Ammannia alcalina Blankinship; A. coccinea Rottbøll subsp. robusta (Heer & Regel) Koehne

Herbs annual, robust, 3–10 dm. Stems: proximal branches often decumbent, length often equaling main stem, unbranched, or branched near base. Leaf blades usually lanceolate-linear, rarely spatulate, 15–80 × 4–15 mm. Inflorescences simple cymes, 1–3(–5)-flowered mid stem; peduncle stout, 0–0.5 mm. Pedicels 0. Floral tube campanulate to urceolate, 3–5 mm; epicalyx segments as long as sepals; sepals 4(–8); petals 4(–8), pale lavender, sometimes with rose purple midvein or rose purple basal spot, 2–3 × 2.5–3.3 mm; stamens 4(–8); anthers light yellow; style slender, well-exserted, 2 mm; stigma capitate. Capsules 4–6 mm diam., equal to or surpassing sepals, splitting irregularly. 2n = 34.

Flowering late spring–late fall. Freshwater marshes, margins of pools and lakes, drying mud and sand flats, ditches, rice fields; 0–2100 m; B.C., Ont.; Ariz., Ark., Calif., Colo., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Mich., Minn., Mo., Mont., Nebr., Nev., N.Dak., Ohio, Okla., Oreg., S.Dak., Tenn., Tex., Utah, Wash., Wyo.; Mexico; West Indies; Central America; South America; Pacific Islands (Guam).

Ammannia robusta flowers spring through fall in the northern part of the range and year-round southward. The species has spread worldwide as a result of introduction through rice cultivation.

Ammannia robusta is documented for New Jersey by a collection from the mid 1800s (P. Knieskern s.n., NY); there is no evidence that it became established in the state.