Oenothera flowers are pollinated by insects, such as moths and bees. Like many other members of the Onagraceae, however, the pollengrains are loosely held together by viscin threads, so only insects that are morphologically specialized to gather this pollen can effectively pollinate the flowers. Bees with typical scopa cannot hold it. Also, the flowers open at a time when most bee species are inactive, so the bees which visit Oenothera are generally vespertine temporal specialists: bees that forage in the evening. The seeds ripen from late summer to fall.

Oenothera are used as food plants by the larvae of some Lepidoptera species. The flower moths Schinia felicitata and S. florida both feed exclusively on the genus, and the former is limited to O. deltoides.

In the wild, evening primroses act as primary colonizers, quickly appearing in recently cleared areas. They germinate in disturbed soils, and can be found in habitat types such as dunes, roadsides, railway embankments, and waste areas. They are often casual and are eventually outcompeted by other species.

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Oenothera rhombipetala, or the diamond-petal evening primrose, is photographed here by Don McClane@Flickr. He photographed this in mid-July of last year near Sedgwick County Park in Wichita, Kansas, USA. Thanks Don!

Also known as the four-point evening primrose, this species is found throughout much of the central continental USA. Like many members of the genus, Oenothera rhombipetala has a biennial life-cycle: in the first year, the germinated seed produces a rosette of leaves. In the second year, an erect leafy stem bearing flowers grows. Opening in the evening, the yellow flowers with their 4 distinctly-pointed petals wither the following morning.

It was observed in the 1960s that Oenothera rhombipetala seemed to be the single pollen source-food for the nocturnal bee species Sphecodogastra texana (flowering can occur all summer long, provided there is enough moisture). More recently, research by R. J. McGinley has shown that female bees of this genus collect pollen almost exclusively from members of the Onagraceae. Onagraceous pollen is unusually-shaped and enveloped in viscin (sticky) threads. Female Sphecodogastra bees have specialized curved pollen-collecting hairs on their hind legs, which allows them to accumulate large loads of the sticky pollen (see: McGinley, R.J. 2003. Studies of Halictinae (Apoidea: Halictidae), II: Revision of Sphecodogastra Ashmead, floral specialists of Onagraceae. (PDF) Smithsonian Contribution to Zoology 610).

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Low-Growing Evening Primrose Here 

How to Grow Evening Primrose Here