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.
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