Annuals, (5-)25-150(-200) cm. Leaves: sessile or petioles (± winged) 5-15(-35+) mm; blades ± elliptic to lanceolate, (20-)40-100(-200+) × 10-30(-70) mm, sometimes laciniately 1-pinnatisect with 1-4+ lobes near bases, bases cuneate, margins entire or dentate to serrate, usually ciliate, apices attenuate, faces glabrous or hirtellous. Heads borne singly or in 2s or 3s. Peduncles 10-35(-80) mm. Calyculi of 2-6+ spreading or ascending, lanceolate, linear, oblanceolate, oblong, or spatulate, ± foliaceous bractlets or bracts 10-25(-35+) mm, margins (entire or serrate) usually ciliate, abaxial faces hispidulous near bases, distally glabrous. Involucres campanulate to hemispheric or broader, (4-)6-12 × (3-)6-9(-15+) mm. Phyllaries (6-)8(-9), elliptic to oblong or ovate, (4-)5-6(-12) mm. Ray florets usually 0, sometimes 1-5+; laminae yellowish, 4-8 mm. Disc florets (5-)20-40(-60+); corollas pale yellow to orange, 2-3 mm (± abruptly ampliate, anthers usually dark, blackish). Cypselae blackish to purplish or brown, usually (at least inner) ± 4-angled, usually cuneate to linear, sometimes ± obpyramidal, outer (3-)4-7 mm, inner (4-)5-8 mm, margins proximally antrorsely to patently, distally retrorsely, barbed, apices ± truncate to concave, faces usually strongly 1-nerved, usually tuberculate, glabrous or setulose; pappi 0, or of (1-)2-4(-6), ± erect to spreading, antrorsely or retrorsely barbed awns (0.2-)2-5 mm. 2n = 48. Flowering Aug-Oct. Marshes and other wet sites; 10-1700 m; N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Conn., Del., Ga., Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Pa., R.I., S.C., S.Dak., Tenn., Va., W.Va., Wis.; Europe; Asia; n Africa; introduced in Pacific Islands, Australia. Bidens connata may be better treated as part of B. tripartita.
Purplestem b.-t. Annual 1-20 dm, glabrous or nearly so; stem green to purplish; lvs simple, serrate, often some of them deeply 3(-7)-cleft, 3-15 cm, to 4 cm wide (exclusive of the lobes), on evident, sometimes winged petioles to 3 cm; heads erect, discoid or with rays to ca 4 mm, rather broadly campanulate to hemispheric, at least the terminal one generally with more than 30 fls, the disk 8-20 mm wide; outer invol bracts 4-9, herbaceous, but not so large as in no. 4 [Bidens comosa (A. Gray) Wiegand]; disk-cors (4)5-lobed, orange-yellow; anthers shortly exserted; achenes 3-7 mm, cuneate or cuneate-obovate, compressed-quadrangular, with the midribs evident and raised at least distally (or the marginal ones flatter), the surface mostly tuberculate and rather sparsely bristly-strigose; pappus of (2-)4(-6) retrorsely (rarely antrorsely) barbed awns, the median ones generally shorter than the lateral; 2n=24, 48, 60. Wet waste places, widespread in c. and e. U.S. and adj. Can., w. to N.D., and Kans., s. to N.C. and Mo., and sporadically elsewhere. Aug.-Oct. Only with difficulty to be distinguished from the Eurasian B. tripartita L., with more consistently cleft lvs and mostly smoother, often flatter achenes.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.