Annual, delicate. Culms very slender, erect, 3–40(–60) cm tall, 2–17-noded. Leaf sheaths keeled, usually hispid, hairs scattered, tubercle-based; leaf blades green when young, becoming reddish, soft, 1.5–5 × 0.1–0.25 cm, hispid with scattered tubercle-based hairs or glabrescent; ligule 0.5–1 mm, lacerate. Racemes 2–3(–5), subdigitate, 1–6(–10) cm, ascending at first, then divergent; rachis triquetrous, scabrid on angles, internodes 1–3 mm; pedicels ca. 0.2 mm, glabrous. Spikelets linear-oblong, 1.7–3.2(–4.5) mm, usually purple or reddish brown; glumes herbaceous with broad scarious margins, scabrid, sometimes with a few long stiff hairs near top of keel, or stiffly pilose throughout, keel usually wingless, occasionally upper glume narrowly winged, apex acute; upper lemma elliptic-lanceolate, 1.6–2 mm; awn 6–11 mm, geniculate; upper palea absent. Anthers 0.4–0.6 mm. Fl. and fr. Sep–Nov.

This is a widespread, polymorphic species, in which a number of infraspecific taxa has been recognized.

Streams, moist places, often gregarious; below 2000 m. Anhui, Fujian, Guangdong, Guangxi, Guizhou, Henan, Jiangsu, Jiangxi, Sichuan, Taiwan, Yunnan, Zhejiang [India, Indonesia, Japan, Korea, Laos, Malaysia, Myanmar, Nepal, Philippines, Thailand, Vietnam; SW Asia (Oman), Australia, Pacific Islands (Polynesia)].