Pl. Horti Univ. Rar. Progr., 7. 1773.
Herbs annual, robust, 2–10 dm. Stems erect, much-branched distal to base. Leaf blades usually lanceolate to linear-oblong, rarely elliptic to spatulate, 20–80 × 2–15 mm. Inflorescences simple or compound cymes, (1–)3–7(–15)-flowered mid stem; peduncle stout, (1–)3–9 mm. Pedicels stout, 1–2 mm. Floral tube campanulate to urceolate, (2.5–)3–5 mm; epicalyx segments as long as sepals; sepals 4(or 5); petals 4(or 5), deep rose purple, sometimes with darker rose purple basal spot, 2 × 2 mm; stamens 4(–7); anthers deep yellow; style slender, well-exserted, 2–3.5 mm; stigma capitate. Capsules 3.5–5 mm diam., equal to or surpassing sepals, splitting irregularly. 2n = 66.
Phenology: Flowering late spring–late fall.
Habitat: Freshwater marshes, margins of pools and lakes, drying mud and sand flats, ditches, rice fields.
Elevation: 10–1500 m.
B.C., Ala., Ariz., Ark., Calif., Colo., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Miss., Mo., Nebr., N.J., N.Mex., N.C., Ohio, Okla., Pa., S.C., S.Dak., Tenn., Tex., W.Va., Mexico, West Indies, Central America, Europe (Italy, Spain), Asia (Afghanistan), Africa (Sudan), Pacific Islands (Guam, Hawaii), widely introduced in s Europe, elsewhere in Asia, Africa, other Pacific Islands, Australia.
Ammannia coccinea is considered an amphidiploid (n = 33) derived from A. auriculata (n = 16 in North America) and A. robusta (n = 17) (S. A. Graham 1979). The species displays a wide range of morphological variability, approaching at extremes each of the putative parents and making identification occasionally difficult, especially in recognizing co-occurring A. coccinea and A. robusta. Differences in petal and anther color are important distinctions, easily determined in the field but not always apparent on herbarium specimens. Occasional specimens of A. coccinea that approach A. auriculata in peduncle length or flower number are best recognized by the larger flowers of A. coccinea. Care must be taken in determining style length because the aging style of A. coccinea tends to break away leaving a short stump that resembles the true short style of A. latifolia.
Ammannia coccinea is native to North America but disjunctly distributed around the world as a result of introductions through rice culture.