1. Parthenium hysterophorus L. (Santa Maria)

Pl. 285 e, f; Map 1210

Plants annual, with taproots. Stems 30–120 cm long, usually much-branched above the midpoint, moderately to densely pubescent with short, stiff, more or less ascending hairs, usually also with minute, sessile, spherical, yellow glands. Basal leaves short- to long-petiolate, the petioles progressively shorter up the stem, the uppermost leaves usually sessile or nearly so. Leaf blades mostly 3–20 cm long, ovate to elliptic, 1 or 2 times deeply pinnately lobed, the lobes oblong-triangular to narrowly oblong or linear, the margins otherwise entire or with few coarse teeth, the surfaces sparsely to densely roughened-pubescent with short, stiff, loosely appressed hairs, often also with minute, sessile, spherical, yellow glands. Blades of uppermost leaves 1–3 cm long, usually linear to narrowly oblong, unlobed, narrowed or tapered at the base and not clasping the stem. Involucre 2–4 mm long, 3–4 mm in diameter, the bracts of the outer series slightly shorter than the others, lanceolate to narrowly elliptic, those of the inner series ovate to broadly ovate. Pappus of 2 ovate scales 0.7–1.5 mm long, these petaloid in color and texture, appressed to and often difficult to distinguish from the corolla. Fruits 1.5–3.0 mm long. 2n=34. August–October.

Introduced, uncommon and sporadic (native of Mexico, Central America, South America, Caribbean Islands; introduced in the eastern U.S. west to Kansas and Texas; also Europe, Asia, Pacific Islands).

Although apparently not a problem in North America, plants introduced and naturalized in India have poisoned cattle and buffalo and caused contact dermatitis in some humans. The agents responsible apparently are sesquiterpene lactones, especially parthenin (Burrows and Tyrl, 2001).