Description from Flora of China
Ammannia rotundifolia Buchanan-Hamilton ex Roxburgh, Fl. Ind. 1: 446. 1820; A. subspicata Bentham.
Herbs, perennial or possibly annual, to 30 cm. Stem creeping or floating, red-tinged; internodes of basal stem portion usually much longer than subtending leaves. Leaves decussate, obovate-elliptic to orbicular or elliptic, 5-13 × 3.5-15 mm, base obtuse, apex obtuse. Bracts distinctly smaller than foliage leaves, ca. 3 × 1.5 mm, ca. equal to floral tube. Flowers in 1-8 terminal, emergent spikes; spikes 1-4(-8) cm, lax in fruit; bracteoles slightly shorter than sepals, scarious. Floral tube 4-merous, campanulate, 1-1.5 mm; sepals 4; epicalyx absent. Petals 4, bright rose, surpassing sepals. Stamens 4; anthers reaching margin of floral tube. Ovary pyriform to globose; style included, shorter than ovary; stigma discoid, ca. 0.3 mm in diam. Capsules globose, ca. 1.5 mm in diam., 4-valved. Seeds ca. 0.5 mm. Fl. and fr. Nov-Jun. 2n = 16*, 28, 30.
Marshes, streamsides, paddy fields, mountains; below 2700 m. Fujian, Guangdong, Guangxi, Guizhou, Hainan, Hubei, Hunan, Jiangxi, Shandong, Sichuan, Taiwan, Yunnan, Zhejiang [Bangladesh, Bhutan, India, Japan, Laos, Myanmar, Nepal, Thailand, Vietnam].