Herbs robust spring annual or, rarely, surviving a second year, caulescent, hirtellous, also glan­dular puberulent; from stout taproot. Stems ascending to erect, stout, unbranched or with lateral stems from basal rosette, densely leafy, 15–30 cm. Leaves 10–14(–14.5) × 1.5–2.3(–3) cm; petiole 4.3–6.6 cm; blade narrowly oblanceo­late, margins irregularly and coarsely dentate, apex acute. Flowers usually 5–10 per stem opening per day near sunset, with heavy, sweet scent; buds erect; floral tube 31–60 mm; sepals 17–26 mm; petals white, fading pale pink, 20–26 mm; filaments 11–16 mm, anthers 8–11 mm; style 65–96 mm, stigma exserted beyond anthers at anthesis. Capsules straight, lanceoloid, obtusely 4-angled, (21–)25–30(–35) × (5–)6–8 mm, tapering to a sterile beak 6–8 mm, dehiscent 1/2–2/3 their length, valve margins with 5–8 conspicuous, irregular tubercles, sometimes 2 or more coalesced into a sinuate ridge, also with conspicuous medial ridge throughout; pedicel 0.5–1 mm. Seeds numerous, usually in 2 distinct rows per locule, sometimes rows partially overlapping, narrowly obovoid, 2.1–2.3 × 1–1.3 mm, embryo slightly less than 1/2 seed volume, surface appearing finely striate but papillose under magnifi­cation; seed collar with membrane intact at maturity, mem­brane rarely splitting and separating from collar, margin entire. 2n = 14.

Flowering May–Jun. On compacted, silty clay to looser rocky and sandy soil in open grassland; of conservation concern; 1400–1900 m; Colo.

Oenothera harringtonii is known only from south­eastern Colorado from western El Paso and eastern Fremont counties, southeast through Pueblo to Otero counties, and south to Las Animas County; it may also occur in adjacent Colfax and Union counties in New Mexico but has not been collected there. Oenothera harringtonii is self-incompatible (W. L. Wagner et al. 1985; Wagner 2005).