- Name also: Marsh Orchid
- Family: Orchid Family – Orchidaceae
- Growing form: Perennial herb. Rootstock long, horizontal, branched. Forms stands.
- Height: 15–50 cm (6–20 in.). Stem reddish, upper part with glandular hairs.
- Flower: Perianth irregular (zygomorphic), outer surface greenish–brownish, inner surface light reddish brown or creamy white (occasionally dark red or pale yellow), darker-veined, 20–25 mm (0.8–1 in.) wide. Tepals 6, in 2 whorls, of which one elaborated into labellum. Labellum under perianth, spurless, white, with red and yellow pattern, deeply constricted in middle, base cup-shaped with clear lateral lobes, widening towards tip, shallowly 2-lobed. Androecium and gynoecium fused into a column, stamens 1, stigmas 2. Inflorescence a quite lax, 4–15-flowered, one-sided raceme.
- Leaves: Alternate, stalkless, amplexicaul, 4–8, becoming smaller towards crown. Lower leaf blades wide, upper ovate–narrowly lanceolate, tapered, parallel-veined, with entire margin.
- Fruit: Nodding capsule. Seeds tiny, like dust.
- Habitat: Fens, fen meadows, beaches.
- Flowering time: July–August.
- Endangerment: Endangered, protected in all of Finland, including the Åland Islands.
Marsh helleborine’s beautifully coloured flowers contain easily-accessible nectar and attract many different kinds of pollinating visitors, including spiders. Some of the small flies that visit the flowers are not big enough to carry the orchid’s large pollinia – they can’t even get airborne with that kind of load. Butterflies and bees that have a long proboscis can reach the nectar without touching the column, so they don’t pollinate the plant either. The most efficient pollinators are burrowing bees and mason wasps, and especially male potter wasps. Marsh helleborine’s specialization in being pollinated by this kind of wasp is evident in the construction of the flower. It is not as highly specialized as its tropical relatives however, as it can also be pollinated by flower flies and certain beetles, such as click beetles and blister beetles. If there is an anthill nearby, its avaricious inhabitants can also become pollinators.
Marsh helleborine also spreads vegetatively via its underground runners and forms stands that are usually small.