The Crack Willow (Salix fragilis) is a willow native to Europe and Asia. It is a rapid growing tree to 27 m tall, usually growing beside rivers. The leaves are bright green, 9-15 cm long and 1.5-3 cm wide, with a finely serrated margin. The flowers are catkins, produced in early spring.

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, particularly 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.