Narrowleaf Cottonwood (Populus angustifolia) aka Willow-Leaved Poplar

From left to right, a plains cottonwood leaf, a narrowleaf cottonwood leaf, and a lanceleaf cottonwood leaf.

About All Cottonwood Trees

Cottonwood trees are a common street tree and are the largest native broadleaf trees in Colorado. Their wide-spreading roots are helpful in reducing erosion and slowing floodwater runoff. They are also a popular habitat for various forms of local wildlife.

Cottonwoods get their name from the white fluff that floats off the trees; these resemble cotton and contain the tree’s seeds.

Many cottonwoods are considered “messy” trees, partially due to the snow-like seeds, but also because they have brittle branches that break easily (and often), as well as suckers that pop up everywhere.

Though some cottonwood trees can live over 100 years, many in urban areas only have a lifespan of around 30 years.

Many communities in the Denver metro area have banned the planting of cottonwood trees because of their large size and invasive root systems. On smaller properties, they just don’t have enough room to grow. Their roots are strong and can break into plumbing lines (especially if there already is a crack) and can contribute to damaging the foundations of buildings. Learn more about common tree root problems in our blog post.

Neighborhoods that do allow cottonwood trees to be planted often insist on the male trees, as they do not produce seeds (and, therefore, no messy cotton fluffs). However, male cottonwood trees are often the ones that negatively impact allergy sufferers the most.

Because cottonwood trees are extremely fast growers (some varieties can grow 6 feet a year), the wood is more brittle than other types of trees. With heavy snowfall, this can contribute to significant branch and limb breakage on cottonwood trees. Proper pruning can help to eliminate any broken or dead branches, but cottonwood trees (like all trees) should never be topped.

Cottonwood trees are not particularly fire-resistant, but their roots are important in keeping soil together to reduce runoff and slowing floodwater. The Colorado State Forest Service has produced this document that has recommendations on how to reduce wildfire risk in cottonwood groves. 

Narrowleaf cottonwoods in the fall.
By Calibas – Own work, CC BY-SA 3.0

About Narrowleaf Cottonwood (also known as Willow-Leaved Poplar)

The narrowleaf cottonwood (also called willow-leaved cottonwood) is native to the Evergreen area of Colorado, where it’s found growing with aspen along or near rivers or streams. Of the three types of cottonwood trees found in the Evergreen area, narrowleaf cottonwoods naturally grow at the highest elevations – usually between 5,000 and 8,000 feet.

Narrowleaf cottonwood will grow a bit taller than most aspen; from a distance, you can see these cottonwood groups standing above the aspen, like a bulge of foliage out of the larger aspen grove.

The narrowleaf cottonwood does not produce the white fluffy seed that cottonwoods are known for. The leaves are willowy, long, and narrow, which is another way to distinguish the narrowleaf cottonwood from other types.

Pest & Diseases to Watch For

As with other poplars, narrowleaf cottonwood is susceptible to aphids, beetle borers, scales, thrips, anthracnose, canker, crown rot, mistletoe, and sooty mold. However, it’s marssonina leaf spot, cytospora canker, and slime flux that are the most serious problems.

Marssonina Leaf Spot

During periods of above-average rainfall and higher humidity, the narrowleaf cottonwood is prone to marssonina leaf spot.

The best long-term treatment for this fungal disease is proper pruning to allow better airflow through the leaves. For trees that have been severely affected in previous years, foliar spraying with fungicide at budbreak will reduce infection.

See this page from Colorado State University for more information.

Cytospora Canker

Cytospora canker is the biggest invasive threat to cottonwood trees, partially because the wood is already weaker than other types of trees.

Cytospora canker is a tree disease caused by several species of Valsa and Leucostoma fungi. The fungus attacks and kills the bark, causing dead or dying areas called cankers. In many cases, cankers can extend around an entire branch, cutting off water and nutrients to the rest of the branch and killing it. In severe cases, it can kill the whole tree.

Read the rest of our post on cytospora canker here.

Slime Flux

Also called bacterial wetwood or bacterial slime, slime flux is exactly what it sounds like – a frothy slime that oozes out of trees, and then dries, leaving a white scum. The bacteria enters through cuts or wounds in the bark, such as from improper pruning cuts, lawnmower blad marks, or animal scrapings. One way to recognize slime flux is that it kills any grass it touches. Inside the tree, the bacteria eats and digests the wood, creating an internal pressure. This pressure is eventually relieved as the slime oozes through a crack in the tree’s bark.

Learn more about slime flux or bacterial wetwood from this publication by the Colorado State University Extension.