This arborescent willow is widespread across temperate North America. In addition to its original swampy habitats, nigra has become common around artificial impoundments, along roadside ditches, and in similar places. Its twigs easily break off, which may lead to local clonal spread (and easy plantung for restoration). Based on Argus (1986; FNA 7), relatively "brittle branches" tend to be a character of section Humboldtiana (in subgenus Protitea) and section Salix (in subgenus Salix). Species that are especially ("very") brittle include nigra, floridana and fragilis. The North American subgenus Protitea differs from the Eurasian subgenus Salix in the following characters: buds with acute apex (versus obtuse) and overlapping (versus fused) scales; leaves with stomata only on lower surfaces (versus both); male flowers with 3 or more stamens (versus 2); and pistillate bracts usually deciduous after flowering (versus persistent). It is often hard to use these characters with available material, and published keys generally need improvement. Another useful but less clearcut character of Ky. species in subgenus Protitiea may be the relatively dense fine sharp serration of leaves; these features tend to be less pronounced in most other species, which are more or less entire in some cases. Also, these species often have slightly falcate leaves.