More than 100 species of willows are native to North America. Most of these are shrubs or dwarf shrubs, but about forty species reach tree size. Willow species commonly hybridize with each other and this, along with their relatively great richness of species, can make some of the willows difficult to identify.
Some of the more common species of willow that can attain the size of trees include the following: the black willow (Salix nigra) is a widespread tree in low-lying and riparian habitats in the eastern United States and southern Ontario; the peachleaf willow (S. amygdaloides) is also widespread in central North America; the Pacific willow (S. lasiandra) is another tree-sized species, occurring widely from southern California to central Alaska.
Shrub-sized species of willows are richer in species and include the following: The sandbar willow (S. interior) occurs widely in eastern and central North America and through the boreal forest to central Alaska. This species often forms thickets in flat, moist, alluvial habitats. The arroyo willow (S. lasiolepis) occurs in moist canyons and along streams in the western United States. The Mackenzie willow (S. mackenzieana) is a northwestern species. The coastal plain willow (S. carolineana) is widespread in the southeastern United States. The Bebb willow (S. bebbiana) is a common shrub of boreal and cool-temperate regions. The feltleaf willow (S. alaxensis) is widespread in the northeastern boreal forest.
Many species of willows are dwarf shrubs, occurring in alpine and arctic tundra. The stems of these tiny willows grow horizontally along the ground, and in some cases they never rise any higher than several centimeters above the ground surface. The most widespread of the dwarf willows is the arctic willow (S. arctica). This species occurs throughout much of the tundra of North America, Greenland, and Eurasia, as far north as the limits of land.