Plants medium to often large trees, sometimes suckering and colonial. Twigs often with prominent lenticels, these mostly circular to oval, sometimes appearing slightly raised, usually lighter than the surrounding surface. Winter buds lateral and usually also terminal, ovoid to ellipsoid, with 3–10 more or less resinous-sticky scales. Leaves mostly long-petiolate, the petiole lacking glands. Stipules usually minute (sometimes prominent on sucker shoots), shed early. Leaf blades usually heterophyllous (with leaves developing in the spring from the winter buds usually differing in shape from those produced later in the year), triangular to broadly ovate or occasionally nearly circular (narrower elsewhere), less than 2 times as long as wide, with 3 or 5 main veins; sometimes with 1–6 glands at the base; the margins faintly to strongly and finely to coarsely toothed, the teeth usually blunt and sometimes uneven. Catkins arched and drooping to pendulous, appearing before the leaves on twigs of previous year’s growth. Flowers each subtended by an irregularly and deeply lobed bract (merely toothed in P. alba), this glabrous or with a patch of hairs on the undersurface, in some species with dense long, silky hairs along the margins, shed early. Perianth appearing as a small disc-shaped to cup-shaped structure subtending the stamens or pistil; nectaries absent. Staminate flowers with 7 to numerous stamens, the filaments free. Pistillate flowers with the pistil composed of 2–4 carpels. Stigmas 2–4, variously linear to disc-shaped, often inrolled or convoluted, sometimes irregularly 2-lobed. Ovules 6 to numerous. Capsules narrowly to broadly ovoid or more or less globose, dehiscing by 2–4 valves. About 30 species, North America, Asia, Africa.

Most of the species of Populus produce leaves with two kinds of morphology. These are associated with differences between so-called early-season leaves, which are produced from meristems in the winter bud and are the first flush of leaves to develop in the spring, vs. so-called late-season leaves, which are produced from lateral meristems as the twigs continue to elongate after the first flush of growth in the spring. The two types of leaves sometimes differ somewhat in size and shape, but principally are distinguished in the number and relative coarseness of the marginal teeth. The early-season leaves usually persist until the end of the growing season, and collectors should attempt to gather both kinds when specimens are collected. The leaves of most Populus species turn yellow in the autumn.

Species of Populus often are ecologically important as primary colonizers of flooded, burned, or otherwise highly disturbed sites, where they are important in soil stabilization. Poplars and cottonwoods generally grow quickly and a number of species are cultivated as ornamental and specimen trees. Some species can develop massive trunks. The wood has been used for flooring, veneers, fence posts (including living fences), palettes, barrel staves, handcrafts, toys, musical instruments, popsickle sticks, and wood chips. Because it tends to burn slowly, it also has been used for match sticks. In some portions of Europe, the fluffy seed hairs were used as a kapok substitute in pillows and life vests, and poplar wood also was a popular material for stakes to strike through a vampire’s heart (Mabberley, 2008). Poplars were of minor medicinal and ceremonial use by some Native American tribes (Moerman, 1998). The staminate catkins produce abundant, wind-dispersed pollen and thus are one cause of hay fever in the springtime.

In the early spring, in addition to honey, bees harvest the resinous exudate from the winter bud scales of several Populus species, which they use as a natural adhesive in hive construction and maintenance, and in chores such as encasement of hive invaders (Hausen et al., 1987a). This fragrant, yellowish brown to dark brown substance, known as propolis or bee glue (one kind of balm of Gilead in Europe), has a long history of use medicinally, as an ingredient in facial creams and ointments, and in high-quality polishes and varnishes used on violins. Ghisalberty (1979) documented hundreds of historical and present-day applications for this substance. Chemically, propolis is a complex mixture of lipophilic compounds, including flavonoid aglycones, substituted benzoic acids and esters, and substituted phenolic acids and esters (Hausen et al., 1987a, b; Wollenweber et al., 1987; Hashimoto et al., 1988; Greenaway et al., 1988). Some of these ingredients have been reported to cause contact dermatitis in susceptible individuals, thus propolis harvested from bee hives or purchased at health food and vitamin stores should be used with care.