Comprehensive DescriptionRead full entry
General: Western yarrow is a member of the Asteraceae (Sunflower) family that is commonly found in natural and disturbed habitats throughout the western U.S. (Hitchcock and Cronquist, 1973). It is a self-incompatible, insect-pollinated species (Clausen et al., 1958) occurring as native forms that may differ in chromosome number (n=18 or n=27); native races originating in the western U.S. (except the Pacific coast) are mostly tetraploid and introduced races mostly hexaploid (Hitchcock et al 1955). Botanists currently acknowledge genetic and ecological differentiation among native ecotypes at the local and regional level (Lavin, 2002; Lesica, 2001).
Taxonomic: Western yarrow is a moderately rhizomatous, long-lived, native, herbaceous perennial forb. It is 30-100 cm tall with few to many unbranched, erect, lanate stems (Cronquist et al., 2002). Leaves are alternate, sessile, pinnately dissected, semi-evergreen, and aromatic with an anthemideous scent (chamomile or dog fennel-like odor). The basal rosette foliage is up to 25 cm long, and cauline leaves are typically up to 10 cm long and 3 cm broad. Inflorescences are arranged in a compound, flat-topped corymb 6-20 cm wide, consisting of numerous, small flower heads 4-6 mm in diameter. Involucre bracts are usually pubescent and greenish, with papery, straw-colored margins (Robinson and Fernald, 1908). Outside ray flower numbers are 3-12, mostly white to cream-colored, 1-2.5 mm long and encircle the center disk flowers. The disk flowers number 10-75 each, and are yellow, tubular, perfect, and seed-producing. Fruit is a flattened, glabrous achene with compressed margins in a mostly reverse egg-shape with no pappus (USDA FS, 1966).