Annuals, (5-)25-35(-55+) cm. Leaves: petioles (narrowly, if at all, winged) 5-20(-35+) mm; blades lanceolate to lance-linear, (20-)30-50(-80+) × (3-)6-20(-30+) mm, sometimes laciniately 1-pinnatisect with 2-4+ lobes near bases, bases cuneate, margins incised to serrate, seldom ciliate, apices acute to attenuate, faces glabrous. Heads borne singly or in 2s or 3s. Peduncles 10-35(-55) mm. Calyculi of (2-)3-5(-7) ± erect, oblanceolate or spatulate to linear, ± foliaceous bractlets or bracts (6-)10-25(-40+) mm, margins rarely ciliate, abaxial faces usually glabrous, sometimes sparsely hispidulous. Involucres usually obconic to broadly campanulate, rarely hemispheric, (4-)6-7 × (4-)5-7 mm. Phyllaries (4-)5-8, oblong to lance-ovate, (4-)5-8+ mm. Ray florets usually 0, sometimes 1-3+; laminae orange yellowish, 2-5(-10) mm. Disc florets (7-)20-70+; corollas yellowish, 2.5-3.5 mm. Cypselae: outer red-brown, ± flattened, sometimes 3-4-angled, broadly cuneate, 4-5(-6) mm, inner blackish, narrowly cuneate, (5-)6-7(-8+) mm, margins glabrous or sparsely antrorsely strigillose, apices ± truncate to convex, faces ± 1-nerved, glabrous or sparsely setulose, usually smooth, sometimes ± tuberculate; pappi 0, or of 2(-4) spreading to divergent, antrorsely or retrorsely barbed awns (0.1-)1-3.5+ mm. 2n = 48. Flowering Aug-Oct. Borders of estuaries, lakes, marshes, streams; 0-10 m; N.B., P.E.I., Que.; Conn. Bidens heterodoxa may be better treated as part of B. tripartita.