Description from Flora of China
Cyperus fuscus f. pallescens Husnot; C. fuscus f. virescens (Hoffmann) Vahl; C. virescens Hoffmann.
Annuals. Roots fibrous. Culms tufted, 6-30 cm tall, slender, compressed triquetrous, soft, smooth, basally few leaved. Leaves shorter than to sometimes subequaling culm; leaf blade 2-4 mm wide, flat, sometimes folded, margin not scabrous. Involucral bracts 2 or 3, leaflike, longer than inflorescence. Inflorescence a compound or sometimes simple anthela; rays 3-5, most to 3 cm, apex of each ray or raylet with 5 to more than 10 densely arranged spikelets. Spikelets very narrowly ovoid to linear, 3-10 × ca. 1.5 mm, slightly compressed, 8-24-flowered; rachilla wingless. Glumes purplish brown, brownish, brown, or pale yellowish on both surfaces but middle yellowish green, slightly laxly imbricate, broadly ovate, ca. 1 mm, obscurely 3-veined, apex obtuse. Stamens 2; anthers ellipsoid; connective not prominent beyond anthers. Style short; stigmas 3. Nutlet ellipsoid, ca. 2/3 as long as subtending glume, 3-sided, base barely stipitate to cuneate, apex apiculate. Fl. and fr. Jun-Oct. 2n = 36.
Cyperus fuscus is naturalized in North America.
Swamps, water margins, ditch margins, river margins, shaded places in valleys, along trails, wet places, paddy fields; 100-2000 m. Anhui, Gansu, Hebei, Heilongjiang, Henan, Jiangsu, Liaoning, Nei Mongol, Ningxia, Shaanxi, Shandong, Shanxi, Sichuan, Xinjiang, Yunnan [Afghanistan, India, Kashmir, Kazakhstan, Kyrgyzstan, Laos, Mongolia, Pakistan, Russia, Tajikistan, Thailand, Turkmenistan, Uzbekistan, Vietnam; N Africa, SW Asia, Atlantic Ocean islands, Europe].