Bidens pilosa
Sp. Pl. 2: 832. 1753.
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.
Phenology: Flowering year round.
Habitat: Disturbed, wettish sites
Elevation: 10–1900 m
Distribution
Introduced; 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, Europe.
Discussion
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.