Willows have many differences in size and type of growth, but are very much alike in other respects. There are about 350 species of this plant, usually found on moist soils in cooler zones in the Northern Hemisphere. Many hybrids are known, both naturally occurring and in cultivation, because willows are very fertile between their own species.
Willows have watery bark sap, charged with salicylic acid (defence against herbivory). They have soft, usually pliant (bendy), tough wood, slender branches, and large, fibrous, often stoloniferous roots. The roots are remarkable for their toughness, size, and are hard to kill. Roots readily sprout from aerial parts of the plant.
Willows are dioecious, with male and female flowers appearing as catkins on separate plants. The catkins are produced early in the spring, often before the leaves.
Salix herbacea, Dwarf Willow, Sweden