Plants cespitose. Culms acutely angled, 30-115 cm, scabrous. Leaves: basal sheaths red-brown, indistinctly ladder-fibrillose; sheaths of proximal leaves glabrous, fronts with red-brown spots, veinless, apex U-shaped; blades 3-5 mm wide. Inflorescences: proximal bract subequal to inflorescence, 2-4 mm wide. Spikes erect; proximal 2-3 spikes pistillate, 1-5 cm × 4-6 mm, base obtuse; terminal 1-2 spikes staminate. Pistillate scales red-brown, longer than perigynia, apex acute or acuminate, awnless. Perigynia divergent, olive-brown with red-brown spots on apical 1/2, veinless, inflated, loosely enclosing achenes, obovoid, 1.5-2.8 × 1.5-2 mm, dull, apex rounded, papillose; beak green, 0.1-0.2 mm. Achenes not constricted, glaucous. 2n = 54. Fruiting Jun-Aug. Wet meadows, moist prairies; 0-1,000 m; N.B., Ont., Que.; Ill., Ind., Iowa, Maine, Mass., Mich., Minn., Mo., Nebr., N.H., N.Y., N.Dak., Ohio, Pa., S.Dak., Vt., Wis. Carex haydenii is a species of seasonally saturated soils in open habitats. It appears to be declining in abundance with changes in land use. Carex haydenii is often confused with C. stricta; it can be identified by the inflated perigynia that are rounded apically, acute scales, and glabrous leaves and sheaths. Only basal sheaths of C. haydenii are ladder-fibrillose. Although often sympatric with C. stricta and C. nigra, no hybrids appear to be formed.