Salix fragilis (Crack Willow) is a species of willow native to Europe and western Asia, usually growing beside rivers.
It is a medium-sized to large deciduous tree, which grows rapidly to 10–20 m (rarely to 29 m) tall, with a trunk up to 1 m diameter and an irregular, often leaning crown. The bark is grey-brown, coarsely fissured in older trees. The leaves are bright green, 9–15 cm long and 1.5–3 cm wide, with a finely serrated margin; they are very finely hairy at first in spring, but soon become hairless. The flowers are produced in catkins in early spring, and pollinated by insects. They are dioecious, with male and female catkins on separate trees; the male catkins are 4–6 cm long, the female catkins also 4–6 cm long, with the individual flowers having either one or two nectaries. The variety Salix fragilis var. decipiens (Hoffm.) K.Koch occurs frequently with the type; it is a smaller shrubby tree, rarely exceeding 5–7 m tall, with completely hairless leaves up to 9 cm long and 2–3 cm broad. According to some botanists, it is a distinct species (treated as Salix decipiens Hoffm.), with, in this view, S. fragilis then being a hybrid between Salix decipiens and Salix alba. Some other botanists regard Salix decipiens as itself being a hybrid between Salix fragilis and Salix triandra. There is little evidence to support either of these suggestions.
It readily forms natural hybrids with White Willow Salix alba, the hybrid being named Salix × rubens Schrank.
The name derives from the twigs which break off very easily and cleanly at the base with an audible crack. The broken twigs and branches take root readily, enabling the species to colonise new areas, where the broken twigs fall into rivers and can be carried some distance downstream. It is particularly adept at colonising new riverside sandbanks formed after floods.