Gaura triangulata Buckley, Proc. Acad. Nat. Sci. Philadelphia 13: 454. 1862; G. hexandra Ortega var. triangulata (Buckley) Munz; G. tripetala Cavanilles var. triangulata (Buckley) Munz

Herbs annual, villous proxi­mally, sparsely villous along veins and on margins, usually glabrate, sometimes strigillose distally; from taproot. Stems ascending, usually well-branched from base and distally, rarely unbranched, 15–60 cm. Leaves in a basal rosette and cauline, 1.5–8 × 0.2–0.6(–1.5) cm, blade very narrowly elliptic to oblanceolate or oblong-elliptic, margins entire or weakly sinuate-dentate. Flowers 3(or 4)-merous, zygomorphic, open­ing at sunset; floral tube 4–5.5 mm; sepals 4.5–6 mm; petals white, fading pink, elliptic-obovate, 3.5–5 mm; filaments 2–3.5 mm, anthers 1.5–3 mm, pollen 35–65% fertile; style 9–10 mm, stigma surrounded by anthers. Capsules narrowly obovoid, 3(or 4)-winged, furrowed between wings, 7–9 × 3–5 mm, narrowed at base; sessile. Seeds (1 or)2–5, yellowish to light brown, 1.5–3.5 × 1–1.5 mm. 2n = 14.

Flowering Apr–Jul. Open, sandy sites; 200–600 m; Okla., Tex.

Oenothera triangulata is a PTH species and forms a ring of 14 chromosomes in meiosis. The species is self-compatible and autogamous (P. H. Raven and D. P. Gregory 1972[1973]). It may have been derived from hybridization between O. patriciae and O. suffulta. The species has a relatively narrow distribution across south-central Oklahoma and north-central Texas (Oklahoma in Cleveland, Comanche, Cotton, Grady, Oklahoma, Rogers, Stephens, and Tulsa counties; Texas in Archer, Baylor, Callahan, Clay, Coleman, Crosby, Eastland, Erath, Jones, Montague, Taylor, Throckmorton, Tom Greene, Wichita, Wilbarger, and Young counties).