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. Flowering (Jul-)Aug-Sep(-Oct). Meadows, along streams; 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.