Description from Flora of China
Alopecurus aequalis subsp. amurensis (Komarov) Hultén; A. aequalis var. amurensis (Komarov) Ohwi; A. aequalis subsp. aristulatus (Michaux) Tzvelev; A. aequalis var. aristulatus (Michaux) Tzvelev; A. amurensis Komarov; A. aristulatus Michaux; A. geniculatus var. aequalis (Sobolewski) Paunero.
Annual, loosely tufted. Culms weak, geniculately ascending, occasionally rooting and branching from lower nodes, 15–40 cm tall. Leaf sheaths smooth, glabrous, slightly inflated; leaf blades light green, soft, 3–10 cm, 2–6 mm wide, abaxial surface smooth, glabrous, adaxial surface closely ribbed, scaberulous; ligule 2–5 mm. Panicle narrowly cylindrical, 2–7 cm, pale gray-green. Spikelets elliptic or oblong, 2–3 mm; glumes membranous, smooth, keels ciliate-hispid, lateral veins hispid on lower part, margins connate at base, apices obtuse; lemma equaling or slightly longer than glumes, margins connate below middle, awned from lower 1/4–1/3, apex obtuse; awn included within spikelet or exserted up to 1.2 mm, straight. Anthers orange, 0.5–0.8 mm. Fl. and fr. Apr–Aug. 2n = 14.
This is a widespread, semi-aquatic weed of N temperate regions of the world, now introduced to Australia and elsewhere.
East Asian forms tend to have more obvious awns than usual, perhaps due to introgression from Alopecurus longearistatus. Alopecurus amurensis and A. aristulatus are both based on such forms. A form from South America with glabrous glumes has been separated as A. hitchcockii Parodi. A specimen of this species has been found on waste ground in Chengdu, Sichuan.
Irrigation ditches, rice fields, damp grasslands, other wet weedy places; below 3500 m. Anhui, Fujian, Guangdong, Guizhou, Hebei, Heilongjiang, Henan, Hubei, Jiangsu, Jiangxi, Nei Mongol, Shaanxi, Shandong, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan, Zhejiang [Bhutan, Japan, Kashmir, Kazakhstan, Korea, Kyrgyzstan, Mongolia, Nepal, Russia, Tajikistan, Turkmenistan, Uzbekistan; SW Asia, Europe, North America].