Shady, ornamental cottonwood trees are a streetside staple in Denver. Learn how to identify different cottonwoods and how to help them grow strong and healthy in our Colorado climate.
Cottonwoods are one of the most common tree species in Denver and represent the largest native broadleaf trees in the state. These towering shady giants have become a major part of the local ecosystem, providing benefits for wildlife habitat, storm water uptake, recreation, and stream bank stabilization.
Cottonwood Tree Species Commonly Found In Denver
Part of the deciduous tree family, all four common varieties of cottonwoods in Denver grow to 45 ft or taller at maturity. Sought after for their shade and protection, these trees also have an ornamental quality. Their patterned bark and fall color makes them aesthetically pleasing as well as practical. Although they share some basic similarities, each cottonwood species is unique. Learn more about the different varieties and see if you can tell them apart on your next stroll around Denver!
The Eastern Cottonwood
This towering cottonwood variety will grow up to 100 ft tall! Fast growing and tolerant of alkaline soils, the Eastern cottonwood grows especially well along streams, rivers, and lowland areas. Due to its large size and penetrating roots, this cottonwood species is usually found in parks and public spaces rather than residential properties. The Eastern cottonwood adapts well to Denver’s wide range of weather in Zone 5. It is known for its ability to withstand drought but is vulnerable to ice damage during winter.
The Narrowleaf cottonwood
Surprisingly, the Narrowleaf cottonwood does not actually produce cotton. It does, however produce showy yellow leaves in the fall. This cottonwood variety has a vertical growth habit and will need vigilant trimming. It is admired for its graceful, willow-like leaves and grows well near streams or other bodies of water in zone 5.
The Plains cottonwood
As its name suggests, the Plains cottonwood thrives on Colorado’s plains, but also does well in the foothills and upper Sonoran areas. This variety grows nearly as broad as it does tall, spreading and growing into an open crown. The best way to distinguish a Plains cottonwood from another species is to look for its dark green triangular leaves. Its massive trunk can often grow to be 4-5 ft in diameter! The Plains cottonwood can be found growing all over the great plains region of the U.S.
The Lanceleaf cottonwood
A natural occurring hybrid of the Plains and Narrowleaf cottonwoods, this variety is known to be especially fast growing. Its thick branches grow upright and give the tree an overall spear shape. The slightly droopy leaves on the Lanceleaf cottonwood turn a vibrant yellow in the fall. Just slightly smaller than other cottonwood varieties, the Lanceleaf stands approximately 40-60 feet tall at maturity and thrives in Denver’s zone 5.
Unique Benefits of Cottonwood Trees
Not only are cottonwood trees adapted to thrive in the Denver riparian zones, near bodies of water such as streams or rivers, according to the Colorado State University Extension they also help the landscape in several ways. Benefits of cottonwoods include:
- Reducing erosion, with roots that hold soil in place
- Capturing and filtering sediment
- Providing wildlife habitat
- Slowing floodwater runoff
- Increasing water infiltration
Cottonwood Tree Diseases and Infestations
Cottonwoods might escape well known pests such as the EAB, but they are susceptible to a wide range of diseases including dieback, cankers, leaf spots, and powdery mildew. They are further plagued by insects like aphids, scales, and borers. Learn more about the threats that cottonwoods face in Denver and how to spot the first signs of disease or infestation.
