Plants annual or facultatively perennial. Vegetative stems submersed with elongate internodes, or emersed and procumbent. Flowering stems 1-9 cm, distal internode 0.5-4 cm. Sessile leaves forming basal rosette, blade linear to oblanceolate, thin, 2.4-3.7 cm × 3-8 mm. Petiolate leaves floating or emersed; stipule 1-5 cm; petiole 2-13 cm; blade reniform, 1-4 × 1-5 cm, length equal to or less than width, apex obtuse. Inflorescences spicate, 2-8-flowered, elongating in 1 day, usually shorter than spathes, terminal flower sometimes extending beyond spathe apex; spathes 0.8-5.5 cm, glabrous; peduncle 0.5-4.2 cm, glabrous. Flowers opening ca. 3 hours after sunrise, wilting by early afternoon; perianth white, salverform, tube 5-10 mm, limbs zygomorphic, lobes narrowly elliptic, 3-6.5 mm, distal central lobe with yellow or green region at base, sometimes with distal brown spot; stamens unequal, lateral stamens 0.9-2.2 mm, filaments linear, pubescent with white multicellular hairs toward apex; central stamen 2.2-4.7 mm, filament sparsely pubescent with multicellular hairs; style pubescent with multicellular hairs. Seeds 8-14-winged, 0.5-0.9 × 0.3-0.5 mm. 2n = 48. Flowering Jun--Oct. Roadside ditches, edges of streams and ponds, freshwater tidal mudflats; 0--2600 m; Ala., Conn., Del., Fla., Ga., Ill., Ind., Ky., La., Md., Miss., Mo., N.J., N.Y., N.C., Ohio, Pa., S.C., Tenn., Tex., Va., W.Va.; Mexico; throughout Central America; scattered in South America (Argentina, Brazil, Paraguay); naturalized in Italy.