- Family: Pea Family – Fabaceae (Leguminosae)
- Growing form: Perennial herb.
- Height: 25–80 cm (10–32 in.). Stem limp, climbing, flat, shallowly winged, sparsely haired.
- Flower: Corolla zygomorphic, purple, 12–20 mm (0.48–0.8 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 a long-stalked, 2–8 flowered raceme.
- Leaves: Alternate, stalked, stipulate. Stalk wingless. Blade pinnate, 2–4-pairs, terminal leaflet modified into a tendril. Leaflets lanceolate, with entire margins. Stipules long and quite narrow.
- Fruit: Flat, 25–50 mm (1–2 in.) long, glabrous, brown, 6–12-seeded pod (legume).
- Flowering time: July–August.
Marsh pea’s habitat is wide, but with gaps: In some places it is common, while in other places – even nearby – it is rare or doesn’t grow at all. It grows mainly on the shores of the Gulf of Bothnia or river banks in south-western Lapland; it is more challenging to find it on the southern coast, the south-western archipelago or inland. Its habitat is submerged so it is not necessarily east to find.
Marsh pea demands rich ground which should remain damp throughout its growing cycle, and often it grows on meadows beside rivers, lakes and the sea, and more rarely in coastal hedgerows. It has probably exploited cutting and grazing on natural meadows to a certain extent, but it hasn’t been able to make the move from its natural habitat to very different environments. Marsh pea is one of those rare species which gets along with common reed vegetation. Marsh pea growing in gaps between the monotonous reeds add a splash of reddish purple colour to the surroundings.
The best identification markers for genus Lathyrus are its small number of leaflet pairs and often winged stem, and marsh pea exhibits both features clearly: its leaves have only 3–4 leaflet pairs and the opposite side of the stem has a narrow wing, a green, thin projection, which is able to assimilate in the same way as its leaves. Marsh pea resembles bitter vetch (L. linifolius), although the latter has no tendrils and doesn’t need damp soil.