willow, common name for some members of the Salicaceae, a family of deciduous trees and shrubs of worldwide distribution, especially abundant from north temperate to arctic areas. The family consists of two genera, Salix and Populus, both of which are propagated easily by cuttings, grow rapidly, and characteristically bear male and female flowers in catkins on separate plants.
Many plants of the narrower-leaved willow genus ( Salix ) flourish in cold, wet ground; willows grow farther north than any other woody angiosperm (flowering plant). The poplars (genus Populus ) usually have heart-shaped or ovate leaves; they include the cottonwoods, aspens, and many species specifically named poplar. The cottonwoods (sometimes also called poplars) characteristically have seeds that are covered with fibrous coats so that when they are released at maturity they clump together in cottony balls. Cottonwoods were a welcome sight to the pioneers pushing westward, for they marked the streams in the otherwise treeless Great Plains. Some of the poplars, especially the aspens, have flattened leaf stalks that permit the pendulous leaves to quiver in the slightest breeze (hence the name quaking aspen). The quaking, or golden, aspen ( P. termuloides ) is a common deciduous tree of the mountains of the W United States; it is often the first tree to reforest burned-over woodlands. Large stands of aspen trees often consist of one or two clones connected at the roots. The hybrid species Populus × jackii is one of the plants called balm of Gilead.
Because the lumber of this family is so soft it finds little use except for paper pulp (mostly the poplars), for biomass and biofuel, for charcoal, and especially in basketry and wickerwork (mostly the willows). The bushes and their twigs used in basketry are often called osiers. Willow buds and bark have also been used medicinally; the chemical predecessor of aspirin was originally isolated from the bark of a willow. The trees are valuable in erosion control along riverbanks because of their rapid growth. The family is most noted for its many species planted as ornamentals, e.g., the Lombardy poplar ( P. nigra cultivar Italica ) and the silver, or white, poplar ( P. alba ), now naturalized in North America from Eurasia; the weeping willow ( S. babylonica ), indigenous to China; and the pussy willow ( S. discolor ) of North America with its silky catkins.
Yellow poplar or tulip poplar is a name sometimes used for the unrelated tulip tree of the magnolia family. Willows are classified in the division Magnoliophyta, class Magnoliopsida, order Salicales, family Salicaceae.