Oenothera trichocalyx Nuttall var. cognata Jepson, Man. Fl. Pl. Calif., 681. 1925; O. deltoides var. cognata (Jepson) Munz

Herbs short-lived perennial, sometimes annual, glabrous or sparsely villous, rarely also strigillose; from a taproot. Stems: central stem often thickened near base, branched from base, branches ascending, not encircling stems in older plants, 20–40 cm. Leaves basal and cauline; blade rhombic-lanceolate, becoming oblanceolate to lanceolate distally, margins usually coarsely sinuate-dentate to subentire, rarely pinnatifid. Flowers: buds weakly quadrangular, without free tips; sepals 20–35 mm; petals 25–40 mm. Capsules 40–60 × 3–5 mm. 2n = 14.

Flowering (Mar–)Jun–Jul(–Sep). Sandy places; 30–500 m; Calif.

W. M. Klein (1964) determined subsp. cognata to be self-compatible. It is known from the San Joaquin Valley. Some specimens from Contra Costa County at the northern edge of the range of subsp. cognata appear to be somewhat intermediate toward subsp. howellii in having minute free sepal tips in bud and the plant is more pubescent.