Creeping primrose-willow, red ludwigia

Isnardia intermedia Small & Alexander; I. natans (Elliott) Kuntze; I. repens de Candolle; I. repens var. rotundata Grisebach; Ludwigia fluitans Scheele; L. natans Elliott; L. natans var. rotundata (Grisebach) Fernald & Griscom; L. natans var. stipitata Fernald & Griscom; L. repens Swartz var. rotundata (Grisebach) M. Gómez; L. repens var. stipitata (Fernald & Griscom) Munz

Herbs creeping and rooting at nodes, often forming loose mats. Stems prostrate, ascending to suberect at tips, terete, sparsely branched, 30–80 cm, glabrous or, sometimes, minutely strigillose on leaf margins and inflorescence. Leaves opposite; stipules narrowly deltate, 0.05–0.1 × 0.05–0.1 mm; petiole narrowly winged, 0.3–2.3 cm, blade narrowly elliptic to broadly lanceolate-elliptic or suborbiculate, 0.8–4.5 × 0.4–2.7 cm, base attenuate, margins entire or sometimes with hydathodal glands, apex acute or apiculate, rarely obtuse, surfaces lustrous, subglabrous or sparingly to densely papillose strigillose; bracts not much reduced. Inflorescences sometimes few-flowered, erect racemes, flowers paired in leaf axils of prostrate stems; bracteoles attached in opposite pairs to pedicel 1–5 mm proximal to base of ovary, lanceolate to narrowly oblong-lanceolate or sublinear, 1–5(–8) × 0.2–1 mm, apex acute, surfaces sparingly minutely strigillose. Flowers: sepals ascending, light green, ovate deltate to narrowly so, 1.8–5 × 1.5–3.5 mm, margins minutely strigillose, apex acuminate to elongate-acuminate, surfaces subglabrous; petals caducous, oblanceolate to elliptic-oblong, 1.1–3 × 0.4–1.4 mm, base attenuate, apex obtuse, often variable in size and shape in same flower; filaments pale yellow, 0.5–1.5 mm, slightly inflated near base, anthers 0.4–0.9 × 0.3–0.8 mm; pollen shed singly or in tetrads; ovary obconic-cylindric, barely 4-angled to subterete, 2–6 × 2.5–3.5 mm; nectary disc elevated 0.3–0.8 mm on ovary apex, yellow, 1.1–3 mm diam., 4-lobed, glabrous; style pale yellow, 0.6–0.9 mm, glabrous, stigma pale yellow, broadly capitate, 0.3–0.5 × 0.3–0.8 mm, usually not exserted beyond anthers. Capsules elongate-obpyramidal, 4-angled, corners sometimes rounded, 4–10 × 2.5–4 mm, hard-walled, irregularly dehiscent, pedicel 0.1–3 mm. Seeds yellowish brown, ellipsoid, 0.6–0.8 × 0.3–0.5 mm, surface cells transversely elongate. 2n = 48.

Flowers Mar–Nov (year-round). Muddy or damp, sandy edges of pools, lakes, swamps, creeks, and roadside ditches, moist soil in solution pits in limerock and hammock clearings in Florida Everglades, shade or sun; 0–1200[–1600] m; Ala., Ariz., Calif., Fla., Ga., Kans., La., Miss., Nev., N.J., N.Mex., N.C., Okla., Oreg., S.C., Tenn., Tex., Va.; Mexico (Chihuahua, Coahuila, México, Morelos, Nuevo León, Puebla, San Luis Potosí, Sonora); West Indies (Bahamas, Cuba, Hispaniola, Jamaica); Bermuda; introduced in Asia (Bangladesh, Japan).

Ludwigia repens occurs primarily on the Atlantic and Gulf coastal plains of the United States from North Carolina to Texas, with more scattered distribution into west Texas, New Mexico, Oklahoma, and Kansas, and disjunct populations in New Jersey, Tennessee, southern Arizona, Nevada, California, and western Oregon (Marion County); it also occurs in northern and central Mexico and the Caribbean region. In some parts of this wide range it is considered an aquatic weed.

Ludwigia repens is one of the most popular species of Ludwigia used in the aquarium trade; this may help to account for its wide distribution.

Much like the related diploid Ludwigia palustris, the hexaploid L. repens is widespread and morpho­logically variable, and infraspecific taxa have been proposed to describe this variation. Given the tendency of species in sect. Isnardia to hybridize, and the lack of a geographical basis for much of the variation, C. I. Peng (1989) declined to adopt any infraspecific classification, and this treatment follows Peng.

As described in C. I. Peng et al. (2005), Ludwigia repens J. R. Forster was conserved with a new type (the original type selected was from Virginia, where the species does not occur); L. repens Swartz (1797) was a later homonym described from Jamaica and pertains here.