1. Bidens alba (L.) DC. var. radiata (Sch. Bip.) Ballard

B. pilosa L. var. radiata (Sch. Bip.) J.A. Schmidt

Pl. 265 e, f; Map 1149

Plants annual, terrestrial, with taproots. Stems 30–150 cm, erect or ascending, the upper portion usually 4-angled, glabrous (sparsely hairy elsewhere). Leaves all more or less similar, short- to long-petiolate, opposite, the blade 2–10 cm long, mostly broadly ovate to ovate-triangular in outline, deeply 1 time ternately or pinnately divided or compound into 3–5(–7) segments or leaflets, these lanceolate to ovate, angled or short-tapered (rarely rounded to shallowly cordate) to a stalked or stalklike, winged base, tapered to a sharply pointed tip, the margins sharply toothed, the surfaces glabrous or the undersurface sparsely short-hairy, especially along the veins. Inflorescences of solitary terminal heads or appearing in loose, open clusters, the heads radiate, not nodding at fruiting. Involucre with the outer series of 8–12 bracts 2–5 mm long, ascending to loosely ascending, narrowly oblong to oblanceolate, the margins with spreading hairs, the outer surface glabrous or sparsely short-hairy; the inner series of 7–13 bracts 4–7 mm long, lanceolate to oblong-oblanceolate, inconspicuously pubescent with short, appressed hairs. Chaffy bracts narrowly oblong, usually purplish-tinged at the tip. Ray florets present, 5–8, the corolla showy, 7–15 mm long, white. Disc florets 20–40(–80), the corollas 3–5 mm long, yellow. Pappus absent or more commonly of 2 or 3 awns 2–4 mm long, these with downward-pointed barbs, usually erect at fruiting. Fruits 4–12 mm long, linear, more or less flattened (outer florets) or 4-angled (inner florets) in cross-section, not winged, the faces each with a pair of longitudinal grooves, reddish brown to black, with short, upward angled, usually pustular-based hairs. 2n=48. July–October.

Introduced, known thus far only from a single collection from the city of St. Louis (southwestern U.S. and Florida; Mexico, Central America, South America, Caribbean Islands; introduced sporadically farther north in the U.S., also Canada, Europe, Asia, Africa). Railroads.

This species was first reported for Missouri by Mühlenbach (1979, as B. pilosa). The present work follows the careful study by Ballard (1986), who separated B. alba, B. odorata, and B. pilosa based on a suite of subtle morphological characters, differences in ploidy, and phytochemistry. True B. pilosa tends to have fewer outer-series involucral bracts, absent or inconspicuous ray florets, and fewer pappus awns, among other differences, and also is a hexaploid (2n=72) taxon.