Cottonwoods are one of the most common tree species in Denver, hence why cottonwood tree care is so vital in Colorado. These towering shady giants are a major part of the local ecosystem, providing benefits for wildlife habitat, stormwater uptake, recreation, and stream bank stabilization.
Below, we’ve assembled important information you’ll need to provide the most complete cottonwood tree care.
How to Identify a Cottonwood Tree
When it comes to identifying a Cottonwood tree, there are many things to look for. They are known for their massive trunks, rising crowns (to 60 to 100 feet in many cases), and releasing cotton. They are easily identifiable by their bark, flowers, leaves, and seeds.
- Bark: Young cottonwood trees typically have shallow furrowed, smooth, yellow green to light grey bark. The more mature trees typically have deeply furrowed, rough, solid grey bark.
- Flowers: The male and female flowers differ slightly from each other, but all cottonwood tree flowers are contained in catkins (cylindrical flower clusters with little to no pedals, usually pollinated through wind). The male flowers are typically red and yellow, and the female flowers are green.
- Leaves: Triangular with a heart-shaped base that tapers into a long, pointed tip.
- Seeds: The seeds are small and brown and usually attached to a white cotton-like thread that is easily carried away with the wind.
Sought after for their shade and protection, these trees also have an ornamental quality. Their patterned bark and color in fall make them aesthetically pleasing. Although they share some basic similarities, each cottonwood species is unique. The two main species of cottonwood trees that are native to the front range of Colorado and the Denver metro area are narrowleaf cottonwoods and plains cottonwoods.
Narrowleaf Cottonwood Trees (Populus augustufolia)
- Bark: Yellow-green and smooth on young trees; thick, gray-brown and furrowed with interlacing ridges at maturity.
- Leaves: Broad-leaf foliage is shiny green with a pale underside; narrow and 2 to 3 inches long; lance shaped with a fine, serrated edge and a pointed tip.
- Fruit: Light brown, hairless fruit; inch long; many broad, egg-shaped capsules that mature in the spring, then split into two parts containing many cotton-like seeds.
- Elevation: 5,000 to 8,000 feet.
- Height: Up to 60 feet.
- Habitat: Moist soils along streams; can often be found with willows and alders in coniferous forests.
- Relation to Fire: Severe fires can easily kill both young and mature trees. Young trees are able to sprout from roots and/or branches after a fire. (Colorado State Forest Service)
- Suckering: Notorious for prolific root shoots.
- Lifespan: Around 100 years, max.
Plains Cottonwood Trees (Populus deltoides)
- Bark: Green-yellow and smooth while young; dark gray, thick, rough and deeply furrowed at maturity.
- Leaves: Broad-leafed foliage is glossy and yellow-green; 3 to 6 inches long, 4 to 6 inches wide; toothed margins.
- Fruit: Inch long with capsules containing 3 to 4 valves; many tiny, cotton-like seeds inside valves.
- Elevation: 3,500 to 6,500 feet.
- Height: 50 to 100 feet.
- Habitat: Found in floodplains, bordering streams, near springs and in moist woodlands; pure stands or with willows.
- Relation to Fire: Generally killed by fire; very poor sprouting response. (Colorado State Forest Service)
- Lifespan: Commonly between 100 and 250 years old, with maturity attainment around 75 years of age.
The Cottonwood’s “Cotton”
Early- to mid-June is when female cottonwoods tend to release their “cotton,” making lawns from Fort Collins to Colorado Springs, and everywhere in between, look like a light snow fell. If you have any level of seasonal allergy, you’re probably aware when the cottonwoods are in season.
Contrary to popular belief, people are not allergic to the cotton produced by female cottonwoods. They are, however, allergic to the pollen produced by male cottonwoods. The reason people assume they are allergic to “cotton” is because at the same time numerous grasses and flowers are generally in full bloom, and the cotton is simply a visual cue.
Fun fact: cottonwood trees are also dioecious, which means they have male and female flowers on different trees. Male trees are not capable of producing the cotton, which is why many neighborhoods that do allow new cottonwood trees restrict planting to only male cottonwood trees.
Is Planting Cottonwoods Illegal?
Speaking of planting restrictions, this may surprise you, but cottonwoods have been prohibited for use in Denver’s public rights-of-way for many years. Basically, they are a massive and invasive tree species and have a habit of wanting to share your living space, and your utilities.
Incoming water sources aren’t the only thing cottonwoods love. They search for any water-type source, so your outgoing sewer line is often just as easily the victim of thirsty cottonwoods.
Furthermore, they have an incredibly strong root system, which poses another threat to homeowners. Cottonwoods planted too close to structures, namely basement walls and garage foundations, will break through the concrete walls over time. We highly recommend checking with your local forestry service, HOA, and/or city authorities to see if cottonwoods are permissible for new plants. If they are, complete cottonwood tree care is of the utmost importance for it to grow healthy and happy.
Benefits of Cottonwood Trees
Not only are cottonwood trees adapted to thrive near Denver’s bodies of water such as streams or rivers, but they also help the landscape in several ways. The benefits of cottonwoods include:
- Reducing erosion, with roots that hold soil in place
- Capturing and filtering sediment
- Providing wildlife habitat
- Slowing flood water runoff
- Increasing water infiltration
Cottonwood Tree Diseases and Infestations
Like many other things in life, these trees can also face some hardships. Disease, infestation, and being at risk for wildfire damage are the most common. Cottonwoods might escape well known pests, but they are susceptible to a wide range of diseases and can be further plagued by insects like aphids, scales, and borers.
