Willow trees can be differentiated by their leaves, their form, their bark and their catkins. Willows are mostly deciduous trees, which means they lose their leaves in the fall. Male and female flowers are found on different trees, and their leaves are usually long and narrow.
One type of willow tree is the white willow, which is native to Europe, northern Asia and northern Africa. It gets its name because the light green leaves have silky white hairs above and white down beneath. The effect is to give the tree a silvery white appearance.
The weeping willow is known for its weeping habit. Native to China, it's now planted in gardens and parks throughout Europe and North America. Its long leaves are hairless, dark green on top and pale beneath.
The grey sallow is a small tree or shrub that's native to Europe. It has slender catkins that open in early spring and brown down on its young shoots. It grows in marshlands and is considered invasive.
The contorted willow can be differentiated from other willows by its strangely twisted twigs and leaves, while the bay willow has leaves that are toothed, dark, glossy green and broader than the leaves of other willows.