Annuals, to 200+ cm (delicate or robust, glabrate or glabrous). Stems erect. Leaves petiolate (proximal, petioles 10-20 mm) or sessile (distal); blades lanceolate or oblanceolate to elliptic or subovate, 30-150 × (7-)10-40 mm, bases (distal) connate, margins serrate, serrate-dentate, or spinulose-serrate. Heads 30-300+, in tight, axillary, sessile glomerules (receptacles of glomerules setose). Calyculi 0. Involucres oblong and cylindric or angular, 3.8-4.5 mm. Phyllaries usually 2, oblong (closely investing and falling with mature cypselae). Ray florets 0-1; laminae pale yellow or whitish, oblique or suborbiculate, 0.5-1 mm. Disc florets 0-1(-2); corolla tubes 0.5-1.4 mm, throats campanulate, 0.5-0.8 mm. Cypselae oblanceoloid to subclavate, 2-2.6 mm (rays longer); pappi 0. 2n = 36. Flowering Mar-Dec. Near water, saline and gypseous areas; 0-1900 m; Ariz., Calif., Fla., Mass., Mo., N.Mex., Tex., Va.; West Indies; Central America (British Honduras); South America (Brazil, Ecuador, Peru, Venezuela); probably introduced in Asia (India, Middle East); Africa; Pacific Islands (Hawaii). Flaveria trinervia is widespread and weedy; it often occurs in saline, gypseous, disturbed areas near permanent or ephemeral water sources in southern Florida and from Texas to southern California. It occurs also in scattered locations in some eastern states and has been reported from Alabama.

The heads of Flaveria trinervia, which usually contain just one floret, are either radiate or discoid; radiate heads tend to occur on the periphery of setose glomerules. Reduction of some of the floral features, including number of florets [0-1(-2)], phyllaries per head (2), and size of ray laminae, suggest that F. trinervia may be the most derived species in the genus.

Plant: Low annual forb; stems dichotomously branched, 15-80 cm Leaves: leaves opposite, lanceolate, toothed, 3-nerved, 3-15 cm, simple, petioled or upper sessile; bases often fused around stem; tip acute to obtuse; surfaces glabrous INFLORESCENCE: primary inflorescence a head, each resembling a flower; heads in dense, sessile, head-like clusters at forks of stem; involucres < 1 mm diam; phyllaries generally 2, 4-4.5 mm, oblong, obtuse Flowers: Ray flowers generally 1; ligule 0.5-1 mm, creamy yellow; Disk flowers: 1-15; corollas 2-2.5 mm, yellow; style tips flattened, obtuse Fruit: 2-2.6 mm, black, 10-ribbed, ± flattened, glabrous, shining Misc: Moist soil in waste places, cultivated areas; < 200 m.