Yellow evening primrose (Calyophus hartwegii)Yellow evening primrose (Calyophus hartwegii). Also known as Oenothera hartwegii, other common names include sundrop and sierra sundrop. This woody perennial prefers full to part sun, and well drained poor soil. It is dormant in winter, but in summer it is an attractive ground covering wildflower that tolerates extremely harsh conditions.This image is copyright and may not be used without specific authorization. Permission is explicitly denied for Pinterest.