Worldwide, there are approximately 350 species of willows, Salix; all are trees or shrubs. They are primarily northern temperate plants, with the greatest diversity found in China, and approximately 100 species found throughout North America. Willows are also found through Europe and in the Andes as far south as Chile. Almost all species will hybridize. White willow, Salix alba,Figures introduced from Eurasia, is widely found throughout Canada and the United States; black willow, S nigra, a native, is found throughout the eastern and midwestern states, as well as in California.4 Willows are easily propagated from shoots and had been cultivated in ancient Greece and Rome. Willows are useful for reclamation and erosion control, thriving under moist conditions and producing an extensive root system. Wood from white willow is light and firm and used for cricket bats, baskets, and paper pulp. Bark extracts contain salicin, similar to acetylsalicylic acid, useful for pain relief and as a substitute for quinine.
White willow is a large tree, 10 to 25 m tall, with wide-spreading branches and an open crown. Leaves are alternate, narrow, and finely serrated, initially light green, darkening with silky white hairs above, and white "down" beneath. Male and female flower on separate trees. Catkins open late April and May and are 3.7 to 5.5 cm long. Female catkins ripen to release white tufted seeds in June. Flowers have nectar glands and odor to attract insects. Willows are amphiphilous, with both insect and wind vectors being important. Although Wodehouse felt willows were of secondary importance in causing hay fever, he acknowledged that the pollen was frequently caught in abundance on samplers. Willow pollen is captured in mid-Europe in significant quantities between March and May. Lewis et al demonstrated more intense skin test reactivity to Salix than to Populus, although sensitization to the latter was twice as common.
Abstracted from: Annals of Allergy, Richard Weber, MD, February 2004.