Kneiffia riparia (Nuttall) Small; Oenothera tetragona Roth var. riparia (Nuttall) Munz

Herbs perennial, sparsely strig­illose, becoming glabrate distally, usually also glandular puberulent distally; from a somewhat fleshy rootstock, forming adventitious roots where submerged. Stems erect or ascending, usually many-branched throughout, proximal branches often somewhat spongy, 50–120 cm. Leaves in a basal rosette and cauline, softly succulent, 4–13.5 × 0.8–2.1 cm, petiole 0–1.5 cm, blade lanceolate to elliptic-lanceolate, margins remotely denticulate. Inflorescences erect, flowers in axils of distalmost few nodes. Flowers opening near sunrise; buds with free tips 0.5–2(–5) mm, erect to spreading; floral tube 14–20 mm; sepals 20–30 mm; petals bright yellow, fading pale pink or lavender, 16–27 mm; filaments 10–15 mm, anthers 7–8 mm, pollen 85–100% fertile; style 15–30 mm, stigma exserted beyond anthers at anthesis. Capsules oblong-clavate to oblong-ellipsoid, 4-angled or 4-winged, and wings 0.1–0.2 mm, 7–15 × 4–6 mm, stipe 2–5 mm; sessile. Seeds 0.8 × 0.4 mm. 2n = 56.

Flowering May–Jul. Isolated colonies in or at edge of water in marshes or slow-running rivers, apparently with at least some tidal influence; of conservation concern; 0–10 m; Ga., N.C., S.C., Va.

Oenothera riparia has not been accepted as a distinct species for a long time. The conservative approach used by G. B. Straley (1977) is here followed in recognizing a broadly delimited O. fruticosa, except that the very distinctive coastal tidal-freshwater semi-aquatic octoploid populations (Straley 1982) are recognized as O. riparia. Plants of these coastal populations, which occur from southern Virginia to North Carolina, are more robust, more branched, and less pubescent than those of the two subspecies of O. fruticosa, and have slightly succulent leaves and more prominent adventi­tious roots (Straley 1982; D. Boufford, pers. comm.). Further field studies and cytological analyses are needed to fully understand the geographical and ecological limits of O. riparia and confirm that it is strictly an octoploid species. K. N. Krakos et al. (2014) determined that Oenothera riparia is self-incompatible and is pollinated by bees (Bombus, Lassioglossum, Megachile, Parallelia, Xylocopa, and Zale).