Description from Flora of China
Ammannia arenaria Kunth; A. auriculata Willdenow var. arenaria (Kunth) Koehne.
Herbs, 15-60 cm tall. Stems branched. Leaves opposite, narrowly lanceolate or oblong-lanceolate, l.5-7.5 cm × 3-15 mm, base cordate-auriculate, clasping. Flowers (1-)3-15, commonly ca. 7, in axillary cymes; pedicels 1-3(-6) mm; peduncle 3-9 mm, commonly ca. 5 mm; bracteoles linear, not reaching floral tube. Floral tube campanulate to urceolate, 1.5-2 mm, conspicuously 4-8-ribbed; sepals 4, broadly deltate; epicalyx segments minute, thickened. Petals 4, rose-purple, suborbicular, ca. 1.5 mm, sometimes absent. Stamens 4-8, long-exserted. Style ca. as long as or longer than ovary. Capsules 1.5-3(-3.5) mm in diam., equal to surpassing floral tube. Fl. Aug-Dec, fr. Aug-Dec. 2n = 30, 32.
Wet places, rice farms. Anhui, Fujian, Gansu, Guangdong, Hebei, Henan, Hubei, Jiangsu, Shanxi, Yunnan, Zhejiang [pantropical].