Evening Primrose

Botanical Profile

Note on species delimitation: Evening-primrose raw materials in literature and trade are not always restricted exclusively to Oenothera biennis.

Oenothera biennis is the common evening primrose, a biennial species of the Onagraceae. Its fatty seed oil is of particular importance, while seeds, herb, root and flowers are considered separately in botanical and historical contexts.

Oenothera biennis L. is a mostly biennial plant originally native to North America and now widely naturalised in Europe. In the first year it forms a basal leaf rosette; in the second, an upright hairy flowering stem with alternate leaves and large yellow flowers. The flowers typically open toward evening and are visited by moths and other insects. The fruit is an elongated capsule containing numerous small seeds. The species often grows on ruderal sites, roadsides, railway embankments, sandy and gravelly areas and other open disturbed habitats. From a raw-material perspective, the seeds and the fatty oil obtained from them are the primary focus. Herb, root and flowers are of more botanical, historical or ethnobotanical significance.

Phytochemistry

The most important raw-material characteristic of Oenothera biennis is its fatty seed oil. It contains predominantly linoleic acid together with relevant proportions of gamma-linolenic acid; other fatty acids, tocopherols, phytosterols and accompanying lipids are also present. Composition depends on species, seed quality, origin, harvest, pressing, refining and storage conditions. Because polyunsaturated fatty acids are susceptible to oxidation, peroxide value, anisidine value, oxidative protection and storage stability are important quality parameters. Other plant parts contain different spectra of phenolic and vegetative constituents but are not central to the monographic consideration of evening-primrose oil.

Traditional Uses

Evening primrose reached Europe from North America and became known there as an ornamental, ruderal and later oil-raw-material plant. Historically, various plant parts were described, including young roots, leaves, flowers and seeds. In modern European phytotherapy, however, the fatty oil obtained from the seeds is the principal focus. The EMA/HMPC describes Oenotherae oleum as a traditionally used herbal preparation for certain dry skin conditions with itching. This classification is based on long-standing use and must be distinguished from general cosmetic, nutritional or experimental statements.

Current Research

Research on Oenothera biennis focuses on evening-primrose oil, fatty-acid patterns, gamma-linolenic acid, oxidative stability, seed quality and clinical or regulatory assessments. The HMPC assessment considers traditional use of evening-primrose oil while also evaluating the available clinical evidence. More recent reviews and studies address skin conditions, inflammatory models, fatty-acid metabolism and quality of plant oils. Robust statements require differentiation among refined oil, cold-pressed oil, seed material, isolated fatty acids and other plant parts.

Sources

• Kew Science, Plants of the World Online: Oenothera biennis L.
• EMA/HMPC: European Union herbal monograph on Oenothera biennis L.; Oenothera lamarckiana L., oleum.
• EMA/HMPC: Assessment report and reference list for Oenotherae oleum.
• Ph. Eur.: Oenotherae oleum raffinatum.