Oenothera fruticosa
They call me Mellow Yellow…
Evening Primroses and Sundrops are very similar in appearance. They both have lance-shaped leaves and an upright growth habit. They both bear large (~2-inch) yellow flowers, each with four petals and a large x-shaped stigma at the center.
The flowers of Evening Primrose are mostly closed during the day but open toward evening (hence the name). The opposite is true of Sundrops. Their flowers open during the day.
I have fond memories of growing Sundrops in my Mississippi garden in years past. It overwinters in the south as a low, basal rosette of evergreen leaves. In late spring, it grows tall again, up to 2 feet in height before coming into bloom. Then, spikes of sassy yellow flowers dependably light up the borders. This is a happy plant that’s commonly gifted from one gardener to another because it transplants so well.
Bloom time in my part of Virginia is early August. The flowers will give way later to club-like, 4-angled fruit capsules (see gallery below).
VIRGINIA WILDFLOWERS 
For 17 years i have been watching the yellow blossoms fully open during the day. Wild in this midcoast Maine ‘yard’, I am certain they are Oenothera biennis – common evening primrose. Every year I try to discover why these plants would be otherwise. I still can’t. Except for the daytime bloom, in every way, the plants here give every identification as evening primrose. They could not be sundrops. Perhaps something else entirely.