Description from Flora of China
Ammannia ramosior Linnaeus, Sp. Pl. 1: 120. 1753; A. monoflora Blanco.
Herbs, annual, terrestrial or amphibious, to 20[-55] cm. Stem erect or ascending, usually branched, weakly 4-angled. Leaves decussate, oblanceolate, narrowly oblanceolate, or elliptic, 1-2.5[-5] × 0.5[-1] cm, base attenuate, apex obtuse. Bracts like foliage leaves. Flowers 4-merous, solitary, sessile, axillary on main stem and branches; bracteoles 1-2.5 mm, less than 1/2 as long as floral tube [or exceeding floral tube]. Floral tube green, tinged with red, narrowly campanulate to subglobose, 2-5 mm; sepals 4, shallowly deltate; epicalyx segments apiculate to deltate, ca. as long as or longer than sepals. Petals absent to 4, pale pink, minute to equaling sepals, caducous. Stamens 4; anthers included. Ovary globose; style shorter than ovary. Capsules globose, 2-3 mm in diam., (3 or)4-valved, slightly exserted from floral tube. Seeds ca. 0.5 mm. Fl. Jul-Sep, fr. Aug-Oct. 2n = 16, 32.
Recently naturalized in paddy fields. Taiwan [native to North America].