Willow

Willows—known scientifically as *Salix*—are a group of about 350 species (plus many hybrids) of trees and shrubs that usually lose their leaves each year. They thrive in damp soils across cold and temperate parts of the world. While most are simply called willows, narrow-leaved shrubby types are often termed osiers, and broader-leaved ones are called sallows (a name that traces back to Old English *sealh*, related to the Latin *salix*). Some species, especially those in the Arctic and high mountains, grow as low, creeping bushes: the dwarf willow (*Salix herbacea*), for instance, rarely tops 6 centimeters (2½ inches) in height but spreads widely along the ground.

The bark sap is watery and rich in salicin. The wood is soft, flexible, and tough; the branches are slender; and the roots are large, fibrous, and often spread by runners. These roots are notably durable, sizable, and tenacious, and they readily sprout from above-ground parts of the plant.

Leaves are usually long and narrow, though they can be round or oval, and often have serrated edges. Nearly all willows are deciduous; semi-evergreen types with leathery leaves are uncommon (examples include *Salix micans* and *S. australior* in the eastern Mediterranean). Every bud grows from the side of a stem—no true terminal bud forms. A single scale covers each bud, typically fused into a cap-like shape, though in some species the scale wraps around with overlapping edges. The leaves are simple, with feather-like veins, and are generally linear to lance-shaped. They are usually serrated, rounded at the base, and pointed or tapering at the tip. Leaf stalks are short, and the stipules (small, round, leaf-like structures) are often very noticeable, sometimes lasting half the summer; on some species, however, they are tiny, inconspicuous, and fall off early. Leaf colors range widely among greens, from yellowish to bluish tones.

Willows are among the first woody plants to leaf out in spring and among the last to drop their leaves in autumn. In the Northern Hemisphere, leafout can happen as early as February, depending on climate, and is triggered by air temperature: if daytime highs reach 10°C (50°F) for several days in a row, a willow will try to produce leaves and flowers. Leaf drop in autumn occurs when day length shortens to about 10 hours and 25 minutes—this varies by latitude, happening as early as the first week of October for boreal species like *Salix alaxensis* and as late as the third week of December for willows in far southern areas.

Except for *Salix martiana*, willows are dioecious: male and female flowers appear as catkins on separate plants, and these catkins emerge early in spring, often before the leaves. Male flowers have no calyx or corolla; they consist simply of 2 to 10 stamens, plus a nectar gland, all attached to the base of a hairy, square scale on the drooping catkin. The anthers are rose-colored in bud but turn orange or purple after opening; they have two cells that open sideways. The filaments are threadlike, usually pale brown, and often hairless. Female flowers also lack a calyx and corolla; they consist of a single ovary, a small flat nectar gland, and a scale on the catkin. The ovary has one cell, a two-lobed style, and many ovules.

The scientific name *Salix* was first used by Carl Linnaeus in 1753. Since Linnaeus didn’t designate types, the type species for the genus was later set as *Salix alba*, based on a conserved type. The word *Salix* comes from Latin and was already used by Romans for various willows. One theory suggests it derives from a Celtic language, combining *sal* (near) and *lis* (water), referring to their habitat. Willows are divided into subgenera, though the exact classification is still debated. Morphological studies typically recognize 3 or 5 subgenera: *Salix* (sometimes split into *Longifoliae* and *Protitae*), *Chamaetia*, and *Vetrix*. Phylogenetic studies suggest that *Chamaetia* and *Vetrix* belong together in one clade. The oldest known willow fossils come from the early Eocene of North America, with the earliest European finds dating to the Early Oligocene.

The genus includes about 350 species of deciduous trees and shrubs. They hybridize freely, and over 160 such hybrids have been named. Well-known willows include many familiar types.

Willows tolerate shade and are usually short-lived. They need disturbances to compete with conifers or large deciduous trees. Their seeds are tiny, abundant, carried by wind and water, and remain viable for only a few days; they need warm, moist conditions to germinate. The plants can also reproduce vegetatively from cut stumps and broken branches. Willows produce modest amounts of nectar, which bees use to make honey, and are especially valued as an early source of pollen for bees. Various animals browse the foliage or find shelter among the plants. Beavers use willows to build dams. The trees serve as food for the larvae of some Lepidoptera, such as the mourning cloak butterfly. Ants, including wood ants, are common on willows inhabited by aphids, coming to collect honeydew, as do wasps at times.

Willow species host more than a hundred aphid species (from *Chaitophorus* and other genera), which form large colonies and feed on plant juices, especially on the underside of leaves. The willow lace bug (*Corythucha elegans*) is another insect pest.