- Name also: Blackening Flat Pea, Black Bitter Vetch
- Family: Pea Family – Fabaceae (Leguminosae)
- Growing form: Perennial herb.
- Height: 30–80 cm (12–32 in.). Stem erect, spreading branched, bristly, wingless, glabrous.
- Flower: Corolla zygomorphic, initially reddish violet, finally bluish, 10–15 mm (0.4–0.6 in.) long. Petals 5; the upstanding the ‘standard’, the lateral two the ‘wings’, the lower two united to form the ‘keel’, overall shape of corolla being butterfly-like. Calyx 5-lobed. Stamens 10. A single carpel. Inflorescence in axils, long-stalked, often one-sided, a 2–10-flowered raceme.
- Leaves: Alternate, short-stalked, stipulate. Blade pinnate, 4–8 pairs, lacking tendrils. Leaflets elliptic, sharp-pointed, with entire margins, glabrous, underside blue-green. Stipules narrow.
- Fruit: 35–60 mm (1.4–2.4 in.) long, glabrous, black, 6–8-seeded pod (legume).
- Habitat: Broad-leaved forests, young forests, forest margins.
- Flowering time: June–July.
Black pea is protected on the Åland Islands.
Pea plants’ original-looking flowers have been compared to a butterfly or sailing boat, and the parts have been named correspondingly. The uppermost and largest petal is called the standard. On either side are two smaller petals, the wings. The lowest and most internal two petals are fused together and form a ‘keel’, like on the hull of a boat. The structure of the flower efficiently keeps out unwanted insects; only powerful insects, mainly bees, are able to get at the nectar. Black pea uses a change in colour to let bees know when there is nectar available. The flowers are initially a beautiful shade of red, and they then turn violet and blue until they wilt into a dirty greyish green. Its most important pollinators respond best to the red colour and collect nectar from the reddest flowers: pollinated blue flowers no longer contain nectar and hold no interest for the bees. The change in colour is the product of the cell fluid changing from acid to alkaline.
Black pea’s stalk also changes colour as it dries, and herbarium specimens are often so dark they are almost black. Black pea is easy to recognize: many vetch and vetchling stalks are so limp that they are unable to carry their own weight and so use tendrils to get support from other plants; black pea – and some of its close relatives, e.g. bitter vetch (L. linifolius), spring vetch (L. vernus) – however lack tendrils and so have robust, erect stalks.