Plant 328

Salix species (Salicaceae)


Early spring brings forth pussy willow, the distinctive flowerheads of shrubs such as the goat willow, Salix caprea. Willows, with over 400 species, are prominent in the world’s temperate and boreal regions, where they occupy habitats ranging from forests through tundra to sand dunes, and climatic zones, from the subtropics to the poles. Salix arctica, which rarely grows more than fifteen centimetres tall, is the world’s most northerly-growing woody plant; it has been found on the northern coast of Greenland. Most willows are pioneers and shade-intolerant; they occupy scrub, marginal or riverine habitats. The association between willows and water is reflected in the generic name (meaning ‘near water’).

Willow species usually have separate male and female plants, and flower in the spring. The clusters of tiny flowers, arranged in catkins, are insect-pollinated, and the capsule-like fruits produce minute seeds with tufts of hair that aid wind dispersal. In natural populations, vast quantities of easily germinated, very short-lived, seed are produced. Clonal propagation, together with coppicing and pollarding, are important for the management of some economically important species.

Willow timber is used for building and pulpwood, whilst female clones of Salix alba ‘Caerulea’ are the sole source of wood for cricket bats. Willow is an important, often species-specific, raw material for the production of hurdles, coracles and baskets; Salix triandra is used to make rods, and Salix purpurea and Salix viminalis are used for thin and coarse withies, respectively. As fuel, willows are used to make charcoal and as short-rotation biomass. Willows may also be planted for habitat stabilisation, reduction of flooding risk, soil conservation, wastewater treatment and as windbreaks. Aesthetically, willows are planted for their stem colours or the weeping habit of some species. However, willow is perhaps most famous as a medicinal bark being the source of salicin, which has been used as a febrifuge and analgesic for centuries; today superseded by salicylic acid (aspirin).

Willow naming is complicated by patterns of morphological variation and hybridisation. Many willow hybrids are fertile, hence individual plants may have complex parentages; an artificial hybrid has been created involving fourteen different Salix species. Moreover, accurate identification requires mature flowers and leaves, which are not usually unavailable at the same time.

Traditionally, willows have been grouped with alders (Alnus), birches (Betula) and walnuts (Juglans) because they are woody and have catkins. Modern classifications show that superficial resemblance based on catkins is due to convergence.

Further reading

Sell P and Murrell G 2018. Flora of Great Britain and Ireland. Volume 1. Lycopodiaceae-Salicaceae. Oxford University Press.

Newsholme, C 1992. Willows: the genus Salix. Timber Press.

Stephen Harris