|As the name Oenothera biennis suggests, common evening primrose has a two-year life cycle. It can thrive in full sun or in partly shaded areas. Sylvan Runkel and Dean Roosa write in Wildflowers of the Tallgrass Prairie,
During the first year, this plant appears as a coarse, flattened rosette of elongate leaves and strong, fleshy roots. In the second year, a stout stem develops that may reach a height of 6 feet, but normally it is less than 4 feet tall. [...]
Bright yellow flowers up to 2 inches across have four notched petals and eight stamens. The stigma is prominently four-lobed. Each blossom lasts only about a day, opening abruptly in late afternoon to dusk. Because the flowers open at dusk, pollination is mainly accomplished by night-flying moths, but there is also some pollination by bees. New blooms continue to appear until frost.
Evening primrose often grows in large colonies. You can see the four notched petals in the flowers near the foreground of this picture.
My camera isn't good enough to show the "crossed stigma typical of many evening primroses," described on the U.S. Wildflowers website.
Here is a picture of common evening primrose blooming near goldenrod:
From Wildflowers of the Tallgrass Prairie:
The fruiting capsules, which are up to 1 1/2 inches long, grow erect. There may be more than 50 capsules clustered along the upper stem.
You can see those capsules in the photos above, but they stand out more against the background of pavement in this shot, taken near New City Market in Des Moines.
Speaking of health food stores, evening primrose oil is used to treat a huge number of conditions and diseases. For that reason, "the king's cure-all" is another common name for Oenothera biennis.
Not all evening primrose plants are tall. This one was blooming close to the ground.
The tiny pink flowers are one of the smartweed species, another group of native North American plants.
Finally, I went back to one of the nightshades I photographed for this post to look for berries. I found some, but they're not the ripe black color yet.