Annuals [perennials], (10-)30-60(-180+)[-250] cm. Leaves: petioles 10-30(-70) mm; blades either ovate to lanceolate, 30-70(-120) × 12-18(-45) mm, or 1-pinnately lobed, primary lobes 3-7, ovate to lanceolate [linear], (10-)25-80+ × (5-)10-40+ mm [blades 2(-3)-pinnatisect], bases truncate to cuneate, ultimate margins serrate or entire, usually ciliate, apices acute to attenuate, faces pilosulous to sparsely hirtellous or glabrate. Heads usually borne singly, sometimes in open, ± corymbiform arrays. Peduncles 10-20(-90) mm. Calyculi (6-)7-9(-13) ± appressed, spatulate to linear bractlets (3-)4-5 mm, margins ciliate, abaxial faces usually hispidulous to puberulent. Involucres turbinate to campanulate, 5-6 × (6-)7-8 mm. Phyllaries (7-)8-9(-13), lanceolate to oblanceolate, 4-6 mm. Ray florets 0 or (3-)5-8+; laminae whitish to pinkish [yellowish], 2-3 or 7-15+ mm. Disc florets 20-40(-80+); corollas yellowish, (2-)3-5 mm. Cypselae: outer red-brown, ± flat, linear to narrowly cuneate, (3-)4-5+ mm, margins antrorsely hispidulous, apices ± truncate or somewhat attenuate, faces obscurely 2-grooved, sometimes tuberculate-hispidulous; inner blackish, ± equally 4-angled, linear-fusiform, 7-16 mm, margins antrorsely hispidulous, apices ± attenuate, faces 2-grooved, tuberculate-hispidulous to sparsely strigillose; pappi 0, or of 2-3(-5), erect to divergent, retrorsely barbed awns (0.5-)2-4 mm. 2n = 24, 36, 48, 72. Flowering year round. Disturbed, wettish sites; 10-1900 m; Ont. Que.; Ala., Ariz., Calif., Conn., Fla., Ga., Ky., La., Md., Mass., Miss., Mo., N.Mex., N.C., Pa., S.C., Tex.; Mexico; Central America; introduced, Europe. R. Ballard (1986) adopted a narrower circumscription of Bidens pilosa than that used here. He used: B. pilosa for plants with outer phyllaries 7-10, ray florets usually 0 (when present, laminae 2-3 mm), disc florets 35-75, pappi of 3(-5) awns 1-3 mm, and 2n = 72; B. alba for plants with outer phyllaries (8-)12(-16), ray florets 5-8 (laminae 5-16 mm), pappi of 2 awns 1-2 mm, and 2n = 48; and B. odorata for plants with outer phyllaries (6-)8(-12), ray florets 5-8 (laminae 3-18 mm), disc florets 12-61, pappi 0, or of 1-2 awns 1-3 mm, and 2n = 24.