Mem. Amer. Acad. Arts, n. s. 4: 86. 1849
Annuals, (10–)20–40 cm. Leaves: petioles 5–40 mm; blades rounded-deltate to ovate overall, 20–50+ × 15–35+ mm, (1–)2–3-pinnatisect, ultimate lobes ± linear, (2–)5–15+ × (0.5–)2–3+ mm, bases ± cuneate, ultimate margins entire, sometimes ciliolate, apices acute to attenuate, faces usually glabrous, sometimes hispidulous. Heads usually borne singly, sometimes in 2s or 3s. Peduncles 20–30(–80) mm. Calyculi of 6–12 ± appressed, linear bractlets 5–7 mm, margins scabro-ciliate, abaxial faces usually pilosulous to hirsutulous, rarely glabrous. Involucres campanulate to hemispheric or broader, (5–)6–7 × 4–8 mm. Phyllaries 8, lance-linear, 5–7 mm. Ray florets 0 or 3–6; laminae yellow, 4–6 mm. Disc florets (5–)12–20(–45); corollas yellow, 2–3 mm. Cypselae: outer red-brown, flat or weakly 4-angled, linear, 6–8 mm, margins not ciliate, apices somewhat attenuate, faces obscurely 2-grooved, often tuberculate, glabrous or ± strigillose; inner similar, blackish (sometimes with pale tips), ± equally 4-angled, linear-fusiform, 8–15 mm, margins not ciliate, apices attenuate, faces 2-grooved, glabrous or, toward tips, strigillose; pappi of 2(–3), spreading to divergent, retrorsely barbed awns (1–)1.5–3 mm.
Phenology: Flowering (Jul–)Aug–Sep(–Oct).
Habitat: Meadows, along streams
Elevation: 1800–2600 m
Ariz., Colo., Idaho, N.Mex., Utah, Mexico (Chihuahua).
Reports of Bidens tenuisecta from Maryland, Massachusetts, and New York may be based on ephemeral introductions.