Kneiffia sessilis Pennell, Bull. Torrey Bot. Club 46: 366. 1919; Oenothera pilosella Rafinesque subsp. sessilis (Pennell) Straley; O. pilosella var. sessilis (Pennell) B. L. Turner

Herbs perennial, densely strig­illose, glabrate proximally; from fibrous roots. Stems ascending, unbranched to few-branched, 30–65 cm. Leaves in a basal rosette and cauline, basal 2.5–7 ×0.7–2.3 cm, petiole 1–1.5 cm, blade oblanceolate, margins subentire, undulate; cauline (3–)6–7(–9) ×(0.3–)0.6–0.8(–1.1) cm, sessile, blade lanceolate to narrowly lanceolate, margins subentire. Inflorescences nodding, flowers in axils of distalmost few nodes. Flowers opening near sunrise; buds with free tips 1–2 mm, connivent to spreading; floral tube 10–15(–20) mm; sepals 10–18 mm; petals bright yellow, fading pale pink, 15–25 mm; filaments 7–9 mm, anthers 5–8 mm, pollen 85–100% fertile; style 10–12 mm, stigma exserted beyond anthers at anthesis. Capsules ellipsoid, 4-angled, 8–10 × 3–4 mm, stipe 0–2 mm; sessile. Seeds 1 × 0.5 mm. 2n = 56.

Flowering May–Jun. Moist remnant prairies in sandy or silty soil; of conservation concern; 0–100 m; Ark., La., Tex.

Oenothera sessilis is relatively rare within its range and has a narrow overall distribution, occurring in Ashley, Phillips, Prairie, and St. Francis counties in Arkansas, Allen, Claiborne, and Tensas parishes in Louisiana, and Galveston County in Texas. Oenothera sessilis appears to be relatively rare and may no longer occur in Texas; it was last collected there in the 1840s by Lindheimer on Galveston Island. It is also rare in Louisiana but has been collected in recent decades. It is currently most common in Arkansas. P. A. Munz (1937, 1965) treated this taxon as O. sessilis, but G. B. Straley (1977) in his revision of sect. Kneiffia placed it as a subspecies of O. pilosella based on a common octoploid (2n = 56) chromosome number, morphology, and field studies. K. N. Krakos et al. (2014), based on new field studies, controlled greenhouse breeding experiments, and phylogenetic data found that this taxon differs morphologically from O. pilosella by having consistently shorter stature and smaller flowers, is self-compatible, and does not form a monophyletic group with O. pilosella in molec­ular analyses, and is here reinstated as a distinct species.

Oenothera sessilis is in the Center for Plant Conservation’s National Collection of Endangered Plants as O. pilosella subsp. sessilis.