Annuals (sometimes persisting), (10-)20-60(-120+) cm. Leaves (sometimes in 3s or 4s) sessile; blades obovate or elliptic to lanceolate or linear, (20-)50-100(-160+) × (5-)10-25(-40+) mm, bases cuneate to rounded, margins usually coarsely dentate to serrate, sometimes ciliate, apices acute to acuminate, faces glabrous. Heads (erect at flowering, sometimes nodding in fruit) borne singly or in open, ± corymbiform arrays. Peduncles (10-)20-60 mm. Calyculi of 5-7(-9+) erect or spreading to reflexed, oblanceolate or lanceolate to linear, often foliaceous bractlets or bracts (6-)10-12(-20+) mm, margins usually ciliate, abaxial faces glabrous or bases hispidulous. Involucres turbinate to hemispheric or broader, (4-)6-8(-10+) × 8-12+ mm. Phyllaries 8-12, ovate or obovate to lance-oblong, (4-)6-8(-10+) mm (tips often orange to purplish, as are tips of paleae). Ray florets usually 7-8, rarely 0; laminae orange-yellow, (10-)15-25(-30) mm. Disc florets (25-)60-100(-150+); corollas yellow to orange-yellow, 3-6.5 mm. Cypselae blackish, red-brown, or stramineous, obcompressed, flattened or unequally 3-4-angled, ± cuneate, outer 6-8 mm, inner 8-10 mm, margins retrorsely ciliate or barbed, apices ± truncate to convex, faces ± 1-nerved, ± striate, glabrous; pappi of 2-4 ± erect, retrorsely barbed awns 3-5 mm. 2n = 22, 24.
Bidens laevis and B. cernua are similar in gross appearance; specimens of one are sometimes misidentified as the other. Perhaps they represent extremes of a single species.
FNA 2006, Wiggins 1964, Kearney and Peebles 1969
Duration: Annual Nativity: Native Lifeform: Forb/Herb General: Annual or (occasionally) perennial herb, up to 100 cm tall; herbage glabrous. Leaves: Opposite or whorled with 3-4 leaves per node, sessile; blades lanceolate to elliptic, 5-10 cm long, glabrous, with coarsely serrate margins. Flowers: Flower heads yellow, radiate, on peduncles 2-6 cm long, solitary or arranged in open flat-topped panicles; calyculi (extra set of bracts just below the involucre) of 5-9 green, oblong bracteoles, up to 2 cm long, with ciliate margins; involucre (ring of bracts wrapped around the flower head) turbinate to hemispheric or broader, 6-8 mm high by 1 cm wide, the bracts (phyllaries) 8-12, ovate to lance-oblong, often with orange to purplish tips; ray florets 7-8, showy, occasionally lacking, the corolla laminae (ray petals) orange-yellow, 1-3 cm long; disc florets 60-100, the corollas yellow to orange-yellow, 3-7 mm high. Fruits: Achenes 6-9 mm long, brown or purplish-brown, flattened, with 2-4 retrorsely barbed awns. Ecology: Found in low, wet places and in shallow water, from 4,000-9,000 ft (1220-2740 m); flowers July-October. Distribution: US southwest, south and east, from CA east to ME and FL; south through MEX to S. Amer. Notes: Bidens is a genus of herbaceous plants with opposite leaves, discoid or radiate flower heads with yellow-orange flowers, and seeds (achenes) with 2-4 barbed awns. B. laevis is distinguished as a large, robust annual wetland plant with no hairs on the foliage; simple, sessile leaves with teeth along the edges; large, showy flower heads with yellow-tipped phyllaries, yellow rays, and darker yellow discs with black stigmas; and seeds with two prongs; the plants can often form abundant stands in wetlands. Similar to B. cernua but the ray flowers are larger and more showy, and the heads are not consistently nodding in fruit. Ethnobotany: Used by Paiute for food. Etymology: Bidens is derived from the Latin bis, twice and dens, tooth, hence meaning 2-toothed, while laevis means smooth or free of hairs. Synonyms: Bidens elegans, Bidens nashii, Helianthus laevis Editor: SBuckley 2010, FSCoburn 2014, AHazelton 2015
Much like no. 2 [Bidens cernua L.] wholly glabrous, sometimes perennial; heads not so consistently nodding; outer invol bracts scarcely leafy, seldom surpassing the disk; rays 1.5-3 cm; receptacular bracts reddish at the tip; achenes 3-4-angled, or often flat; pappus of 2-4 retrorsely barbed awns; 2n=22, 24. Low, wet places, sometimes in shallow water, chiefly but not wholly coastal; N.H. and Mass. to Fla. and Calif., s. to S. Amer. Aug.-Nov. Passes into no. 2.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.