396. Ageratum Linnaeus, Sp. Pl. 2: 839. 1753; Gen. Pl. ed. 5, 363. 1754.
[Greek a, not, and geras, old age, apparently alluding to long-lasting nature of flowers]
Guy L. Nesom
Annuals and perennials, mostly 20–120 cm. Stems often decumbent (rooting at proximal nodes), sparsely to densely branched. Leaves cauline; all or mostly opposite; petiolate; blades mostly 1-nerved, deltate to ovate, or elliptic to lanceolate, margins entire or toothed, faces glabrous or ± pilose, puberulent, or strigoso-hispid, sometimes gland-dotted. Heads discoid, in dense to open, cymiform to corymbiform arrays. Involucres campanulate, 3–6 mm. Phyllaries persistent, 30–40 in 2–3 series, usually 2-nerved, lanceolate, ± equal (often indurate, margins scarious). Receptacles conic, epaleate [paleate]. Florets 20–125; corollas white or bluish to lavender, throats ± campanulate (lengths 2 times diams.); styles: bases not enlarged, glabrous, branches ± linear to clavate (usually papillose and dilated distally). Cypselae prismatic, 4–5-ribbed, glabrous or sparsely strigoso-hispidulous; pappi persistent, of 5–6 aristate scales, or coroniform, or 0. x = 10.
Species ca. 40 (4 in the flora): United States, Mexico, Central America; 2 species widespread as adventives.
Johnson, M. F. 1971. A monograph of the genus Ageratum L. (Compositae–Eupatorieae). Ann. Missouri Bot. Gard. 58: 6–88.