Annuals. Roots fibrous. Culms tufted, 8-80 cm tall, slender to slightly stout, compressed triquetrous, base few leaved. Leaves much shorter than culm; sheath reddish brown to brownish purple; leaf blade 2-5 mm wide, slightly folded or flat. Involucral bracts 3-5, leaflike, lowermost 2 or 3 longer than inflorescence. Inflorescence a compound or rarely simple anthela; rays 4-9, mostly to 12 cm, each with 5-10 or more spikes. Spikes ovoid to oblong-ovoid, 1-4 cm, with 5-20 or more spikelets. Spikelets laxly arranged, ascending to slightly spreading, narrowly ovoid, very narrowly ovoid, or oblong, 4-10 × ca. 2 mm, compressed, 6-22-flowered; rachilla almost wingless, with successive glumes 0.7-0.9 mm apart. Glumes yellow to straw-colored, lax, broadly obovate, ca. 1.5 mm, membranous, 3-5-veined, keel middle abaxially green, margin apically white hyaline, apex retuse and mucronulate. Stamens 2(or 3); anthers ellipsoid; connective not prominent beyond anthers. Style very short; stigmas ± short. Nutlet dark brown, obovoid to subellipsoid, 1.2-1.4 mm, nearly as long as subtending glume, 3-sided, densely prominently puncticulate. Fl. and fr. Jun-Oct. 2n = 108, 112, 116, 128.

Cyperus iria is naturalized in Central, North, and South America.

Forest margins, under shrubs or forests, grasslands by water, mountain slopes, along trails in valleys, river margins, wet places, paddy fields; below 100-2000 m. Anhui, Chongqing, Fujian, S Gansu, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, S Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, SE Xizang, Yunnan, Zhejiang [Afghanistan, Bangladesh, Bhutan, India, Indonesia, Japan, Kashmir, Korea, Laos, Malaysia, Myanmar, Nepal, Pakistan, Papua New Guinea, Philippines, Sri Lanka, Thailand, Turkmenistan, Uzbekistan, Vietnam; tropical Africa, SW Asia, Australia, Indian Ocean islands, Madagascar, Pacific islands].