1. Achillea L. (yarrow, milfoil)

Plants perennial herbs (annual elsewhere), with rhizomes, weakly to strongly aromatic with an unpleasant odor. Stems erect or ascending, unbranched below the inflorescence or less commonly few-branched, sometimes with short, leafy branches in the leaf axils, finely ridged to angled, sparsely to densely pubescent with white, woolly hairs. Leaves alternate and basal, the basal leaves sometimes withered by flowering time, sessile or short-petiolate with somewhat broadened, more or less clasping bases. Leaf blades narrowly oblong to lanceolate or oblanceolate, unlobed or deeply 2(3) times pinnately lobed, glabrous or sparsely to densely pubescent with woolly hairs. Inflorescences panicles at the stem tips, rarely reduced to a cluster of heads, dense, flat-topped to dome-shaped, the stalks sparsely to densely hairy, more or less bractless (a few reduced leaflike bracts sometimes present along the branches). Heads radiate. Involucre narrowly nearly cylindrical to hemispherical or broadly cup-shaped, the bracts in more or less 3 overlapping series, the outer ones shorter than the inner ones, elliptic-lanceolate to narrowly oblong or narrowly ovate-triangular, bluntly to sharply pointed at the tip, cobwebby-hairy, often becoming tan to brown with age, usually with a narrow, pale, raised midrib, this surrounded by a narrow to broad, green area, the narrow, paler margins thin and papery. Receptacle slightly convex to hemispheric, solid, chaffy. Ray florets 3–12 (numerous in doubled forms), pistillate, the corolla sometimes glandular or appearing minutely pebbled, white, rarely pink. Disc florets perfect, 10 to numerous, the corolla white to grayish white, sometimes glandular, the 5 lobes without resin canals, persistent, the tube often somewhat flattened toward the tip, slightly swollen at the base. Pappus absent. Fruits 1–2 mm long, oblong-obovate to slightly wedge-shaped in profile, flattened, broadly rounded to nearly truncate at the tip, the margins somewhat thinner and sometimes appearing as blunt wings, the surface otherwise smooth or with several faint, longitudinal lines, glabrous, tan to light brown with lighter margins. About 115 species, North America, Europe, Asia, Africa, most diverse in Europe, Asia.