10. Bidens tripartita L. (beggar-ticks, swamp beggar-ticks)

B. comosa (A. Gray) Wiegand

B. connata Muhl. ex Willd.

B. connata var. petiolata (Nutt.) Farw.

Pl. 273 j, k; Map 1158

Plants annual, terrestrial, with taproots. Stems 10–90(–200) cm, erect or ascending, sometimes straw-colored or purplish-tinged, glabrous. Leaves all more or less similar, sessile or short- to long-petiolate, the petiole often partially winged, opposite, the blade 2–8(–15) cm long, lanceolate to elliptic (in unlobed leaves) or broadly ovate to ovate-triangular (in divided leaves) in outline, mostly unlobed, but occasionally those of the larger leaves deeply and sometimes irregularly 1 time ternately or pinnately lobed into 3(5) lobes or segments or leaflets, these lanceolate to elliptic, angled or tapered but not stalked at the base, tapered to a sharply pointed tip, the margins sharply and usually coarsely toothed, rarely nearly entire, the surfaces glabrous or sparsely to moderately pubescent with fine, short hairs. 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 4–9 bracts 7–35(–70) mm long, loosely ascending to spreading, mostly leaflike, oblanceolate to elliptic, oblong-lanceolate, or linear, the margins entire or finely toothed, often also with spreading hairs, the outer surface glabrous or sparsely short-hairy toward the base; the inner series of 7 or 8(–12) bracts (4–)7–12 mm long, narrowly ovate to ovate, glabrous. Chaffy bracts narrowly lanceolate, usually purplish-tinged at the tip. Ray florets absent or less commonly present, if present then 1–5, the corolla inconspicuous, 3–8 mm long, yellow. Disc florets 20–40(–80), the corollas 3–4 mm long, sometimes only 4-lobed, yellow to orangish yellow. Pappus absent or more commonly of (2–)3 or 4 awns mostly 2–3 mm long, these with upward- or more commonly downward-pointed barbs, erect or somewhat spreading at fruiting. Fruits 3–11 mm long, linear to narrowly wedge-shaped, more or less flattened and somewhat 3- or 4-angled in cross-section, not winged, the faces each with a longitudinal nerve, dark brown to purplish black, glabrous or with sparse, short, upward angled, fine hairs, sometimes also with minute tubercles. 2n=24, 48, 60, 72. July–October.

Scattered nearly throughout the state but absent or uncommon in the southern portion of the Ozark Division (U.S., Canada, Europe, Asia, Africa; introduced in the Pacific Islands, Australia). Banks of streams and rivers, margins of ponds and lakes, sloughs, swamps, bottomland forests, and fens; also ditches and railroads.

The B. tripartita complex comprises both Old World plants and those native to the New World that have been segregated variously under the names B. comosa and B. connata, as well as a number of infraspecific names (Hall, 1967; Cronquist, 1980). Morphological differences between populations that various authors have used to separate taxa include stem color, leaf divisions, number of disc corolla lobes, relative size and shape of the involucral bracts, and pappus details, but these do not seem to correlate well enough to allow the segregation of discreet species or varieties based on present knowledge. However, not all of the plants currently called B. tripartita may represent the same biological entity, as evidenced by the broad geographic range, great morphological variation, and multiple ploidy levels reported in the literature. Crowe and Parker (1981) studied this and related species of Bidens in Ontario and suggested that populations they called B. connata probably arose through past hybridization between B. cernua and B. frondosa, which had regained fertility by switching to an apogamous life cycle. Further studies are needed involving genetic variation in plants from throughout the range of the complex.