Willow are very cross-compatible, and numerous hybrids occur, both naturally and in cultivation. A well-known ornamental example is the weeping willow ("Salix × sepulcralis"), which is a hybrid of Peking willow ("Salix babylonica") from China and white willow ("Salix alba") from Europe. The hybrid cultivar 'Boydii' has gained the Royal Horticultural Society's Award of Garden Merit. Numerous cultivars of "Salix" L. have been developed and named over the centuries. New selections of cultivars with superior technical and ornamental characteristics have been chosen deliberately and applied to various purposes. Most recently, "Salix" has become an important source for bioenergy production and for
Salix fragilis also spreads by root suckers, expanding into pure 'groves'. "Salix fragilis" is cultivated as a fast-growing ornamental tree. The cultivar "Salix fragilis" 'Russelliana' (syn. "S. fragilis" var. "russelliana" (Sm.) Koch) is by far the most common clone of crack willow in Great Britain and Ireland, very easily propagated by cuttings. It is a vigorous tree commonly reaching tall, with leaves up to 15 cm long. It is a female clone. "S. fragilis" has escaped cultivation to become an invasive species in various parts of the world, including: New Zealand; the upper half of the United States; and South Africa. In