Oenothera deltoides var. ambigua Munz, Amer. J. Bot. 18: 315. 1931, based on O. ambigua S. Watson, Proc. Amer. Acad. Arts 14: 293. 1879, not (Nuttall) Sprengel 1825; Anogra simplex Small; O. albicaulis Pursh var. decumbens S. Watson; O. deltoides var. decumbens (S. Watson) Munz; O. simplex (Small) Tidestrom

Herbs annual, strigillose, especially distally; from a taproot. Stems: central stem often thickened near base, branched from base, branches ascending to decumbent, not encircling stems in older plants, 5–25 cm. Leaves basal and cauline; blade rhombic-obovate to -lanceolate, becoming oblanceolate to lanceolate distally, margins subentire, sometimes coarsely sinuate-dentate distally. Flowers: buds quadrangular in distal 1/2, with free tips 1–3 mm; sepals 20–30 mm; petals 18–35 mm. Capsules 40–70 × 1.5–5 mm. 2n = 14.

Flowering (Mar–)Jun–Jul(–Sep). Sandy soil in Mojave Desert with Ambrosia, Cylindropuntia, or Larrea; 200–1300 m; Ariz., Nev., Utah.

Subspecies ambigua is restricted to a relatively small area of Mojave Desert in southeastern Nevada and adjacent areas in southwestern Utah and Arizona. W. M. Klein (1964) determined subsp. ambigua to be self-incompatible.