Herbs, perennial, aquatic to semiterrestrial, stoloniferous, to 50 dm. Stems prostrate, forming mats, often fistulose, glabrous. Leaves sessile; blade narrowly elliptic, elliptic, or oblanceolate, 3.5-7.1 × 0.5-2 cm, herbaceous, apex acute to obtuse, glabrous. Inflorescences terminal and axillary, pedunculate; heads white, globose, 1.4-1.7 cm diam.; bracts not keeled, less than 1/2 as long as tepals. Flowers: tepals monomorphic, white, lanceolate or oblong, 6 mm, apex acute, glabrous; stamens 5; pseudostaminodes ligulate. Utricles not seen. Seeds not seen. Flowering early spring-summer. Ponds, ditches, streams, bayous; 0-200 m; introduced; Ala., Ark., Calif., Fla., Ga., Ill., Ky., La., Miss., N.C., S.C., Tenn., Tex., Va.; West Indies; native to South America. Alternanthera philoxerioides is said to reproduce only vegetatively in North America. I have seen no fruit or seed.
