Neue Entd. 2: 135. 1821.
Perennials (perhaps flowering first year) to 200+ cm (aquatics). Leaves sessile; submersed blades multifid (2–3+-pinnatisect), ultimate lobes filiform, mostly 0.1–0.3 mm diam.; aerial blades ovate to lanceolate, 10–45+ × 5–20+ mm, bases cuneate, margins pectinately incised to serrate or entire, not ciliate, apices obtuse to acuminate or attenuate, faces glabrous. Heads usually borne singly. Peduncles (10–)20–100 mm. Calyculi of 5–6 usually spreading, oblong to obovate bractlets 5–8 mm, margins entire, not ciliate, abaxial faces usually glabrous. Involucres ± hemispheric, 7–12 × 12–15 mm. Phyllaries 7–8+, ovate, 7–10 mm. Ray florets 8; laminae yellow, 10–15 mm. Disc florets 10–30+; corollas pale yellow, 5–6 mm. Cypselae (outer and inner ± alike) yellowish to greenish brown, nearly terete or weakly 4-angled, ± linear, 10–15 mm, margins not barbed or ciliate, apices truncate, faces smooth or ± striate, glabrous; pappi of 2(–6) divergent to patent, retrorsely barbed (on distal 1/4 or so) awns 13–25(–40) mm. 2n = 26.
Phenology: Flowering Jul–Sep.
Habitat: Still or slow-moving waters
Elevation: 0–300+ m
B.C., Man., N.B., N.S., Ont., Que., Sask., Conn., Idaho, Ill., Ind., Iowa, Maine, Mass., Mich., Minn., Mo., Mont., N.H., N.J., N.Y., Ohio, Oreg., Pa., R.I., Vt., Wash., Wis.
M. L. Roberts (1985) presented a strong case for treating Bidens beckii as Megalodonta beckii.