Hort. Berol. 1: 7, plate 7. 1803.
Herbs annual, slender, 1–4(–8) dm. Stems erect, usually much-branched distal to base. Leaf blades narrowly linear-lanceolate to linear-oblong, 20–80 × 2–15 mm. Inflorescences simple or compound cymes, (3–)7–15-flowered mid stem; peduncle slender (nearly filiform), 3–9 mm. Pedicels slender, 1–3 mm. Floral tube campanulate to urceolate, 1–3 mm; epicalyx segments shorter than sepals or absent; sepals 4; petals 4, deep rose purple, 1.5 × 1.5 mm; stamens 4(–8); anthers deep yellow; style slender, well-exserted, 1+ mm; stigma capitate. Capsules (1–)1.5–2.5(–3.5) mm diam., equal to or surpassing sepals, splitting irregularly. 2n = 30 (Egypt), 32 (Mexico, Tanzania).
Phenology: Flowering late spring–late fall.
Habitat: Freshwater marshes, margins of pools and lakes, drying mud and sand flats, ditches, rice fields.
Elevation: 50–1500 m.
Kans., Nebr., N.Mex., Okla., S.Dak., Tex., Mexico, West Indies, Central America, South America, Asia, Africa, Australia.
Ammannia auriculata, the most widely distributed species in the genus, is common and probably native to Africa but sporadic in occurrence, depending on the availability of suitable habitats, which may change from year to year. The species in the flora area is most common in the central United States, where it often grows with A. coccinea or A. robusta.