Cottonwood is a large tree with long, straight trunk and massive branches forming a rounded top.
Leaves are alternate, simple, 3–7 inches long, broadly triangular, abruptly pointed, with coarse teeth with tiny hairs, small glands at base of leaf blade; upper surface green, shiny; lower surface paler, smooth; leaf stalk slender, flattened.
Bark is thin, smooth, yellow-green when young; thick, corky, brown to gray, with deep, straight grooves and wide, flat ridges with age.
Twigs are stout, angular, yellowish to brown, smooth; pores prominent; bud at tip about ½ inch long, brown, with sticky bud scales.
Flowers March–May, male and female flowers in catkins on separate trees before leaves emerge; petals absent.
Fruits May–June, drooping catkins 5–10 inches long; capsules widest at base, about ¼ inch long, splitting into 2–4 parts; seeds brown, small, numerous, each with tuft of long cottony hairs.
Similar species: Six species in genus Populus have been found growing wild in Missouri. Cottonwood is the most common and widespread of the six. Swamp cottonwood (P. heterophylla) occurs naturally in swampy locations in the Bootheel; its triangular leaves are proportionally longer and have blunt tips. Bigtooth or large-toothed aspen (P. grandidentata) occurs only in a few locations in Missouri, and some of those might represent relict populations from when Missouri had a colder climate. Quaking aspen (P. tremuloides) is quite uncommon in our state, occurring only in a few locations in extreme northeastern Missouri. Lombardy poplar (P. nigra var. italica) and silver poplar (P. alba) are non-natives used in landscape plantings; both persist in groups of clones at old home sites.