Oenothera deltoides var. piperi Munz, Amer. J. Bot. 18: 314. 1931; O. trichocalyx Nuttall var. piperi (Munz) Jepson

Herbs annual, usually villous, hairs relatively long, curly, especially distally and on buds, sometimes glabrous; from a taproot. Stems: central stem thickened proximally, unbranched or with several lateral, ascending to decumbent branches, 3–30(–40) cm. Leaves basal and cauline; blade rhombic, becoming lanceolate distally, margins deeply sinuate-dentate to pinnatifid. Flowers: buds fluted or strongly quadran­gular in distal 1/2, without free tips; sepals 13–22(–27) mm; petals 15–25(–30) mm. Capsules 15–25(–30) × 3–5 mm. 2n = 14.

Flowering (Mar–)Jun–Jul(–Sep). Sandy soil or dunes in Great Basin Desert with Artemisia, Ericameria, or Sarcobatus; 900–1900 m; Calif., Nev., Oreg.

W. M. Klein (1964) determined subsp. piperi to be self-incompatible. It occurs in the northern part of the range of O. deltoides, from northeastern California to southern Oregon and the western half of Nevada.