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 spirally up the flowering stems; they are entire to 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. 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.

In the wild, evening primroses acts 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.

It was originally assigned to the genus Onagra, which gave the family Onagraceae its name. Onagra (meaning "(food of) onager") was first used in botany in 1587, and in English in Philip Miller's 1754 Gardeners Dictionary: Abridged. Its modern name Oenothera was published by Carolus Linnaeus in his Systema Naturae, also has a donkey-related origin, meaning "ass-catcher". In Greek oeno means "donkey", while thera means "to catch, trap, pursue". It is believed that the name refers to the toxicity of the plant that can be used to trap donkeys and other animals. William Baird suggests, however, that oeno could be interpreted as "wine" in Greek. He believes that it refers to the fact that the root of the edible Oenothera biennis was used as a wine flavor additive.

Cultivation and uses

Young roots can be eaten like a vegetable (with a peppery flavour), or the shoots can be eaten as a salad. The whole plant was used to prepare an infusion with astringent and sedative properties. It was considered to be effective in healing asthmatic coughs, gastro-intestinal disorders, whooping cough and as a sedative pain-killer. Poultices containing O. biennis were at one time used to ease bruises and speed wound healing. One of the common names for Oenothera, "Kings cureall", reflects the wide range of healing powers ascribed to this plant, although it should be noted that its efficacy for these purposes has not been demonstrated in clinical trials.

The mature seeds contain approximately 7-10% gamma-linolenic acid, a rare essential fatty acid. The O. biennis seed oil is used to reduce the pains of premenstrual stress syndrome. Gamma-linolenic acid is also said to be effective against breast cancer.[citation needed]

Evening Primroses are very popular ornamental plants in gardens. For propagation, the seeds can be sown in situ from late spring to early summer. The plant will grow successfully in fertile soils if competing species are kept at bay. Evening primrose species can be planted in any ordinary, dry, well-drained garden soil (preferly sandy loam) in an open site that is sunny to partly shady. They are fairly drought-resistant.

The first plants to arrive in Europe reached Padua from Virginia in 1614 and were described by the English botanist John Goodyer in 1621. Some species are now also naturalized in parts of Europe and Asia, and can be grown as far north as 65° N in Finland. The UK National Council for the Conservation of Plants and Gardens, based at Wisley, maintains an Oenothera collection as part of its National Collections scheme.