Oenothera biennis (Common evening primrose or Evening star) is a species of Oenothera native to eastern and central North America, from Newfoundland west to Alberta, southeast to Florida, and southwest to Texas, and widely naturalized elsewhere in temperate and subtropical regions.
It is a biennial flowering plant growing to 30–150 cm tall. The leaves are lanceolate, 5–20 cm long and 1–2.5 cm broad, produced in a tight rosette in the first year, and spirally on the stem in the second year. The flowers are pale yellow, 2.5–5 cm diameter, with four petals; they are hermaphrodite, and produced on a tall spike from late spring to late summer. They open in the evening, hence the name "evening primrose", and close by the following noon. The flower has a bright nectar guide pattern, invisible in visible light, but apparent under ultraviolet light, which assists its pollinated by Lepidoptera (moths and butterflies) and bees. The fruit is a capsule 2–4 cm long and 4–6 mm broad, containing numerous 1–2 mm long seeds, released when the capsule splits into four sections at maturity.
It is also known as Weedy evening-primrose, German rampion, hog weed, King's cure-all, and fever-plant.