Plants shrubs or small to trees, sometimes colonial from stem or root suckers, stem fragmentation, or stem layering. Branchlets often with prominent lenticels, these variously shaped, lighter or darker than the surrounding surface. Winter buds lateral and sometimes also appearing terminal, variously shaped, with a single scale, this not sticky, consisting of 2 opposite scales that are fused totally into a conical cap or with free, overlapping margins on the side facing the stem. Leaves short-petiolate, the petiole sometimes with glandular dots or short, flat glands. Stipules usually present (in some species minute or apparently absent, especially on early-season shoots), herbaceous, variously shaped, often shed early. Leaf blades not or only slightly heterophyllous (leaves developing in the spring from the winter buds sometimes differing slightly in size and/or shape from those produced later in the year), variously linear, narrowly lanceolate to ovate, narrowly oblanceolate to obovate, or narrowly elliptic to elliptic, 2 or more times as long as wide, with a single midvein, the margins entire or more commonly faintly to strongly toothed, the teeth blunt (sometimes appearing scalloped) or sharply pointed, sometimes gland-tipped (the glands sometimes also present along the margins on the untoothed portions). Catkins straight or slightly curved, erect to spreading, appearing before or as the leaf buds open, terminal on short, axillary branches or lateral on branchlets of usually the second year’s growth, sessile or nearly so. Flowers each subtended by an entire or somewhat uneven-margined to toothed bract, this usually hairy on the margins and/or surfaces, and persistent or shed early. Perianth absent, apparently replaced by 1 or rarely 2 nectaries, these entire or sometimes irregularly lobed, distinct or fused into a cup. Staminate flowers with (1)2–8 stamens, the filaments distinct or fused toward the base. Pistillate flowers with the pistil composed of 2 carpels. Stigmas 2, usually linear, occasionally 2-lobed. Ovules 2 to numerous. Capsules ovoid to obovoid, ellipsoid, or somewhat club-shaped, dehiscing by 2 valves. About 450 species, nearly worldwide, but absent (except for introductions) from Oceania and Australia.

Some species of Salix are ecologically important as primary colonizers of seasonally flooded or otherwise highly disturbed, wet sites, where they are important in soil stabilization. Steyermark (1963) noted that they are used by beavers in lodge construction, and they are also an important food source for these mammals. Willows also are hosts for a bewildering array of parasitic insects whose larvae form galls on the stems and leaves (Redfern and Askew, 1992). These include mainly sawflies (Hymenoptera: Tenthredinidae) and gall midges (Diptera: Cecidomyiidae). Nyman et al. (2000) noted that more than 200 species of sawflies alone form galls on various Salix species.

A number of species of willows are cultivated commonly as ornamentals and specimen plants, as well as for wind breaks, erosion control, bioremediation, and reclamation of severely impacted sites, such as mines and quarries. The leaves of most species turn yellow in the autumn. Many cultivars have been developed for some of the more commonly grown species and a number of hybrids exist as well. To the chagrin of many home owners, the roots of willows are adept at invading sewer lines, thus planting of this genus near underground pipes is to be avoided. The wood is relatively soft, but tends to resist warping and splitting, and has been used (at least historically) for packing crates, palettes, furniture, cricket bats, and paper pulp. Many children of a bygone era learned the penalty for misbehavior, which involved application of a willow switch.

The bitter inner bark of Salix species is one of the original sources of salicylic acid (2-hydrobenzoic acid), and willow bark has an extremely long history of medicinal use for fevers, aches, skin conditions, and head aches, and also is an effective anti-clotting agent (Jeffries, 2005). Edmund Stone (1763) noted its astringent properties and experimented with the use of dried, powdered bark of S. alba in reducing fevers; he brought the plant to the attention of the European scientific community. Today, salicylic acid is a common ingredient in some acne, psoriasis, wart, and callus treatments, as well as in some dandruff shampoos, and also is used as an antiseptic additive in some toothpastes. Aspirin (acetylsalicylic acid) is a biochemical derivative of salicylic acid. It was first synthesized by the French chemist, Charles Gerhardt (1853). A team of German chemists subsequently purified the compound and determined its biochemical structure (Schröder et al., 1869). During the period of 1897–1899, the German pharmaceutical firm of Bayer developed the commercial drug, for which it coined the name aspirin (Jeffries, 2005). Aspirin apparently was the first mass-market drug developed for commercial profit and continues to be one of the most-used medicines in the world.

Species of Salix have a reputation for hybridization. Elsewhere in the United States this may be true, but in Missouri hybrids appear to be uncommon. Although several different hybrids have been reported, these mainly are documented by historical specimens. This may be correlated with the dramatic decline of wetlands in the state during the first half of the twentieth century.

Note that in the present treatment the term branchlets to denote first-season growth at the tips of branches is used in place of the more frequently encountered term twig, as this terminology has been used in other recent literature on the genus.