Dieback is an injury that occurs when parts of the tree freeze and die off during the winter. These parts of the tree no longer produce new cells or growth. Tree limbs that suffer from winter dieback are vulnerable to breakage and falling without warning. This becomes a big liability in high winds and spring storms! Outward signs of winter dieback:
- Vertical cracks or holes where bark is missing
- Dry brittle wood that easy breaks off
- Large amounts of fungus
- Exposed smooth wood with almost no bark
- Sores or cankers on branch
Powdery Mildew is caused by a fungus that overwinters in buds and emerges during humid, warm weather progressively throughout the growing season. Symptoms include:
- Whitish-gray powdery mold or felt-like patches on buds, young leaves, and twigs
- Leaves may crinkle and curl upward
- New shoots are stunted
Slime Flux is a bacterial infection that infects the heartwood of cottonwood and elm trees. It gets its name from the foul smelling white slime that oozes out of the tree bark after infection. The slime also kills any grass around the affected area! Wounds in the bark provide the source of entry. Signs of infection include:
- Cracks in bark caused by internal pressure
- Oozing slime
- Dying bark
This fungal disease can cause serious damage to your cottonwoods. The Leucostoma kunzei fungus enters the tree through stressed or injured openings in the bark. As the fungus spreads, sunken cankers form. If left untreated, the cankers eventually girdle the branches, restricting flow of water and nutrients, and killing that area of the tree. This disease is slow to develop. Be on the lookout for signs of infection:
- cracked, dry or discolored cankers
- sunken, discolored bark
- oozing resin
Leaf Spot Diseases
A mainly cosmetic disease, Leaf Spot is caused by a variety of fungi, including Phyllosticta, Cercospora and Gloeosporium. The fungi create unsightly dark leaf spots. Symptoms normally appear in hot, humid weather conditions. Here’s what to look for:
- Abnormal discolored spots appear on the foliage
- Spots may drop out and leave holes in the leaves
- Severely infected leaves may fall from the tree
Aphids are found on almost all types of plants and a few species can cause plant injury. Some aphid species can curl the new leaves of some types of plant. Feeding aphids excrete honeydew, a sticky fluid that can attract other unwanted pests.
Scale insects can be difficult insects to recognizable because female and juvenile scales do not have wings, legs or recognizable body parts. The scales are minuscule and hide on the underside of leaves, sucking fluids from plants. Serious damage will occur only if a large number of scales develops, because they excrete honeydew that leaves a harmful residue on plant growth.
Cottonwood Trees and Wildfire Prevention
The Colorado State University Extension recognizes cottonwood trees for being especially at risk of damage due to wildfires. Although they can usually survive low-level wildfires, cottonwoods are not highly fire resistant and do not have the ability to stand up to high-intensity fires. The CSU extension recommends these steps to help Denver residents protect cottonwoods from the threat of fire:
- Do not stack branches or woody material under trees or large shrubs. This increases wildfire intensity near the trees, which can damage or kill them.
- Remove invasive woody plants from underneath cottonwoods.
- Remove ladder fuels by pruning off tree branches from ground level up to a height of 10 feet above ground, or up to one-third the height of the tree, whichever is less.
- Thin less-desirable trees to decrease competition and increase vigor of remaining trees in the stand.
- Leave enough young cottonwood growth for habitat and tree regeneration.
Cottonwood Tree Care And Treatment Options
Keeping up with tree care is an important investment in your tree’s longevity. Here are our top recommendations for annual care that will help your cottonwoods thrive:
- Watering: proper watering is an overlooked but very important tree care practice. Some cottonwoods are prone to fungi related diseases if their soil becomes too saturated with water. Leaf scorch and other damage can occur if there isn’t enough water. We can provide expert consultation for how much water you should be giving your cottonwoods, and how often.
- Fertilization: for many cottonwood species, even the ones commonly found in Denver, the high alkaline soil can lead to problems with chlorosis. The best way to neutralize this threat is with the right fertilizer application. Balancing the soil and getting your trees the nutrients they crave will keep them strong and healthy.
- Tree trimming: annual pruning and trimming helps cottonwoods keep their beautiful shape and foliage.
- Insecticides: although cottonwoods are not prey to destructive pests like the EAB, an insecticide may still be applicable for large infestations of scales or aphids. Pest control helps your cottonwoods maintain their beautiful foliage.
If you are concerned that your cottonwood is showing signs of disease or infestation, contact one of our certified arborists immediately! At Fielding Tree & Shrub Care we take a four-step approach for treating cottonwood trees:
- Step 1: A complimentary on-site disease diagnosis of your trees and/or shrubs. Sending a picture in advance helps speed things up!
- Step 2: We work to cure the disease or at least slow it down with the most viable treatment option available for your tree or shrub’s unique situation.
- Step 3: A proactive approach is implemented to protect your existing trees from disease or infestation.
- Step 4: Unfortunately, sometimes trees and shrubs die, even with our best, safest, and most proven treatment solutions. If your tree or shrub dies, we try to remove the dead plant as soon as reasonably possible. This keeps your property safe from unnecessary infestations and unsightly rotting trees or shrubs.
To learn more, schedule your complimentary on-site diagnosis today! One of our arbor experts will walk you through our four-step approach for treating cottonwood trees and create a custom plan for the unique needs of your trees!