Species: Oenothera biennis
Oenothera biennis L.
* Oenothera muricata L.
* Oenothera suaveolens Desfontaines
* Onagra biennis (L.) Scopoli
* Onagra muricata (L.) Moench
* Species Plantarum 1:346. 1753
* USDA, ARS, National Genetic Resources Program. Germplasm Resources Information Network - (GRIN) [Online Database]. 
Ελληνικά: Οινόθηρα η διετής
English: Common Evening Primrose
Русский: Ослинник двулетний
Türkçe: Eşek otu
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 pollinators: 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.
Cultivation and uses
The mature seeds contain approximately 7-10% gamma-linolenic acid, a rare 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 .
1. ^ Germplasm Resources Information Network: Oenothera biennis
2. ^ Borealforest: Oenothera biennis
3. ^ Plants of British Columbia: Oenothera biennis
4. ^ Jepson Flora: Oenothera biennis
5. ^ Ultraviolet Flowers: Oenothera biennis
6. ^ Blanchan, N. (1922). Wild Flowers Worth Knowing. Project Gutenberg Literary Archive Foundation.