Annuals, (2-)20-100(-400+) cm (stems sometimes rooting at proximal nodes). Leaves (rarely in 3s) sessile; blades lance-ovate or oblanceolate to lanceolate or linear, 40-100(-200+) × (2-)5-25(-45+) mm, bases cuneate to rounded, margins usually coarsely dentate to serrate, sometimes entire, sometimes ciliate, apices acute to acuminate, faces glabrous. Heads (erect or nodding at flowering, usually nodding in fruit) borne singly or in open, ± corymbiform arrays. Peduncles 10-40(-100+) mm. Calyculi of (3-)5-8(-10) spreading to reflexed, oblong to lance-linear, often ± foliaceous bractlets or bracts (3-)8-12(-25+) mm, margins usually ciliate, abaxial faces usually glabrous, bases sometimes hispidulous. Involucres ± hemispheric or broader, (3-)6-10 × (8-)12-20+ mm. Phyllaries 6-8+, ovate or lance-ovate to lanceolate, 2-10 mm. Ray florets usually 6-8, sometimes 0; laminae orange-yellow, 2-15(-18) mm. Disc florets (10-)40-100(-150+); corollas orange-yellow, 3-4 mm. Cypselae blackish or brown, usually ± flattened, sometimes ± 4-angled, ± cuneate, outer (3-)5-6+ mm, inner 4-8 mm, margins (± thickened or winged) retrorsely ciliate, apices ± truncate to convex, faces ± striate, glabrous or tuberculo-strigillose; pappi of (2-)4, ± erect, retrorsely barbed awns (1-)2-4 mm. 2n = 24, 48. Flowering late (Jul-)Aug-Sep(-Oct). Swamps, marshes, peat and sedge bogs, flood plains; 0-2300 m; Alta., B.C., Man., N.B., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Europe, Asia. Bidens cernua is used medicinally to treat urinary-tract infections.

Plant: Annual; stem 1-9 dm, erect, glabrous to short-rough-hairy Leaves: simple, sessile, opposite; bases fused around stem; blades 4-20 cm, linear-lanceolate to lanceolate, acuminate, serrate, glabrous INFLORESCENCE: primary inflorescence a head, each resembling a flower; heads radiate or discoid, erect in flower, nodding in fruit; peduncles 1-7 cm; involucre 1-2 cm diam, hemispheric; outer phyllaries 5-8, 1-3 cm, linear-lanceolate, spreading; inner phyllaries 8-10 mm, ovate-lanceolate; chaff scales 6-8 mm, yellowish Flowers: Ray flowers 0 or 6-8; ligules 8-15 mm, yellow; Disk flowers: corollas 3-4 mm, yellow Fruit: achenes, 5-7 mm, narrowly wedge-shaped, 4-angled, flat; angles thick, barbed; pappus awns generally 4, 2-3 mm Misc: Freshwater wetlands; < 2000 m.