Oenothera biennis, the common evening-primrose, is a species of flowering plant in the family Onagraceae, 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.
Common names The plant is known by various common names, such as evening star, sundrop, German rampion, hog weed, and fever-plant.
Description
Oenothera biennis usually has a life span of two years (biennial) growing to 1.6 m (5 ft 3 in) tall in the seeding year. The leaves are lanceolate, 8–18 cm (3–7 in) long and 2–6 cm (3⁄4–2+1⁄4 in) wide, produced in a tight rosette the first year, and spirally on a stem the second year. However, plants with annual life cycles are documented, and have been selected for under experimental conditions in environments with low competition.
Growing cycle Most commonly, O. biennis grows as a low, spreading herb in the first year, just a few centimetres tall, with its leaves spreading out in a rosette to cover the ground, whilst storing energy in a thick root. In the second year, it grows up much taller, to around 1.6 metres (5.2 ft) tall, on a semi-woody stem, which then flowers and seeds. This will usually die at first frosts, but occasionally grows another stem in the third year.
Blooming and fruiting 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 a bright nectar guide pattern, invisible to the naked eye. This pattern is apparent under ultraviolet light and visible to its pollinators, moths, butterflies, and bees. The fruit is a capsule 2–4 cm (3⁄4–1+1⁄2 in) long and 4–6 mm (0.16–0.24 in) broad, containing numerous 1–2 mm (0.04–0.08 in) long seeds, released when the capsule splits into four sections at maturity.
Taxonomy Oenothera biennis was given its scientific name in 1753 by Carl Linnaeus in the book Species Plantarum. It has no accepted subspecies or varieties, but it has 127 synonyms according to Plants of the World Online.
Ecology This plant is a weedy species that favors disturbed environments. It has been introduced and become established on all continents except Antarctica. The seeds of the plant are important food for birds, including American goldfinch, Northern bobwhite, and mourning dove, and it is a larval host for both the primrose moth and the white-lined sphinx moth. Bumblebees and honeybees also visit the flowers. The primrose moth (Schinia florida) is a common consumer of Oenothera biennis. The adults lay eggs on the flower, and the emerging caterpillars feed on the plant.
The seeds of O. biennis can remain viable in the soil seed bank for 70 years or more.
Uses Over the centuries, Indigenous people in North America have used the plant as food and traditional medicine. The evening primrose was introduced to Europe in the early 17th century as an ornamental plant in botanical gardens where its flowers are favored for nectar by pollinators, such as bees, and its seeds are food for birds.
Food uses Most of the plant parts are edible, having a taste that is mild. The roots can be eaten raw or cooked like potatoes. The leaves can be eaten raw in salads or cooked like spinach or in soups. Anishinaabe tribes traditionally make tea from the evening primrose leaves for use as a dietary aid and to reduce fatigue. The flowering stems are preferably used when they are still young in June. They have to be peeled and can then be eaten raw or fried. The flower buds are regarded as a delicacy and can be harvested from June to October. The seeds have a protein content of about 15%, an oil content of 24%, and contain about 43% cellulose. The proteins are especially rich in the sulphur-containing amino acids, methionine and cysteine, and tryptophan. Gamma-linolenic acid (GLA), and linoleic acid are present. Mainly for these polyunsaturated fatty acids, evening primrose oil is sold as a dietary supplement.
Research
There is not enough clinical evidence to support the use of evening primrose oil as a therapy for any medical condition. A Cochrane review concluded that there was no effect of primrose oil on eczema. The Mayo Clinic stated there was no good evidence that it affected eczema, diabetic neuropathy or premenstrual syndrome. The American Cancer Society stated that there was little evidence for its effectiveness as an anti-cancer agent, for which it is sometimes promoted, and "neither GLA nor other GLA-rich supplements (such as evening primrose oil) have been convincingly shown to be useful in preventing or treating any other health conditions."
Adverse effects Evening primrose oil is considered likely as safe in recommended doses. It may increase the risk of bleeding, a concern for patients with bleeding disorders or taking drugs that may increase bleeding. The Mayo Clinic recommends caution in people with seizure disorders or mania, and by pregnant or breastfeeding women, and publishes a long list of possible side-effects. Oral use of evening primrose oil may cause headaches or nausea.
Traditional medicine The leaves and stems were used by Native Americans to treat skin, digestion, and throat conditions.
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