Below, we provide knowledge about the threats that cottonwoods face in Colorado 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. Particularly, 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 are:
- Vertical cracks or holes where bark is missing.
- Dry brittle wood that easily breaks off.
- Large amounts of fungus.
- Exposed smooth wood with almost no bark.
- Sores or cankers on branches.
This fungal disease can cause serious damage to your cottonwoods. Fungus enters the tree through stressed or injured openings in the bark. More fungus spreading causes sunken cankers to form. If left untreated, the cankers eventually girdle the branches, restricting flow of water and nutrients, and killing that area of the tree. Since it is slow to develop, be on the lookout for signs of infection:
- Cracked, dry or discolored cankers
- Sunken, discolored bark
- Oozing resin
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.
Bacterial wetwood is a bacterial infection that affects the heartwood of cottonwood and elm trees. The infection produces a foul-smelling white slime that oozes out of the tree bark. 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.
Leaf Spot Diseases
A mainly cosmetic disease, Leaf Spot is caused by a variety of fungi. The fungi create unsightly dark leaf spots. Symptoms normally appear in hot, humid weather. 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 plants. Additionally, feeding aphids excrete honeydew, a sticky fluid that can attract other unwanted pests.
Scale insects can be difficult insects to recognize because they do not have wings, legs, or recognizable body parts. Note that these insects 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.
If you are concerned that your cottonwood is showing signs of disease or infestation, contact one of our certified arborists immediately at Fielding Tree Care!
Wildfire Prevention for Cottonwood Trees
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. Here are recommended steps to prevent the threat of wildfire damage:
- Do not stack branches or woody material under trees or large shrubs.
- 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 out the 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
Keeping up with cottonwood tree care is an important investment in your tree’s longevity and protects your property. Cottonwood trees are heavy and can be dangerous if neglected, which is why we want to share this information with you.
Specifically, these trees grow at a rapid pace, as much as six feet per year for new seedlings. With Colorado’s heavy spring snows and high winds, cottonwood trees can be susceptible to breakage. A good rule of thumb is to never “top” your cottonwood tree. This practice is extremely harmful to your tree’s overall health, and it doesn’t provide any specific benefit to the branches below. In fact, topping your tree may create a maligned canopy that doesn’t allow your cottonwood tree to grow as designed.
Cottonwood Tree Care Recommendations
Here’s a list of 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. Additionally, the roots need oxygen, and they get this by the soil drying out between waterings. On the other hand, leaf scorch and other damage can occur if there isn’t enough water. Generally, you should water your mature cottonwood tree every couple of weeks, but this can vary greatly depending on the season, the maturity of the tree, and it’s location.
We can provide expert consultation for how much water you should be giving your cottonwoods, and how often.
- Fertilization: For many cottonwood species, the high alkaline soil can lead to problems with chlorosis. The best way to neutralize this threat is with the right fertilizer. Balancing the soil and getting your trees the nutrients that 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 generally not prey to destructive pests, an insecticide may still be applicable depending on the size of your tree and level of infestation.
Hire an Arborist in Your Area
Cottonwood tree care in Denver and the surrounding metro area is what we do best. Our team loves seeing big, beautiful cottonwoods thrive and get the care they need. Contact us for a free consultation or estimate.
Why Your Cottonwoods are Dying in the Denver Area
Cottonwood trees are dying across the Denver area. Native cottonwoods are highly resilient and thriving in many Denver neighborhoods while hybrid cottonwoods are slowly dying in some of Denver’s older neighborhoods.
A late freeze can cause a significant tremor to a tree’s vascular system and shock a tree back into hibernation. Then there’s Canker, as mentioned above, which affects stressed trees, which would include cottonwoods that have experienced a late frost. The best route of action is prevention by eliminating lawnmower wounds, drought conditions, and improper pruning to help prevent Canker from affecting your cottonwoods.
If your cottonwood hasn’t started blooming by the Fourth of July, it’s most likely curtain time for your tree. Try watering around the base up to five feet away from the trunk. This care for cottonwood trees may kickstart a late spring growth. Unfortunately, the tree nearing the end of its natural life could be a possibility. Life expectancy of hybrid cottonwoods is around thirty years. In older Denver neighborhoods, like Greenwood Village or Cherry Creek, your cottonwoods may be hitting the end of their life cycle.
Providing Complete Cottonwood Tree Care
To provide the best care for your cottonwood trees, we recommend:
- Mastering cottonwood tree identification.
- Learning the rules in your area pertaining to cottonwood trees.
- Understanding how to identify, prevent, and treat diseases and pets that affect cottonwoods.
- Practicing correct wildfire prevention.
- Providing the correct watering, fertilization, trimming for cottonwood trees.
Whether your cottonwood needs a little ‘haircut’ or it’s dead and needs proper disposal, we will treat your tree and your property with the utmost respect. That’s why we’re Denver’s favorite arborist for cottonwood tree care! Schedule your complimentary on-site inspection of your cottonwood tree today.