The fruits produced by the female plants are double samaras, or winged capsules composed of two easily-separable lobes. All willows have simple leaves that are alternately arranged on spindly stems. Leaf margins vary from smooth to serrate. Fruits are bivalved capsules. In winter, leaf buds are covered by a single, nonresinous scale.
Several species can be aggressive in certain habitats.
A close relative of the willow, the eastern cottonwood shares some of its characteristics. Also a dioecious plant, it bares pistils and stamens in catkins, although its catkins tend to droop, whereas willow catkins are normally upright or spreading. The cottonwood grows only as a tree, with spreading branches and a tall crown reaching heights of over 100 feet. Multi-stemmed cottonwoods may reach diameters of over 10 feet. The tree bark is nearly black, deeply furrowed, and decorated with resinous, many-scaled leaf buds in winter.