Herbs, annual, cespitose. Culms (1-)10-30(-60) together, roundly trigonous proximally, trigonous distally, (0.5-)3-6(-12) cm × (0.2-)0.4-0.8 mm, glabrous. Leaves V-shaped, (1-)3-5(-10) cm × (0.5-)1-2 mm. Inflorescences: spikes loosely ovoid, 8-15(-25) mm wide; rachis 1-3 mm; rays (1-)2-4(-6), (0.5-)1-3(-6) cm; 2d order rays absent; bracts (1-)2-4(-5), ascending at 30-45°, V-shaped, (1-)2.5-6(-11) cm × 0.5-1.8 mm. Spikelets (2-)4-6(-10), linear-lanceoloid, strongly flattened, (5-)8-12(-18) × 1.2-1.5 (1.8) mm; floral scales (5-)10-16(-32), laterally clear, stramineous or light reddish brown, medially green, laterally ribless, medially 3(-5)-ribbed, ovate, (1.2-)1.3-1.6(-2) × 1-1.2(-1.4) mm, apex obtuse, mucronate to minutely mucronulate. Flowers: stamens 2; anthers 0.1-0.2 mm; styles 0.1-0.3(-0.4) mm; stigmas 0.3-0.4 mm. Achenes light to dark brown, glossy or iridescent, stipitate, oblong-obovoid, 0.9-1.1 × 0.4-0.5 mm, stipe 0.1 mm, apex truncate, apiculate, surfaces punctate.

Fruiting early summer. Wet, disturbed sandy soils in full sun; 0-200 m; La., Tex.; West Indies (Cuba); Central America; South America (Ecuador).

Cyperus fugax has been treated as a form of C. polystachyos (G. Kükenthal 1935-1936; A. B. Ayers 1946); nevertheless, specific status is appropriate. The most conspicuous differences are the annual habit and small size (less than 12 cm) of C. fugax; C. polystachyos is a perennial of moderate size (mostly taller than 20 cm). Other differences are summarized below.

Cyperus polystachyos is characterized as a shortly rhizomatous perennial; floral scales oblong, 1.8-2.4 mm, closely imbricate; styles 0.6-1 mm; stigmas 1.4-2 mm; anthers 0.6-0.8 mm; achenes substipitate to cuneate, slightly compressed laterally.

Cyperus fugax is characterized as a nonrhizomatous annual; floral scales ovate, 1.3-1.6 mm, loosely imbricate; styles 0.1-0.2 mm; stigmas 0.3-0.4 mm; filaments 1.2-1.4 mm; anthers 0.1-0.2 mm; achenes stipitate, strongly compressed laterally.