According to Flora of North America, Oenothera villosa ssp. strigosa is the locally common erect evening-primrose with relatively small (compared to Hooker’s evening-primrose) yellow flowers.  It is the native member of a complex of difficult-to-distinguish species, with the possibility of ssp. villosa and Oenothera biennis also being present, as introductions from the eastern United States and Europe respectively.  Our native plants differ from both in having more open inflorescences and reddish sepals, which usually have both soft glandular hairs and stiffer hairs with pustulate bases.  This species is encompassed in an inclusive O. biennis in Flora of the Pacific Northwest (2nd ed.)