- Family: Willow Family – Salicaceae
- Growing form and height: Shrub. 0.5–1(–1.5) m (1.5–3(–5) ft.).
- Flower: Male and female flowers on separate plants. Inflorescence a roundish, unstalked catkin. Individual flowers in axils of catkin scales, small, lacking perianth. Catkin scales narrow, dark or with dark tip, and densely brown-hairy. Stamens 2, small, filaments hairless, anthers yellow. Pistil formed from 2 fused carpels, ovary hairy, style long.
- Leaves: Alternate. Fairly long-stalked, lacking stipules. Blade 3–7 cm (1.2–2.8 in.) long, narrowly to broadly elliptic, taper-pointed, greyish-green and wrinkled above, grey, downy and prominently veined beneath, and with entire or rarely basally shallow-toothed margins that are rolled under. Vein pairs 7–12.
- Buds: Yellowish-brown, rounded, usu. hairy.
- Fruit: Densely short-hairy roundish capsule. Seeds plumed.
- Habitat: Flooded fens and shores, waterside thickets, mire margins, fjeld scrub.
- Flowering time: May–June. Flowers before coming into leaf.
Willows are insect-pollinated, sympodially growing, dioecious trees, shrubs, or dwarf shrubs. Their buds have a single protective scale, and their leaves are entire and stipulate. The inflorescence is a catkin which falls off in one piece.
Downy willow is an erect shrub with stout branches. Its old branches are hairless, whereas the young shoots are grey-downy. Together with gray willow (S. glauca) and woolly willow (S. lanata) this species forms characteristic “grey-willow thickets” along fjeld streams and rivers. Downy willow grows on wet sites and favours places affected by flowing water.
Hybrids between willow species are common. Downy willow crosbreeds with many other willow species, most commonly with goat willow (S. fragilis), swamp willow (S. myrtilloides) and grey willow (S. cinerea) and rarely with tea-leaved willow (S. phylicifolia) and creeping willow (S. repens).