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 humboldtiana Salix humboldtiana Salix humboldtiana, the Humboldt's willow, is a tree species of willow native to North and South America, growing along watercourses. The species is evergreen or deciduous, depending on climate. It can grow up to 25 meters tall, with a narrow triangular or columnar crown shape. The trunk has a maximum d.b.h. of 80 cm and dark brown to gray fissured bark. Narrow lanceolate leaves up to 15 cm long, with serrate margin and light green color, in temperate climates, it turns yellow in autumn. Catkins 4–10 cm long; male flowers yellowish green, with an ovate-lanceolate bract, six stamens;