Description from Flora of China
Annuals. Culms densely tufted, 6-25 cm tall, slender, compressed, with few leaves at base. Leaves shorter than culm; sheath pale brown, densely pilose; leaf blade less than 1 mm wide, flat, both surfaces pubescent. Involucral bracts 3-7, leaflike, shorter to slightly longer than inflorescence. Inflorescence a simple, compound, or decompound anthela; rays few to many, to 3 cm. Spikelets solitary, ovoid, narrowly ovoid, or oblong, 3-7 × 1.2-3 mm, many flowered, squarrose or not. Glumes spirally imbricate, yellowish brown, oblong to oblong-ovate, 1.5-2 mm (including awn), membranous, 3-veined forming an abaxial green keel, apex obtuse and awned, awn either long and recurved or short and straight or slightly recurved. Stamen 1; anther oblong, apex apiculate. Style long and compressed, basally inflated with long filiform pendent hairs covering top of nutlet, apically ciliate; stigmas 2. Nutlet yellowish white, shortly stipitate, obovoid, 0.5-1 mm, biconvex, almost smooth or with very indistinct hexagonal reticulation. Fl. Sep.
Fimbristylis squarrosa was treated as a synonym of F. aestivalis by D. A. Simpson and T. Koyama (Fl. Thailand 6(4): 333-334. 1998).
Watersides, wet places; 100-2200 m. Anhui, Fujian, Guangdong, S Guangxi, NW Guizhou, Hainan, Hebei, Heilongjiang, Henan, Jiangsu, Shandong, Taiwan, SE Xizang, Yunnan, Zhejiang [India, Indonesia, Japan, Korea, Laos, Myanmar, Nepal, Philippines, Thailand, Vietnam; Africa, SW Asia, Australia, S Europe, Pacific islands (New Zealand)].