Salix alba L.

Standardized Common Name: White Willow

Family: Salicaceae

Taxonomy: There are over 500 species of Salix, mostly native to cold or temperate parts of the Northern Hemisphere. The taxonomy of the genus is therefore complicated, with numerous sections recognized. Salix alba is classified in Subg. Salix, Sect. Salix, along with species such as S. fragilis L. (Brittle Willow) and S. babylonica L. (Weeping Willow). It has been reported to hybridize with these and other species, including S. nigra Marsh. (Black Willow) and S. lucida Muhl. (Pacific Willow). It is a native of Eurasia and is widely naturalized in eastern North America.

Description: Tree to 25 m high, with short trunk and ascending, spreading crown; bark grayish, irregularly ridged or fissured; twigs thin and flexible, often hanging downward, olive to brown or reddish brown, soft-pubescent. Leaves alternate, petiolate, narrowly lanceolate to lanceolate or narrowly oblong, 4–13 cm long; base attenuate to cuneate; apex acute to long-acuminate; margins minutely serrate; upper surface grayish-green, lower surface whitish; young leaves softly pubescent on both surfaces, becoming nearly glabrous at maturity especially above. Flowers minute, unisexual, borne in unisexual catkins 3–5 cm long, each flower subtended by a scale. Perianth absent; male flowers consist of 2 stamens, female flowers of an ovary with 2–4 stigmas. Fruit a capsule, 3–5 mm long, 1-loculed, 2–4 valved; seeds numerous, with long downy hair.

Parts in Commerce: Bark, primarily from 2–3-year-old branches, or young twigs

Identification:

• Pieces from small branches channeled, 1–2 cm broad; larger pieces nearly flat
• 1–2(–3) mm thick
• Outer surface greenish to grayish brown; young bark smooth, old bark dull, slightly wrinkled and sometimes dark brown
• Inner surface pale reddish or cinnamon, longitudinally striated to nearly smooth
• Fracture short in outer portion, coarsely fibrous in inner portion
• Cork very thin, brown; cortex and secondary phloem reddish to cinnamon-colored, sometimes appearing multilayered, sometimes separating irregularly into fibrous layers especially in small pieces
• Phloem in transverse section contains many thin interrupted layers of fiber cells; this may be visible (with a dissecting scope) on a moistened cut surface as lines of tiny, elongated, slightly darker spots broken into segments by very narrow, slightly paler perpendicular rays
• Young twigs, if included, have whitish xylem
• Sometimes has weak aromatic odor
• Taste astringent, slightly bitter

Adulterants: Numerous other species of willow are used medicinally, including those mentioned above, S. purpurea L. (Purple Willow), and S. daphnoides Vill. (Violet Willow). Some of these could be confused with S. alba in the wild. According to European authorities, all species of acceptable chemical content (which will have a similar taste) may be sold collectively as “willow bark.” Indeed, many species are more potent than S. alba in terms of salicin content. In the United States, Herbs of Commerce specifies that each such species should be sold under a different common name. Distinguishing among some of these species with certainty, given only the bark, may be possible only by chemical fingerprinting (assuming that reliable differences among species can be found). Some useful microanatomical features have been identified; for example, unlike many of the commercially valuable species, S. alba has stone cells only in the cortex, not in the phloem. However, it seems unlikely that numerous closely related species of Salix could be adequately differentiated through bark anatomy alone.

References:

Argus GW. The Genus Salix in the Southeastern United States. Ann Arbor, MI: American Society of Plant Taxonomists; 1986. Systematic Botany Monographs, vol. 9.

Argus GW. Infrageneric Classification of Salix (Salicaceae) in the New World. Ann Arbor, MI: American Society of Plant Taxonomists; 1997. Systematic Botany Monographs, vol. 52.

British Herbal Pharmacopoeia, p. 188.

Elias TS. The Complete Trees of North America. Field Guide and Natural History. New York, NY: Gramercy Publishing Co.; 1987.

Fang Z, Zhao S, Raven PH. Salicaceae. In: Wu Z-Y, Raven PH, eds. Flora of China, vol. 4. Cycadaceae through Fagaceae. Beijing: Science Press and St. Louis, MO: Missouri Botanical Garden Press; 1999:139–275.

Fernald ML. Gray’s Manual of Botany, 8^th ed. New York: American Book Company; 1950:488–519.

Greenish HG. Materia Medica. Jodhpur: Scientific Publishers (India); 1920 reprinted 1999:285–287.

Rechinger KH, rev. Akeroyd JR. Salix. In: Tutin TG, Heywood VH, Burges NA, et al., eds. Flora Europaea. 2^nd ed., vol. 1. Cambridge: Cambridge University Press; 1993:53–64.

Triest L. Hybridization in staminate and pistillate Salix alba and S. fragilis (Salicaceae): morphology versus RAPDs. Pl Syst Evol. 2001;226:143–154.

Wichtl M, ed. Herbal Drugs and Phytopharmaceuticals, 3^rd English ed. Stuttgart: medpharm Scientific Publishers and Boca Raton, FL: CRC Press; 2004:534–537.

Youngken HW. Text-Book of Pharmacognosy, 5^th ed. Philadelphia, PA: The Blakiston Company; 1943:250–251.