Herbs, perennial, stoloniferous; stolons (2-)5-12 cm × 1-2 mm, bearing tubers 3-8(-12) mm diam., wiry, springy when dried, indurate. Culms trigonous, 10-35(-40) cm × 0.7-3.4 mm, basally indurate, glabrous. Leaves V-shaped to flanged V-shaped, 5-30 cm × 2-6 mm. Inflorescences: spikes 1(-3), broadly ellipsoid, (12-)15-25(-30) × (12-) Fruiting summer-fall. Croplands, disturbed soils usually; 0-400 m; Ala., Ariz., Ark., Calif., Fla., Ga., La., Miss., Mo., N.Mex., N.C., S.C., Tenn., Tex., Va.; Mexico; Central America; South America; Asia; Africa; Australia.
Cyperus rotundus is documented in Connecticut, Delaware, New York, and Pennsylvania; there is no evidence of persistent populations. Cyperus esculentus and C. rotundus are the only two species of subg. Cyperus in the New World that produce tuberiferous stolons. The two species also have persistent floral scales and persistent rachillas, a combination of characteristics not found in any other New World species of Cyperus. Cyperus rotundus is distinguished from other species of the genus in the New World by its open spikes composed of linear reddish spikelets borne on a conspicuous slender rachis. Cyperus rotundus is usually acknowledged to be the world´s worst weed (cf. G. C. Tucker 1987). In the United States, it does not grow north of the mean 1°C January isotherm. Cyperus esculentus (preceding species) is a serious weed in much of the world, especially in cooler regions where the more tropical C. rotundus does not grow. Cyperus esculentus is able to tolerate lower air temperatures (as low as -18°C). The two species apparently differ also in their thermal optima for growth.
Perennial with numerous slender rhizomes ending in small tubers; stems 2-5 dm, smooth; lvs basally disposed, mostly shorter than the stem, 3-6 mm wide; bracts few, all or all but one shorter than the infl; rays 3-7, simple or sometimes branched at the top, to 10 cm; spikelets 3-10 in short-cylindric spikes, 10-40-fld, to 4 cm; scales closely imbricate, ovate, 2.5-3.5 mm, obtuse, keeled, 7-nerved (3 in the midvein, 2 nearby on each side), the sides purple-brown or red- brown; rachilla persistent, winged; achenes nearly black, narrowly trigonous-obovoid, 1.5 mm; 2n=108. A weed of sandy soil; pantrop., n. to Va. and Ky., and occasionally adventive farther n.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.