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. Flowering Jul-Sep. Still or slow-moving waters; 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.
From Flora of Indiana (1940) by Charles C. Deam
Floating in still, shallow water of bayous of lakes and rivers. This species has been reported from Fulton, Kosciusko, Lake, Marshall, Starke, Steuben, and Whitley Counties. Doubtless it was formerly found throughout the lake area but the settlement of all lake fronts has destroyed it. Another reason why it is not commonly reported is because it is inconspicuous except at its flowering time, which is of short duration.
Aquatic perennial; lvs 2-4 cm, the submersed ones filiform- dissected, the emersed ones simple, lanceolate to ovate, sessile, serrate; heads terminal and solitary, the disk 1 cm wide; rays 1-1.5 cm; outer invol bracts more herbaceous and less striate than the rather similar inner ones; achenes subterete, 10-14 mm; pappus of 3-6 awns, longer than the achenes, retrorsely barbed above; 2n=26. In ponds and streams; Que. to N.J., w. to Man. and Mo.; disjunct in Oreg., Wash., and B.C. July-Oct. (Megalodonta b.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.