Oenothera biennis (common evening-primrose, evening star, sundrop, weedy evening primrose, German rampion, hog weed, King's cure-all, or fever-plant) is a species of Oenothera native to eastern and central North America.
Oenothera biennis has a life span of two years (biennial) growing to 30–150 cm (12–59 in) tall. The leaves are lanceolate, 5–20 cm (2–8 in) long and 1–2.5 cm (1⁄2–1 in) broad, produced in a tight rosette the first year, and spirally on a stem the second year. Blooming lasts from late spring to late summer. The flowers are hermaphrodite, produced on a tall spike and only last until the following noon. They open visibly fast every evening producing an interesting spectacle, hence the name "evening primrose." The blooms are yellow, 2.5–5 cm (1–2 in) diameter, with four bilobed petals. The flower structure has an invisible to the naked eye bright nectar guide pattern. This pattern is apparent under ultraviolet light and visible to its pollinators, moths, butterflies, and bees.