4. Bidens bipinnata L. (Spanish needles)

Pl. 274 i, j; Map 1152

Plants annual, terrestrial, usually with taproots. Stems 15–60(–150) cm, erect or ascending, glabrous or sparsely to moderately pubescent with minute, more or less spreading hairs. Leaves all more or less similar, short- to long-petiolate, opposite, the blade 3–20 cm long, lanceolate to ovate in outline, all except rarely those of the uppermost leaves 2 or 3 times pinnately lobed, divided, and/or compound into 7 to numerous ultimate lobes or segments, these lanceolate to obovate, mostly angled or tapered at the base, without a stalklike base, tapered to a sharply pointed tip, the margins otherwise entire or coarsely few-toothed, sometimes minutely hairy, the surfaces glabrous or the undersurface sparsely and minutely hairy along the main veins. Inflorescences of solitary terminal heads or appearing in loose, open clusters, the heads discoid or appearing discoid, not nodding at fruiting. Involucre with the outer series of 7–10 bracts 3–5 mm long, ascending to more commonly spreading, mostly not leaflike, linear to narrowly oblong-oblanceolate, the margins entire but usually with minute, ascending hairs, the outer surface glabrous; the inner series of 8–12 bracts 4–9 mm long, narrowly lanceolate to linear, glabrous. Chaffy bracts narrowly oblong to linear (elongating somewhat as the fruits mature), usually with a minute fringe of white hairs at the otherwise greenish tip. Ray florets absent or 1–5, the corolla inconspicuous, 2–4 mm long, yellow. Disc florets 12–27, the corollas 1–2 mm long, yellow. Pappus of 2–4 awns 1–4 mm long, these with downward-pointed barbs, erect to slightly spreading at fruiting. Fruits 10–18 mm long, linear, strongly 4-angled (more or less square in cross-section), the angles glabrous or with a few minute, stiff, ascending hairs (these denser in immature fruits), the faces each with a pair of slender longitudinal grooves, dark brown to black, often somewhat mottled, glabrous. 2n=24, 72. August–October.

Scattered, mostly south of the Missouri River (eastern U.S. west to Nebraska and Arizona; Canada, Mexico, Central America, South America, Asia, Madagascar, Malesia, Australia). Upland prairies, glades, openings of mesic to dry upland forests, bottomland forests, and banks of streams and rivers; also ditches, pastures, fallow fields, gardens, railroads, roadsides, and open, disturbed areas.

The natural range of this species is not well understood as it long ago became established as a weed in many parts of the world. Sherff (1955) speculated that the native range prior to the period of European expansionism was in the eastern United States and eastern Asia. The distributional summary above does not attempt to discriminate native vs. adventive components of the overall range of the species.