Herbs, annual, cespitose. Culms 1-15, trigonous, 7-30 cm × 1.2-2.5 mm, soft (flattened in pressing), glabrous. Leaves 2-7, flat, (2-)7-22 cm × 2.2-4 mm. Inflorescences: heads dense, 7-17 mm diam.; when rays short, heads sessile or nearly so, then densely irregularly lobate, 12-35 mm diam.; rays 1-5, 2-32 mm; bracts 2-4, longest bract erect or nearly so, appearing as continuation of culm, other bracts horizontal to ascending, 1-22 cm × 0.5-3.5 mm, margins and keel minutely scabridulous. Spikelets 30-120, greenish brown to purplish brown, oblong-ellipsoid, compressed, (2-)3-5(-6) × 0.8-1.2 mm; floral scales (6-)12-20(-30), laterally clear margins, stramineous to deep purple, medially greenish, stramineous, or purplish, laterally ribless, medially 3-ribbed, obovate to orbiculate, 0.6-0.8 × 0.6-0.8 mm, apex mucronulate. Flowers: stamens 1 or 2; anthers ovoid-ellipsoid, 0.1 mm, connective not prolonged; styles 0.1 mm; stigmas 0.1-0.3 mm. Achenes light brown, obovoid-ellipsoid, 0.6-0.8 × 0.3-0.4 mm (as long as subtending scale), base cuneate, apex obtuse, apiculate, surfaces finely reticulate, papillose. Fruiting summer. Disturbed, muddy soils, shallow waters; 0-1000 m; introduced; Ala., Ariz., Calif., Fla., Ga., Idaho, Ky., La., Miss., N.J., N.Mex., N.C., Oreg., Pa., S.C., Tenn., Tex., Va.; Mexico; West Indies (Puerto Rico); Central America (Nicaragua, Panama); South America; Eurasia; Indian Ocean Islands; Pacific Islands; Australia. Cyperus difformis is naturalized in the New World and native to the Old World, where it ranges from southern Europe to southern Africa and eastward to Southeast Asia and Australia.