Description from Flora of China
Scirpus aestivalis Retzius, Observ. Bot. 4: 12. 1786.
Annuals. Rhizomes absent. Culms densely tufted, 3-25 cm tall, slender, flatly 3-angled, smooth, with few leaves at base. Leaves shorter than culm; leaf blade filiform, 0.5-1 mm wide, flat, both surfaces sparsely pubescent, margin slightly involute; sheath brown, short, white pilose. Involucral bracts 3-5, filiform, shorter to longer than inflorescence, sparsely hairy. Inflorescence a compound anthela, lax; rays 3-7, to 3 cm, slender. Spikelets solitary, ovoid, oblong-ovoid, or narrowly ovoid, 2.5-6 × 1-1.5 mm, many flowered. Glumes spirally imbricate, reddish brown, ovate to oblong, ca. 1 mm, membranous, 3-veined forming an abaxial green keel, apex mucronate. Stamen 1; anther narrowly ovoid. Style long and compressed, basally inflated, apically ciliate; stigmas 2, short. Nutlet yellow, subsessile, obovoid, ca. 0.6 mm, biconvex, almost smooth or sometimes with very indistinct hexagonal reticulation. Fl. May-Aug.
Grasslands, marshy places, paddy fields; 400-2200 m. Anhui, S Chongqing, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Heilongjiang, Hubei, Hunan, Jiangxi, S Shaanxi, Sichuan, Taiwan, S Yunnan, Zhejiang [Bhutan, India, Indonesia, Japan, Laos, Nepal, Papua New Guinea, Philippines, Russia (Far East), Sri Lanka, Thailand, Vietnam; Australia, Pacific islands].