One of the summer garden's most romantic of flowers, the evening primrose (Oenothera biennis) opens its blossoms by dusk and closes by sunrise the next morn. Having only four petals, they are first pale yellow but deepen to gold by sunrise. Moths pollinate the night-receptive flowers, leading to production of seeds that sprout and bloom in their second year of growth, a true biennial plant.
The common evening primrose is a member of the willowherb or evening primrose family, Onagraceae. Besides being called common evening primrose, it may also be referred to as king's cure-all or sundrop.
Growing freely in dry, rocky plains, along lake shores and in open woods, the evening primrose's native range is expansive across much of southeastern North America. Its range extends from Canada's Newfoundland to Alberta and then southward to Texas and Georgia in the United States.
The common evening primrose is an erect annual or biennial herb with a large rosette, or radial cluster of leaves at the plant base. Each leaf is mid-green, lance-like, slightly sticky to the touch and has reddish veins. In summer and autumn, the stem of the plant produces bowl-shaped flowers that smell lightly of lemons. The blooms open in the late afternoon, initially pale yellow and as they age overnight become more golden yellow. They are withered by the following morning. The plant dies after two years, replaced by plants that sprout nearby from the seeds produced by flowers. It matures at 2 to 6 feet in height.
Best in full sun locations, the evening primrose tolerates partial sun to partially shaded areas in the landscape, with anywhere from two to 10 hours of direct sunlight daily. It is tolerant of hot summer temperatures and high humidity as well as nutrient-poor soils that are rocky or gravelly. In fact, if soil is too fertile, the plant grows very quickly and often dies within one year. Avoid highly alkaline soils or those that do not drain freely after rainfall. Common evening primrose is hardy in USDA hardiness zones 5 through 9.
Other species of Oenothera also may be given the name evening primrose. Showy evening primrose (Oenothera speciosa) is native to the southwestern United States and Mexico, producing white to pink flowers. The tooth-leaf evening primrose (Oenothera serrulata) has white blossoms. The Missouri or Ozarks primrose (Oenothera missouriensis or Oenothera macrocarpa) may also be called an evening primrose with its showy bright yellow flowers. Any plant in the botanical group Oenothera may be commonly called an evening primrose or sundrop, depending on where you live.