How to Care For Your Aspen Trees

Colorado has a higher percentage of the species growing within its borders than any other state. Aspens thrive in higher elevations. Planting these trees in Denver with our heavy clay soils and hot lowland weather takes its toll, setting up Aspens for a host of troubles. However, homeowners continue to plant these trees because they are so Colorado. They provide shade in summer, color in autumn, and beautiful white bark in winter. Below is a short guide to Aspen tree care in Denver.

Keeping Aspens Happy in the City

Nearly all Aspens for sale in Denver came from the mountains for transplanting. So, the best way to keep your trees happy is to recreate the environment from which they came. Aspens like slightly moist and loose soils, so the first step is to plant them in a berm of sandy loam soil high enough to keep their roots out of Denver’s clay soil. The raised area should be mulched and attached to a yard’s irrigation system to keep the soil slightly damp. The berm should be away from any foot traffic to avoid soil compaction. Even after careful preparation and care, Aspen only live about 25 years in a city landscape.

Aspen Shoots

Homeowners with Aspens on their property probably notice that the trees send little shoots up from the ground. Each node becomes another Aspen tree if not cut down. These shoots originated from the trees’ roots and are considered clones of the parent plant. You can mow them down when needed. However, property owners should not use chemicals to kill these plants because they remain part of the original tree. One Aspen grove in Utah is 106 acres in size, 80,000 years old, and considered the largest living organism on earth.

Aspen trees’ bright white bark looks good in all seasons. Their white trunk-covering is unique because it performs photosynthesis for the tree. Wildlife in the mountains, sometimes during brutal winters, have trouble finding food. Since the white bark produces sugars during the winter, it becomes a last resort food source for hungry deer and elk.


Common Aspen Tree Pests in Denver

According to the Denver Post article titled “Got Aspens in your yard? Here’s how to keep them happy, healthy“, here is a list of the tree’s most common disorders and how to remedy them.


aspen tree aphids

Aphids are a nuisance with their honeydew blackening leaves in the late summer and fall. Also, this sticky substance falls on cars, furniture, and other yard fixtures. Applying dormant oils in the winter kills the insect’s eggs. Insecticide soaps and systemic injections control Aphids during the growing season.

Oystershell Scale

aspen tree oystershell scale

These insects look like oyster shells that develop on the bark of trunks and limbs of Aspen, Ash, Oaks, and Poplar trees. These scales suck the fluids of cells underlying the bark, often killing the surrounding tissue. In high populations, the scale causes dieback and some cases, tree death. When the eggs hatch in late spring, their nymphs or crawlers have legs and search for feeding spots. Surviving crawlers molt into adults, losing their legs, and become scales. Since the Colorado State Service has not found any males in the state, this insect probably reproduces asexually.

Applying dormant oils in the winter kills the eggs of this insect. Using summer oils and crawler sprays control the insect in its nymph stage, but these treatments are not effective once they turn into scales. Crawlers are small and hard to see, making it hard to time spraying. Systemic bark sprays and drenches control this insect, but these products are only available to licensed commercial applicators. Homeowners can scrape off the scales using plastic utensils, but they need to proceed with care to avoid damaging the tree.

Poplar Twiggal Fly

The Poplar Twiggall Fly produces lumpy twigs on Aspens. These small, dark flies insert eggs into new twigs in the spring, where they grow protective woody galls. These insects do not harm the tree, but some find the swollen galls unattractive. The fly is native to Colorado, and sometimes it is best to let local birds and parasitic wasps control this tree pest.

Below are other pests that attack Aspens.

  • Aspen Leaf Miners
  • Sawflies
  • Western Tent Caterpillar
  • Poplar Borer

Common Aspen Tree Aliments in Denver

Fungi and mineral depletion are responsible for many maladies affecting Aspens in Denver. They include



denuded Aspens in Denver

This summer, our arborists are seeing denuded Aspen trees which usually means Cytospora cankers. Denver experienced 232 days between snowfalls, tying a record dating back to the 1880s. Winter drought stress makes thin bark trees such as Aspen, Cottonwood, Mountain ash, Willow, and Elm susceptible to canker diseases. Aspens in Denver are being hit hard by Cytospora this year.

A canker is an inverted bark blister full of fungi that forms on branches and trunks. Affected trees’ leaves turn yellow and may drop to the ground. For treatment, we remove any limbs with cankers from the tree. Trees with large cankers on their trunks will eventually die and need removal. Prevention is the best tack to keeping trees canker free because no chemicals are available to control the fungi. Homeowners should water and fertilize their Aspens throughout the year, which helps the trees’ natural defenses to ward off this disease.

Marssonina Leaf Blight

aspen tree marssonina leaf blight

The Marssonina fungus grows in the yard debris leftover from the previous year. Fungus spores land on Aspen leaves, forming yellow-ringed dark spots that gradually enlarge and merge. With light infections, the tree sheds infested leaves, but in extreme cases, tree defoliation occurs. To prevent the growth of this fungus, rake up leaves in the fall. Also, thinning of Aspen stands improves air circulation making the trees more resistant to this fungus.

Iron Chlorosis

aspen iron chlorosis

Denver’s clay soils make it difficult for Aspens to take up enough iron to produce chlorophyll, which turns the trees’ leaves yellow. Drought, root stress, and compacted soils can cause Chlorosis, which can kill trees. The injection of chelated iron products controls this disease.

Below are other Aspen tree ailments commonly found in Denver.

  • Leaf and Shoot Blight – The Leaf and Shoot fungus curls the outer stems of Aspens and produce dark black spots on foliage.
  • Ink Spot -The disease progresses across the leaf in a continuous line of attached brownish circles.
  • Septoria Leaf Spot – The fungus produces randomly arranged circular and tan spots on leaves. The blemishes may join to create irregular brown and black splotches across the leaf surface.
  • Leaf Rust – This fungus does not kill mature Aspens but can be harmful to newly planted saplings. Leaf Rust produces small yellowish growths clinging like tiny barnacles to the leaves.

5 Ways to Prevent Aspen Disease in Denver

Homeowners can improve the health of their Aspens by following these tree care tips

  1. Raised Berms

    Aspens grown in loose, slightly damp soils avoid many of the issues described above.

  2. Pruning

    Homeowners with Aspens diseased branches should hire a tree service company for pruning.

  3. Dormant Oils and Systemics

    These products control many of the insect pests and ailments that plague Aspens planted in Denver.

  4. Watering & Feeding

    Keeping a tree’s root system healthy by watering and feeding, even in the winter, reduces stress and is probably the best way to keep Aspens healthy.

  5. Keep Trees Dry

    Excess moisture on the leaves and branches spread fungus and spores. Aim sprinklers so they do not soak the trees.

Contact the tree experts at Ross Tree Service to schedule inspection, treatment, trimming, and other services for the Aspen trees on your property. Call 303-871-9121 or click here to fill out a tree service request form.