Description from Flora of China
Cyperus pumilus Linnaeus, Cent. Pl. 2: 6. 1756; C. nitens Retzius.
Annuals. Roots fibrous. Culms tufted, 2-20[-35] cm tall, slightly slender, compressed 3-angled, smooth, with a few leaves basally. Leaves shorter to longer than culm; leaf blade 1-2 mm wide, folded or flat. Involucral bracts 3-5, leaflike, longer than inflorescence. Inflorescence a simple anthela; rays 3-6, mostly to 2 cm, sometimes very short into a capitulum, each with 10-20 or more spikelets congested into a subglobose spike. Spikelets oblong to linear-oblong, 3-15 × 1.5-2 mm, compressed, 8-30(-40)-flowered; rachilla straight, wingless. Glumes pale to yellowish white, densely imbricate, ovate to ovate-elliptic, 1.2-1.5 mm, 3-5-veined, keel abaxially green, laterally membranous and veinless, apex truncate-emarginate and with a 0.3-0.5 mm mucro. Stamens 1 or 2; anthers oblong, ca. 0.2 mm. Style ca. 0.5 mm; stigmas 2, ca. 0.3 mm. Nutlet grayish brown, obovoid to oblong, ca. 0.6 mm, 1/3-2/5 as long as subtending glume, biconvex, densely prominently puncticulate, apex obtuse. Fl. and fr. Aug-Nov.
Pycreus pumilus is naturalized in North and South America.
Water margins, wet places in fields; below 100-500 m. Fujian, Guangdong, SE Guangxi, Hainan, Hunan, Jiangxi, S Taiwan [Bangladesh, Bhutan, India, Indonesia, Kashmir, Malaysia, Myanmar, Nepal, Pakistan, Papua New Guinea, Philippines, Sri Lanka, Thailand, Vietnam; Africa, N Australia, Indian Ocean islands, Madagascar].