the tree, which can be unsightly and reduce photosynthesis.
• Spider mites: Often mistaken for fungal diseases, especially in drier locations. They feed on leaf cells, causing stippling and discoloration.
• Poplar borers: Damage the trunk by creating holes. You’ll see pieces of wood around and below the hole. These holes may leak brown resin (aspen sap) that runs down the trunk.
• Oystershell scale: Appear as tiny black dots on the bark. When viewed with a hand lens, they resemble miniature oyster shells. They feed on bark tissue, potentially weakening the tree.

Proper aspen tree maintenance and regular inspections can help catch these pest problems early, allowing for more effective treatment and better overall aspen tree health.

Treatment for Insect Pests

If you plan on spraying an insecticide, such as Bt (Bacillus thuringiensis), follow all directions to ensure you are spraying at the right time to reduce insect populations. Early in the growing season you can also wrap a wide strip of flexible paper around the trunk and coat it with a sticky barrier to trap insect larvae that crawl up into foliage.

To Spray or Not to Spray?

You may find that spraying entire large trees is unfeasible, or that you require a tree care professional to spray (which is what we recommend for larger trees as it’s very difficult to adequately cover the entire tree without specialized spray equipment).

If you don’t want to spray trees, remember that during their growing season, aspens regenerate leaves quickly, so if you can, endure some leaf damage knowing it will be replaced with new leaves.

A Certified Arborist can also apply insecticide sprays at the end of the season (when a systemic treatment wouldn’t be effective) and/or when the population of a specific pest needs to be quickly reduced to avoid extensive aspen damage.

Aspen Issue #4: Fungal Problems

Aspens are affected by a variety of fungal problems that can impact their health and appearance. Common fungal issues include:

• Leaf spot: Causes discolored spots on leaves, potentially leading to early leaf drop.
• Aspen leaf blight: Can defoliate the tree, making it look unhealthy and reducing its aesthetic appeal.
• Leaf rust: Causes orange or yellow spots on leaves, sometimes leading to defoliation.
• Cytospora canker: A common aspen tree problem that causes patches of bright orange bark, often oozing brown liquid. Over time, the affected bark falls off, revealing dark brown or black areas beneath.

Most of these diseases can defoliate the tree if severe enough. While it typically requires multiple incidents of leaf blight and dieback to kill a tree, annual occurrences can significantly impact the tree’s health and ruin its appearance. These fungal issues can reduce the desirable characteristics of aspens, such as their delicate branches and green, fluttering leaves.

What can you do about fungal problems?

To prevent and manage fungal issues in aspen trees:

• Irrigate only in early morning, and do not wet foliage with irrigation
• Maintain moist, but not wet, soil – water regularly during dry spells, including in winter
• Clean up and discard diseased leaves, twigs and branches that have been affected by any kind of fungal problems, especially in fall
• Prune out diseased twigs and branches, and repair damaged or broken areas ASAP
• Plant trees with enough space between them to allow for air and sun to reach them
• Avoid trunk damage from mowers and string trimmers
• Fertilize to ensure adequate nutrient levels, but don’t over-fertilize
• Spray with fungicide when necessary

Treatment for Fungal Problems

If you choose to spray your aspens with fungicides, do it early in the season (at bud break when leaves are about the size of a dime, which is usually in late May to early June). We recommend a second application about 12 to 14 days later. Usually, two (or sometimes three) applications per season will control fungal leaf diseases on aspen.

These sprays will keep new infections from establishing. Sprays cannot repair existing damage or “cure” a fungal disease, so removing damaged twigs and leaves is an important part of keeping down the level of fungal spores that can spread the infection.

If you’re unsure what or when to spray, check with the local experts at LAM Tree Service to make sure you use the best fungicide for your specific leaf disease.

Aspen Issue #5: Animal Damage

Animals can seriously damage aspen trees, primarily by eating the shoots of young suckering trees that sprout from the aspen’s root system, as well as by eating bark.

In an undisturbed or balanced forest, trees regulate new growth and maintain trunk spacing and sunlight levels. But when animals over-browse aspen stands, they reduce understory species, damage tree bark, and leave open areas within the stand. Evergreen species such as conifers take advantage of these openings to quickly grow, shading out the surrounding aspens and causing them to decline.

We also see problems with deer and elk rubbing their antlers against the bark, causing large wounds and even killing the tree altogether. Damage is particularly prevalent during the elk rut which usually begins in the fall (roughly mid-September through mid-October) although it sometimes lasts through November.

How can you prevent animal damage to aspens?

In residential properties, a strong barrier fence is the best defense against browsing deer and elk.

Aspens can also be wrapped with a tough plastic that lessens the amount of antler damage (although a large bull elk can knock down a smaller aspen, regardless of whether or not it’s wrapped). Wraps should be removed in spring.

Aspen Issue #6: Heat & Drought Stress

Aspens like cold winters, warm summers, and moist soil, which is why you typically find them in the higher elevations of the Front Range. Lower, valley conditions are usually hotter and drier, stressing the trees and making them more susceptible to fungi and insect damage.

We see so many aspens planted in a dry, south-facing landscape where they’re exposed to conditions that are well outside their natural environment. It’s no wonder they don’t do well!

What to Do in Hot, Dry Conditions

To help aspens thrive in challenging conditions:

• Monitor soil moisture levels to ensure your aspens have enough water
• Water aspens in winter (when there’s no snow on the ground) to keep them hydrated
• Spread mulch on the soil around trees to retain soil moisture; just be sure to keep mulch away from trees’ trunks
• If you store cut firewood, keep the wood away from the aspens and cover the pile with a waterproof tarp
• Bury the edges of the tarp in soil to prevent any insect larvae from crawling out and reaching the aspens

The Benefits of Wildfire for Aspens

Aspens respond to forest fires by sprouting suckers in the new, open burn areas. If a fire has burned a stand of trees, sprouts from their underground root system appear soon after to refill the stands. Fire also removes any conifer species that have sprung up into the stand and threaten to shade out the aspens.

So, while fire mitigation is an important part of living in this beautiful part of Colorado, wildfires are not necessarily a bad thing for aspens.

h2 Alternatives to Plant Instead of Aspen Trees

If your property doesn’t have conditions that are best for aspen groves, but you want to approximate the feel of them, consider alternatives such as:

• American Mountain-Ash (Sorbus americana): Flowers in spring and produces bunches of red berries. Deciduous leaves have bright fall color.
• Oakleaf mountain ash (Sorbus x hybrida): Flowers and produces berries, but has leaves reminiscent of an oak tree.
• Ash Trees (Fraxinus sp.): Have similar pinnate leaves to the sorbus, and smooth gray bark and fall color.
• River Birch (Betula occidentalis): Has smooth reddish bark and leaves similarly shaped to aspens, that turn gold in fall.
• Mountain Alder (Alnus tenuifolia): Likes moist soil, has green leaves with a white underside, and smooth gray bark.
• Serviceberry (Amelanchier x grandiflora): Has a similar look to an aspen and fits well in tight spots in the ornamental landscape.

Or check out our full list of aspen alternatives for Colorado landscapes.

Make sure to check with your local nursery to find out if these alternatives are suitable for your garden, and what their maintenance needs will be.

Protecting Your Aspen Trees

Aspens are beautiful trees that give the Rocky Mountains part of their character and appeal. However, in the Evergreen area and at lower elevations they’re prone to a range of problems that affect their appearance and health.

Maintaining the health of aspens in our area will likely require professional assistance from a Certified Arborist. If you’re concerned about your trees, please give us a call at 303-502-2188 or request an inspection.