Oenothera speciosa

Oenothera speciosa is a species of evening primrose known by several common names, including pinkladies, pink evening primrose, showy evening primrose, Mexican primrose, amapola, and buttercups (not to be confused with true buttercups in the genus "Ranunculus").
Distribution.
It is a herbaceous perennial wildflower. Originally native to the grasslands of Kansas, Missouri, Nebraska, northeastern New Mexico, Oklahoma, and Texas, it has been naturalized in 28 of the lower 48 U.S. states as well as Chihuahua and Coahuila in Mexico. It frequently escapes from gardens. The specific name, "speciosa", means "showy".
The plant's wild habitat includes rocky prairies, open woodlands, slopes, roadsides, meadows and disturbed areas. While it makes an attractive garden plant, care should be taken with it as it can become invasive, spreading by runners and seeds. This drought-resistant plant prefers loose, fast-draining soil and full sun.
Habit.
The pink primrose has glabrous (smooth) to pubescent stems that grow to in height. The pubescent leaves are alternate with very short or no petiole (sessile), reaching long to broad. They are variable in shape, from linear to obovate, and are toothed or wavy-edged. It produces single, four-petaled, cup-shaped flowers on the upper leaf axils. These fragrant shell-pink flowers bloom throughout the summer into early autumn. The flowers start out white and grow pink as they age. 
The flower throats, as well as the stigmas and stamens, have a soft yellow color. It blooms both day and night, but typically in the pre-dawn hours, closing when the full sun hits them. They bloom from March to July, and occasionally in the fall. The flowers are frequented by several species of insect, but moths are the most common as the flowers are mostly open at night.
This plant is also frequently referred to as a buttercup, though it is not a true buttercup (genus "Ranunculus"), or even in the buttercup family Ranunculaceae.
Usage.
The pink evening primrose is used in the temperate latitudes as an ornamental plant. However, it does not survive a severe winter. It is a ground cover. With the USDA climates 4 to 9, the pink evening primrose in most areas of Central Europe should be sufficiently hardy. The green plant parts can be cooked or eaten as a salad; Harvest, if possible, before flowers develop, because then the taste is pleasant.