What comes to mind when I describe a tree that has heart-shaped leaves, is in the poplar genus, grows well in Colorado and turns a brilliant gold in the fall? Chances are, you didn’t immediately think of cottonwoods. Yet, when it comes to putting on a show, cottonwoods are every bit as spectacular as their close kin, the aspen. In many ways, they are the other iconic Colorado tree. Find any stream, pond, or irrigation ditch, and chances are there will be at least one cottonwood growing next to it. They are riparian trees, and prefer to have their feet wet.
All cottonwoods* are easy to identify. In addition to the characteristics noted above, they have deeply furrowed, gray bark, and often (especially in the wild) branch low along the trunk. Unlike other species, the Narrowleaf Cottonwood (P. angustifolia) has leaves that are longer and more lance-like. Cottonwoods can reach one hundred feet in height, with their canopy spreading just as wide. The trunk of a mature tree can grow to over five feet in diameter. Clearly, these are large trees requiring a lot of space. Due to their rapid growth, cottonwoods achieve their huge size quickly, but that speed comes at a cost. Their wood is brittle and subject to both breakage and rotting.
Cottonwoods are dioecious, meaning that the sexes are separate. Male trees produce pollen, a bane of allergy sufferers. Female trees produce catkins of unimpressive flowers that ripen into cascading clusters of egg-shaped fruit. This fruit eventually opens, releasing seeds borne in the a blizzard of cottony fluff that gives these trees their name.
A number of serious pests make their livelihood chomping on cottonwood trees. Various borers, the cottonwood leaf beetle, leaf miners, several species of sawfly… the list goes on and on. In the wild, for the most part, nature keeps these pests in check; trees grown in urban areas may require intervention on the part of the property owner.
When you consider the cost of irrigation, their huge size, the sheer abundance of “cotton” (or pollen) released each spring, the strong likelihood of broken branches, and the host of hungry insects that feed on them, it’s clear that cottonwoods make a poor choice for suburban yards. Happily, there are plenty of these beautiful trees growing naturally all over Colorado, so we don’t have to grow our own.
*There are several species of cottonwood in Colorado, and their taxonomy is a subject of debate. A separate species (Populus deltoides) grows in the East.