2. Bidens aristosa (Michx.) Britton (tickseed sunflower)

B. polylepis S.F. Blake

B. aristosa var. fritcheyi Fernald

B. aristosa var. mutica (A. Gray) Gatt. ex Fernald

B. aristosa f. mutica (A. Gray) Wunderlin

B. aristosa var. retrorsa (Sherff) Wunderlin

B. polylepis var. retrorsa Sherff

B. aristosa f. involucrata (Nutt.) Wunderlin

Pl. 273 g–i; Map 1150

Plants annual or biennial, terrestrial, sometimes with taproots. Stems 15–100(–150) cm, erect or ascending, glabrous or sparsely pubescent with short, ascending hairs. Leaves all more or less similar, short- to less commonly long-petiolate, opposite, the blade 2–15 cm long, lanceolate to ovate or triangular-ovate in outline, all but occasionally those of the uppermost leaves 1 or 2 times pinnately deeply divided or compound into 3–7 segments or leaflets, these linear to lanceolate or narrowly elliptic, angled or more commonly tapered at the base, the middle one frequently with a stalklike base, tapered to a sharply pointed tip, the margins sharply and finely to coarsely toothed, also minutely hairy, the surfaces glabrous or sparsely hairy. Inflorescences of solitary terminal heads or appearing in loose, open clusters or small panicles, the heads radiate, not nodding at fruiting. Involucre with the outer series of 8–25 bracts 4–12(–25) mm long, spreading to more commonly reflexed, mostly not leaflike, linear, the margins entire but usually with short, spreading hairs, the outer surface glabrous or with short, spreading hairs; the inner series of 6–8(–12) bracts 4–12 mm long, narrowly lanceolate to narrowly ovate, glabrous. Chaffy bracts narrowly lanceolate, usually with broad, yellowish margins and tip, occasionally purplish-tinged at the tip. Ray florets 5–8, the corolla showy, 10–25 mm long, yellow. Disc florets 20–80(–120), the corollas 2.0–3.5 mm long, yellow. Pappus absent, reduced to a minute crown with 2 teeth, or more commonly of 2 awns variously 0.5–4.0 mm long, these with upward- or downward-pointed barbs, erect to somewhat spreading at fruiting. Fruits 4–8 mm long, wedge-shaped to obovate (mostly 1.5–2.5 times as long as wide), more or less flattened and somewhat 3-angled in cross-section, the angles roughened with short, stiff, ascending, pustular-based hairs, the faces sometimes with few to several faint longitudinal lines, dark brown to black, sparsely to moderately pubescent with short, fine, mostly appressed hairs. 2n=24. August–October.

Scattered to common throughout the state (eastern U.S. west to Minnesota, Colorado, and New Mexico; Canada). Bottomland prairies, upland prairies, bottomland forests, margins of ponds and lakes, sloughs, and fens; also crop fields, fallow fields, railroads, roadsides, and open, disturbed areas.

Bidens aristosa is highly variable morphologically. The present treatment accepts a broader species circumscription for the species than that found in much of the North American botanical literature. Most authors have treated the complex as comprising two species, B. aristosa and B. polylepis, with a number of additional infraspecific taxa (Sherff, 1955). Bidens polylepis has been said to differ from B. aristosa in its involucral bracts of the outer series more numerous, larger, and hairier, as well as its slightly smaller fruits. However, most authors who have studied the group have conceded that there is strong overlap between the two extremes (Steyermark, 1963). In a short note preliminary to an account of Illinois Asteraceae, Wunderlin (1972) reduced the main varieties of B. polylepis to forms of B. aristosa var. retrorsa (although current rules would require that the as-yet unpublished name var. polylepis be used instead). Subsequently, Lipscomb and Smith (1977) completed a detailed numerical analysis of morphological variation in some Arkansas populations, concluding that there is no rational basis for the continued recognition of two entities at any taxonomic level. Their treatment is followed here. Occasional plants with the pappus awns having downward-pointed (vs. upward-pointed in f. aristosa) barbs have been called f. fritcheyi, var. fritcheyi, or var. retrorsa, whereas plants with the awns absent or highly reduced have been called f. mutica or var. mutica.