Salix alba, the white willow, is a species of willow native to Europe and western and central Asia.[1] [2] The name derives from the white tone to the undersides of the leaves.
It is a medium to large deciduous tree growing up to 10 - 30 m tall, with a trunk up to 1 m diameter and an irregular, often-leaning crown. The bark is grey-brown and is deeply fissured in older trees. The shoots in the typical species are grey-brown to green-brown. The leaves are paler than most other willows because they are covered with very fine, silky white hairs, in particular on the underside; they are 5 - 10 cm long and 0.5 - 1.5 cm wide. The flowers are produced in catkins in early spring and are pollinated by insects. It is dioecious, with male and female catkins on separate trees; the male catkins are 4–5 cm long, the female catkins 3–4 cm long at pollination, lengthening as the fruit matures. When mature in midsummer, the female catkins comprise numerous small (4 mm) capsules, each containing numerous minute seeds embedded in silky white hairs, which aids wind dispersal.[1] [2] [3]
Like all willows, Salix alba is usually to be found in wet or poorly-drained soil at the edge of pools, lakes or rivers. Its wide-spreading roots take up moisture from a large surrounding area.[4] White willows are fast-growing but relatively short-lived, being susceptible to several diseases, including watermark disease caused by the bacterium Brenneria salicis (named because of the characteristic 'watermark' staining in the wood; syn. Erwinia salicis) and willow anthracnose, caused by the fungus Marssonina salicicola. These diseases can be a serious problem on trees grown for timber or ornament.
It readily forms natural hybrids with crack willow Salix fragilis, the hybrid being named Salix × rubens Schrank.[1]
Several cultivars and hybrids have been selected for forestry and horticultural use:[1] [2]
The following have received the Royal Horticultural Society's Award of Garden Merit[5]
The wood is tough, strong, and light in weight, but has minimal resistance to decay. The stems (withies) from coppiced and pollarded plants are used for basket-making. Charcoal made from the wood was important for gunpowder manufacture. The bark tannin was used in the past for tanning leather.[1] [2] The wood is used to make cricket bats.
S. alba wood has a low density and a lower transverse compressive strength. This allows the wood to bend, which is why it can be used to make baskets. Willow bark contains indole-3-butyric acid, which is a plant hormone stimulating root growth; willow trimmings are sometimes used to clone rootstock in place of commercially synthesized root stimulator.[11] It is also used for ritual purposes by Jews on the holiday of Sukkot.[12]
Willow (of unspecified species) has long been used by herbalists for various ailments, although it is a myth that they attribute to it any analgesic effect.[13] One of the first references to White Willow specifically was by Edward Stone, of Chipping Norton, Oxfordshire, England, in 1763. He 'accidentally' tasted the bark and found it had a bitter taste, which reminded him of Peruvian Bark (Cinchona), which was used to treat malaria. After researching all the 'dispensaries and books on botany,' he found no suggestion of willow ever being used to treat fevers and decided to experiment with it himself. Over the next seven years he successfully used the dried powder of willow bark to cure 'agues and intermittent fevers' of around fifty people, although it worked better when combined with quinine.[14]
Stone appears to have been largely ignored by the medical profession and herbalists alike. There are reports of two pharmacists using the remedy in trials, but there is no evidence that it worked.[15] By the early 20th century, Maud Grieve, a herbalist, did not consider White Willow to be a febrifuge. Instead, she describes using the bark and the powdered root for its tonic, antiperiodic and astringent qualities and recommended its use in treating dyspepsia, worms, chronic diarrhoea and dysentery.[16] She considered tannin to be the active constituent.
An active extract of the bark, called salicin, after the Latin name Salix, was isolated to its crystalline form in 1828 by Henri Leroux, a French pharmacist, and Raffaele Piria, an Italian chemist, who then succeeded in separating out the acid in its pure state. Salicylic acid is a chemical derivative of salicin and is widely used in medicine. Acetylsalicylic acid (aspirin) is, however, a chemical that does not occur in nature and was originally synthesised from salicylic acid[17] extracted from Meadowsweet, and is not connected to willow.[18]