Oenothera biennis (common evening primrose, evening star, or sun drop) 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. Evening primrose oil is produced from the plant.
Growth and flowering
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 (2.0–7.9 in) long and 1–2.5 cm (0.39–0.98 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 (0.98–2.0 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.
The fruit is a capsule 2–4 cm (0.79–1.6 in) long and 4–6 mm (0.16–0.24 in) broad, containing numerous 1–2 mm (0.039–0.079 in) long seeds, released when the capsule splits into four sections at maturity. 
Primrose moth (Schinia florida) in flower
It is also known as Weedy evening-primrose, German rampion, hog weed, King's cure-all, and fever-plant.
Cultivation and uses
The mature seeds contain approximately 7–10% gamma-linolenic acid, an essential fatty acid. The O. biennis seed oil is used to reduce the pains of premenstrual stress syndrome and is beneficial to the skin of the face. Also, poultices containing O. biennis were at one time used to ease bruises and speed wound healing.
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- Germplasm Resources Information Network: Oenothera biennis
- http://www.umm.edu/altmed/articles/evening-primrose-000242.htm (Retrieved 6/17/13)
- http://nccam.nih.gov/health/eveningprimrose (Retrieved 6/17/13)
- Borealforest: Oenothera biennis
- Plants of British Columbia: Oenothera biennis
- Jepson Flora: Oenothera biennis
- Ultraviolet Flowers: Oenothera biennis
- Blanchan, N. (1922). Wild Flowers Worth Knowing. Project Gutenberg Literary Archive Foundation.
- Gaertner, Erika E. (1968). "Additions to the list of wild edible plants preservable by the deep freeze method". Economic Botany 22 (4): 369. doi:10.1007/BF02908133
- Bamford, JT; Ray, S; Musekiwa, A; van Gool, C; Humphreys, R; Ernst, E (2013 Apr 30). "Oral evening primrose oil and borage oil for eczema.". The Cochrane database of systematic reviews 4: CD004416. PMID 23633319.
- Profile: Yellow Evening-primrose (Oenothera biennis) Photos, Drawings, Text. (Wild Plants of Winnipeg from Nature Manitoba)