Evening Primrose — Oenothera is a genus of about 125 species of annual, biennial and perennial herbaceous flowering plants, native to North and South America. It is the type genus of the family Onagraceae. Common names include evening primrose, suncups, and sundrops.
The species vary in size from small alpine plants 10 cm tall (e.g. O. acaulis from Chile), to vigorous lowland species growing to 3 m (e.g. O. stubbei from Mexico). The leaves form a basal rosette at ground level and spiral up to the flowering stems; the leaves are dentate or deeply lobed (pinnatifid). The flowers open in the evening, hence the name “evening primrose”, and are yellow in most species but white, purple, pink or red in a few; there are four petals. One of the most distinctive features of the flower is the stigma with four branches, forming an X shape. Pollination is by Lepidoptera (moths) and bees; like many members of the Onagraceae, however, the pollen grains are loosely held together by viscin threads (see photo below), meaning that only bees that are morphologically specialized to gather this pollen can effectively pollinate the flowers (it cannot be held effectively in a typical bee scopa). Furthermore, the flowers are open at a time when most bee species are inactive, so the bees which visit Oenothera are also compelled to be vespertine temporal specialists. The seeds ripen from late summer to fall.
Oenothera species are used as food plants by the larvae of some Lepidoptera species including Schinia felicitata and Schinia florida, both of which feed exclusively on the genus, the former exclusively on O. deltoides.
In the wild, evening primrose act as primary colonizers, springing up wherever a patch of bare, undisturbed ground may be found. This means that they tend to be found in poorer environments such as dunes, roadsides, railway embankments and wasteland. It often occurs as a casual, eventually being out-competed by other species.
The genus Oenothera may have originated in Mexico and Central America. During the Pleistocene era a succession of ice ages swept down across North America, with intervening warm periods. This was repeated for four ice ages, with four separate waves of colonization, each hybridizing with the remnants of the previous waves. This generated a present-day population that is very rich in genetic diversity, spread right across the North American continent.