Easily confused with : other Evening-primroses (Oenothera) species.

Not to be semantically confused with : Fragrant Agrimony (Agrimonia procera), Fragrant-Orchid (Gymnadenia conopsea) [plants with similar names belonging to differing families]

Hybridizes in a (unique to the UK) way with : Evening-primrose (Small-flowered) (Oenothera cambrica), Evening-primrose (Intermediate) (Oenothera x fallax), Evening-Primrose (Large-Flowered) (Oenothera glazioviana) and with any of their hybrids or itself to produce a 'hybrid swarm', whenever two (or more) of those are in proximity. Many of these hybrids have no common nor botanical name. See  Hybrid Swarms.

The identifying features to look out for in this species, Oenothera biennis are:

• Stems: Green (can have red parts).
• Flower buds: Green
• Flower size: Medium
• Stigma versus Stamen length: Stamens longer
• Leaves: Flat(ish)
• Rarity: No longer the commonest Evening-primrose in the UK.

It is more frequent in the North of Britain to Central Scotland, but is not as common as it once was, and is no longer the commonest Evening-primrose in the UK. It grows on sand-dunes near the coast.

Of those Evening-primroses which comprise a hybrid-swarm on the Sefton Coast, this species is the most variable of them. It's anthers and stamen are held close together so is more likely to self-fertilise than take part in hybrid-swarm orgies.

Evening Primrose Oil (aka EPO) is usually obtained from Common Evening-primrose, but that is now rarer in the UK than other more common species/hybrids. But that does not matter, since it is not usually harvested here but is grown for this oil (which is obtained from the seeds by a cold-pressing process) in 30 different countries. It is widely used as a dietary supplement for conditions such as PMS, atopic eczema, psoriasis, multiple sclerosis, cancer, coronary heart disease, diabetic neuropathy, gastrointestinal symptoms and autoimmune conditions, and consists of a variety of essential fatty acids such as 70-77% Linoleic Acid, 9-10% γ-Linolenic Acid, 5-11% Oleic Acid, 5-7% Palmitic Acid and 1.5-2.5% Stearic Acid with small contributions (<1% combined) from α-Linolenic Acid, Eicosanoic Acid and Eicosenoic Acid. γ-Linolenic Acid degrades by oxidation, thermal degradation and photooxidation to the unsaturated aldehyde Hexenal as the dominant odorous component.

It is expensive and used both externally by massaging into the skin or in aromatherapy and perhaps more controversially internally. Concerns have been raised about its potentisl adverse effects such as occasional headaches, abdominal pain, nausea, loose stools and some seizures.