Herbs, perennial, cespitose, 2.5--6.5 dm. Culms erect, terete, 1--3 mm diam., smooth. Cataphylls 0--1, maroon or dark green, apex obtuse. Leaves: basal 1, cauline 2--3; auricles 1--2.1 mm, apex rounded, membranaceous; blade maroon or dark green, terete to compressed, 3--20 cm x 1--2.4 mm. Inflorescences terminal panicles of 30--70(--130), 5--20 cm, branches spreading; primary bract erect; ; heads (1--)2--10-flowered, hemispheric or narrower, 5--10 mm diam. Flowers: tepals green to straw-colored, lanceolate, apex acute; outer tepals (2--)2.6--3.2 mm; inner tepals (1.8--)2.3--3 mm; stamens 3, anthers 1/2--2/3 filament length. Capsules exserted, , straw-colored, 1-locular, linear-lanceoloid, 4--5.2 mm, apex acute, valves separating at dehiscence. Seeds oblong-ellipsoid, 0.3--0.4 mm, not tailed; body clear yellow-brown. Fruiting summer. In soft mucky substrates, marshy shores, sloughs, occasionally in wet wooded places, often in shallow water; commonly abundantly colonizing wet, sandy- alluvial outwash in ditches and clearings; 10--1000 m; Ala., Ark., Calif., Conn., Fla., Ga., Ill., Ind., Kans., Ky., La., Md., Miss., Mo., N.C., Ohio, Okla., S.C., Tenn., Tex., Va., Wash., W.Va.; probably introduced, South America.