Description from Flora of China
Hypaelyptum microcephalum R. Brown, Prodr. 220. 1810.
Annuals, without a rhizome. Culms 5-40 cm tall, ca. 0.7 mm thick, flat, longitudinally sulcate, puberulent. Leaves basal; sheath tubular, 1.5-2.5 cm, membranous, glabrous, basal ones without a blade, apical ones with a blade; leaf blade linear, 1/4-1/2 as long as culm, 0.7-1.5 mm wide, papery, both surfaces glabrous, midvein obsolete, margin involute. Involucral bracts 2 or 3, leaflike, slightly broad at base, not sheathing. Spikes 2- or 3(or 4)-fascicled, ovoid, 3-5 × ca. 3 mm, with numerous bractlets and spikelets; bractlets oblanceolate to spatulate, ca. 2 mm, membranous, apex abruptly contracted and formed into recurved mucro. Spikelets with 2 glumes and 1 bisexual flower; basal glume white, elliptic, ca. 1 mm, hyaline, 5-veined, apex obtuse; apical glume oblong, ca. 1 mm, membranous, several veined, with a bisexual flower, apex acute. Stamens 2; anthers linear-oblong, ca. 0.3 mm. Style slender, exserted from subtending glume; stigmas 3, pubescent. Nutlet straw-colored, narrowly oblong, 0.9-1 mm, slightly curved, 3-sided, densely puncticulate, shiny, apex mucronate. Fl. and fr. Jun-Oct. 2n = 46.
Water margins, wet places, swamps; 400-2100 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Liaoning, Shandong, Sichuan, Taiwan, S Yunnan, Zhejiang [Cambodia, India, Indonesia, Japan, Korea, Laos, Malaysia, Myanmar, Papua New Guinea, Philippines, Thailand, Vietnam; Australia, Pacific islands].