Oenothera deltoides Torrey & Frémont var. arizonica Munz, Amer. J. Bot. 18: 315. 1931; O. avita (W. M. Klein) W. M. Klein subsp. arizonica (Munz) W. M. Klein; O. californica (S. Watson) S. Watson subsp. arizonica (Munz) W. M. Klein

Herbs winter-annual, younger parts sparsely to densely strigillose and sparsely to densely hirsute, older stems glabrate; from a taproot. Stems ascending to erect, with decumbent branches, thickened at base, tapering toward apex, 10–35(–60) cm. Leaves in a basal rosette and cauline, basal 5–10(–26) × 0.6–1.5(–3.5) cm, cauline 5–8(–15.5) × 1–2 cm; petiole 0–12 cm; blade lanceolate to oblanceolate, margins pinnatifid or sometimes coarsely serrate. Flowers 1–several opening per day near sunset; buds nodding, weakly quadrangular, without free tips; floral tube 26–31 mm; sepals 19–26 mm, conspicuously maroon-spotted, each spot at base of a long hair; petals white, fading pink to deep pink, broadly obovate or obcordate, 16–26(–36) mm; filaments 9–15 mm, anthers 7–9 mm; style 45–50 mm, stigma exserted beyond anthers at anthesis. Capsules spreading, woody in age, curved upward, or distal end recurved, cylindrical, obtusely 4-angled, especially toward base, tapering gradually from base to apex, 30–80 × 2.5–3.5 mm; sessile. Seeds numerous, in 1 row per locule, light brown to yellowish brown with dark purple splotches, obovoid, 1.6–2 mm. 2n = 14.

Flowering (Oct–)Feb–May. Gravelly or sandy soil, along watercourses, disturbed sites; 200–1400 m; Ariz.; Mexico (Sonora).

Oenothera arizonica occurs in southern Arizona from Maricopa and Yuma counties to Cochise County, and from scattered localities in northern Sonora, Mexico, including Cerro Tepopa, Puerto Libertad, and Tastiota. The populations from southwestern Arizona (Yuma County) southward to Sonora often grow on low dunes.

Populations from sand dunes in Yuma County, Cabeza Prieta National Wildlife Refuge, appear to be a large phenotype of Oenothera arizonica that differ from all other specimens in the size of vegetative parts and flowers, and comprise all of the atypical measurements given in the description. Oenothera arizonica typically grows on dunes in Sonora, but rarely so in Arizona. Populations growing on dunes should be studied further and compared to non-dune populations in the northern and eastern portion of the range. Oenothera arizonica is self-compatible (W. L. Wagner et al. 2007; K. E. Theiss et al. 2010).