Arroyo willow is a large, sprawling, multi-trunked shrub or small tree, usually less than 30 feet (10 m) high. It is winter deciduous, but often retains some leaves throughout the year. Arroyo willows are riparian, needing a consistent source of fresh water. They often form dense groves where there is a shallow water table or seep.
Leaves are elongate, usually less than 6 inches (15 cm) long and 1 inch (2.5 cm) wide; mature leaves are often oblanceolate. Leaf tip is narrowly to broadly acute. Leaf margins may be smooth or shallowly, irregularly serrate or scalloped. Leaves are shiny green above, made pale below by waxy particles and short white hairs.
Flowers are dense, cylindrical clusters of tiny flowers, catkins, appear February – May1 before new leaves appear. Petals and sepalsare absent. Male and female flowers occur on different plants. At the base of each male or female flower is a small, dark flower bract that is covered with long, shaggy, whitish hairs. Also, each flower has a small gland that secret nectar (nectaries) to attract pollinating insects. Male catkins are 1-3 inches long (3-7 cm) colored yellow or greenish yellow by the abundant pollen (“yellow = fellow100). Each flower consists of two stamens that are united at their bases. Female catkins are slightly smaller and green in color. Female flowers consist of a green, bowling-pin-shaped pistil and a small stalk. The ovary + style tapers to two, small, two-lobed stigmas.
Seeds are tiny (<1/32 inch or 1 mm) and wind-dispersed by means of long, silky-white hairs at one end.