Herbs, annual, cespitose. Culms trigonous, (5-)20-60 cm × (0.2-)0.6-2.4 mm, glabrous. Leaves (1-)3-4, V-shaped, 15-40 cm × 3-6 mm. Inflorescences: spikes 2-5(-8), oblong to ovoid, 2-10 cm × 3-12 mm; rays 4-8, to 11 cm; bracts (4-)5(-7), ascending at 45°, V-shaped, (1.5-)8-30(-50) cm × 1-6 mm; rachilla persistent, wingless (successive floral scales 1.1-1.5 mm apart). Spikelets 10-30, spreading, compressed, linear, 4-20 × 1.5-2.1 mm; floral scales deciduous, (2-)6-26, laterally brown to golden brown, edges clear, medially green, 5-ribbed, outermost ribs more noticeable than others, obovate to nearly orbiculate, 1.3-1.8 × 1.2-1.8 mm, margins emarginate, apex mucronulate, mucro 0.2-0.25 mm. Flowers: stamens 2(-3); anthers (0.2-)0.3-0.4 mm, tips of connectives reddish, less than 0.1 mm; styles less than 0.1 mm; stigmas 0.2-0.4(-0.5) mm. Achenes brown, slightly stipitate, obovoid, 1.2-1.4(-1.5) × 0.5-0.7 mm, apex apiculate, surfaces puncticulate. Fruiting summer. Disturbed soils; 0-100 m; introduced; Conn., Ky., Mass., N.J., N.Y., Ohio, Pa.; e Asia. The name Cyperus amuricus Maximowicz has been misapplied to C. microiria by some American authors (e.g., M. V. McGivney 1938).