Pittonia 4: 72. 1899.
Annuals, (15–)30–50(–150) cm. Leaves: petioles 10–50 mm; blades ± deltate to ovate overall, 50–100(–150+) × (15–)30–80(–120+) mm, usually laciniately 1-pinnatisect or 3–5-foliolate, primary lobes or leaflets ± lanceolate, 20–80(–120) × 10–25(–40+) mm, blades rarely 2–3-pinnatisect, bases cuneate, ultimate margins dentate to serrate, little, if at all, ciliate, apices acute to attenuate, faces glabrous or ± hispidulous. Heads borne singly or in 2s or 3s or in open, corymbiform arrays, erect. Peduncles (10–)40–150+ mm. Calyculi of 10–16(–21) ascending to spreading, spatulate to linear, seldom foliaceous bractlets or bracts 10–20(–40) mm, margins usually hispid-ciliate, abaxial faces ± hispidulous. Involucres hemispheric or broader, 5–6 × 8–10 mm. Phyllaries 10–12, ovate to lanceolate, 6–9 mm. Ray florets 0 or 3–5+; laminae pale yellow, 2.5–3.5 mm. Disc florets 40–60(–150+); corollas yellow, 2.5–3.5 mm. Cypselae purplish, brown, olive, or stramineous, ± flattened, obovate to cuneate, outer 6–10 mm, inner 8–12 mm, margins (sometime ± winged) proximally antrorsely, distally retrorsely barbed, apices ± truncate, faces obscurely 1-nerved, sometimes tuberculate, glabrous or sparsely strigillose; pappi of 2 erect to divergent, retrorsely barbed awns 3–4(–7) mm. 2n = 24, 48.
Phenology: Flowering Aug–Sep(–Oct).
Habitat: Ditches, shores of lakes and streams, swamps, marshes, moist woods, roadsides, railroads, fields, waste areas
Elevation: 10–1000 m
Alta., B.C., Man., N.B., N.S., Ont., Que., Sask., Ark., Calif., Colo., Conn., Del., D.C., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo., introduced in Europe.
Bidens vulgata is similar to B. frondosa; it is more robust. Both are phenotypically plastic. Locally, B. vulgata often matures earlier than B. frondosa.