Herbs, annual, cespitose, with fibrous roots. Culms 1-20, trigonous, 2-16 cm × 0.5-0.8 mm, glabrous. Leaves 1-3, flat to V-shaped, (1-)5-10(-15) × 0.5-2.5 mm. Inflorescences: spike 1, loosely to densely ovoid to oblong, 6-20(-40) × 9-15(-20) mm; rays absent or 1-3(-6), 4-40 mm; bracts (1-)2-4, longest ± erect, V-shaped, 1-15 cm × 0.5-3 mm; rachilla ± deciduous, wingless. Spikelets (2-)6-20(-40), greenish to reddish brown, somewhat compressed, ovoid-lanceoloid to oblong, 2.5-10(-20) × 1.3-2.2 mm (excluding awns); floral scales deciduous, (4-)10-20(-34), greenish to stramineous or brownish red laterally, greenish medially, (5-)7-9(-11)-ribbed nearly to margins, oblong-lanceolate, (1.2-)1.3-1.8(-2.2) × (0.5-)0.7-0.8(-1) mm, apex cuspidate, excurved awn additional 0.5-1(-1.3) mm. Flowers: stamen 1, filaments 1.5 mm; anthers oblong, 0.2-0.3 mm, connective apex reddish, minute; styles 0.3-0.5 mm; stigmas 0.4-0.7 mm. Achenes light brown to nearly black, stipitate, obovoid (occasionally linear-spatulate or linear oblong, infrequently constricted in middle), 0.7-0.8(-1.1) × (0.2-)0.3-0.4(-0.5) mm, stipe 0.05-0.1 × 0.1 mm, apex truncate, apiculate, surfaces papillose. Fruiting summer. Moist, disturbed soils, gravelly roadsides, flood plains, edges of puddles, muddy places; 0-2500 m; Alta., B.C., Man., N.B., Ont., Que.; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah., Vt., Va., Wash., W.Va., Wis., Wyo.; Mexico; West Indies; Bermuda; Central America; South America; Eurasia; Africa; Atlantic Islands; Indian Ocean Islands; Pacific Islands; Australia. Cyperus squarrosus can be recognized by its small size and annual habit combined with its oblong-lanceolate floral scales bearing five to eleven conspicuous ribs and excurved awns. Some collections have been misidentified as C. acuminatus, an annual species of subg. Pycnostachys that has ovate-lanceolate, three-ribbed floral scales and digitately clustered spikelets.