Seeds of the 24 common Salix species of the Alaskan boreal forest and tundra were set to germinate in laboratory and field experiments, and seed dispersal times were observed. During the growing season, 16 species disperse short-lived, nondormant seeds. At the end of the growing season, eight other species, all tundra willows, disperse conditionally dormant seeds. These fall-dispersed seeds are fully developed in the same length of time as summer-dispersed seeds, but they develop dormancy while being held on the plant until the leaves senesce. At the time of dispersal, some seeds are capable of germinating at high temperatures, but no seeds can germinate at the low soil temperatures occurring then. Cold stratification gradually widens the range of temperatures at which seeds can germinate, and seeds germinating at low soil temperatures in the spring shortly after snowmelt. Salix species dispersing dormant seeds during the fall appear to have evolved, as an adaptation to short growing seasons in cold climates, from taxa dispersing nondormant seeds during the summer.