Lavauxia hamata Wooton & Standley; L. triloba (Nuttall) Spach; L. watsonii Small; Oenothera hamata (Wooton & Standley) Tidestrom; O. rhizocarpa Sprengel; O. roemeriana Scheele; O. triloba var. parviflora S. Watson; O. triloba [unranked] watsonii (Small) F. C. Gates

Herbs winter-annual, sometimes biennial, acaules­cent or very short-caulescent, sparsely to moderately strigillose and glandular puberulent, sometimes one hair type predominant, rarely glabrate, sometimes also very sparsely hirsute, especially on leaf veins; from a slender or, sometimes, stout taproot. Stems (when present) ascending, 1–several, densely leafy, 0–20 cm. Leaves in a basal rosette, sometimes also cauline, (2.5–)6–25(–32) × (0.6–)1.5–4(–5) cm, thin; petiole (0.5–)1–8 cm; blade oblanceolate to elliptic, margins irregularly pinnatifid, sometimes subentire, apex acute to obtuse or rounded. Flowers 1–4 opening per day near sunset, without noticeable scent; buds with subequal free tips 2–7 mm; floral tube (20–)28–95(–138) mm; sepals (6–)10–30(–35) mm; petals pale yellow, fading pale orange, drying lavender, (10–)12–30(–38) mm; filaments (5–)8–15(–18) mm, anthers (3.5–)4–11 mm; style (3.4–)4.2–11.5(–16.3) mm, stigma usually sur­rounded by anthers, sometimes (especially in some Texas populations) exserted beyond anthers. Capsules woody in age, rhombic-obovoid, winged, wings broadly triangular, 5–10 mm wide, often terminating in a hooked tooth, (10–)15–25(–28) × 4–8 mm (excluding wings), valve surface reticulate, dehiscent 1/8–1/3 their length. Seeds asymmetrically cuneiform, (2.1–)2.5–3(–3.3) mm. 2n = 14.

Flowering (Feb–)Mar–May(–Jul). Scattered to common in clay, sandy or rocky soil, playas, floodplains, creek beds, slopes and flats, moist sites, disturbed sites, roadsides, old fields, in Larrea deserts, prairies, glades; 300–1900 m; Ala., Ark., Colo., D.C., Ill., Ind., Kans., Ky., Md., Mo., N.Mex., Ohio, Okla., Pa., Tenn., Tex., Va.; Mexico (Baja California, Chihuahua, Nuevo León).

Oenothera triloba is primarily a species of the high plains from eastern Socorro County, New Mexico, east through all but eastern Texas, Oklahoma, to southern Kansas, east of Meade and Pawnee counties and south of Douglas and Saline counties. It becomes more sporadic eastward into Missouri south of the Missouri River, northwestern and north-central Arkansas, central and eastern Tennessee, northern Alabama, and Logan and Warren counties, Kentucky; also known from disjunct sites in northern Mexico from Nuevo León, Chihuahua, and Baja California, Mexico; and, introduced in Illinois, Indiana, Ohio, Kentucky (Campbell and Fayette counties), Pennsylvania, Virginia, Maryland, and the District of Columbia. Areas where it was introduced are represented by old collections; no current information indicates their continued presence in any of these areas. It was recently collected in Baca County, Colorado.

Capsules of dead plants sometimes form pineconelike clusters of ten to 100 or more capsules.

The illegitimate names Lavauxia nuttalliana Spach and L. triloba (Nuttall) Spach var. watsonii Britton pertain here.