- Name also: European Purple Lousewort (USA)
- Family: Broomrape Family – Orobanchaceae
(formerly Figwort Family – Scrophulariaceae)
- Growing form: Biennial herb. Taproot strong, straight. Hemiparasite.
- Height: 15–40(–80) cm (6–16(–32) in.). Stem almost glabrous, often brownish red, usually branched, branches often flowering.
- Flower: Corolla zygomorphic, red, sometimes yellowish white, 15–22 mm (0.6–0.88 in.) long, fused, bilabiate, with long tube. Upper lip flat-sided, tip sharply convex; lower lip 3-lobed, central lobe smaller than lateral lobes, round. Calyx bowl-shaped, bilabiate, unclearly 5-lobed. Stamens 4. Gynoecium fused, single-styled. Inflorescence a long terminal spike, lax in the lower part.
- Leaves: Alternate; with basal rosette. Rosette leaves long-stalked, blade triangular, 2 times pinnately lobed. Stem leaves short-stalked, blade ovate–linear, pinnately lobed, lobes toothed or lobed.
- Fruit: Quite elliptic, with tapered tip, brown, capsule opening from one side.
- Habitat: Seashore and flood-influenced meadows, lake shores, riversides, moist meadows, boggy margins, rich swamps.
- Flowering time: June–August.
Marsh lousewort is common in Finland, but rarely abundant. Its reddish brown, decorative shoots and red flowers stand out from a distance. Only the most powerful insects, such as bumble and honey bees, are able to get at its nectar. Bumble bees land on the corolla’s lower labellum, push their way inside and push the upper labellum forcefully in order to get at the nectar. In doing so the insect reveals its stamens and pollinates the plant while it loads up on nectar.
Marsh lousewort is a hemiparasite, meaning that it sucks extra nutrition from its neighbour’s roots. The plant’s stem goes woody and stands up all through the winter. Marsh lousewort is divided in Finland into three subspecies, which can be differentiated from each other on the basis of the area they grow in and their flowering time. Ssp. palustris in quite low, abundantly branched, flowers in June, is large-flowered (18–22 mm, 0.72–0.88 in.), and grows in southern and central Finland; ssp. borealis grows in northern and northern parts of central Finland, is branchless, has a slightly smaller flower (approx. 15 mm, 0.6 in.) and it flowers in July; ssp. opsiantha is abundantly branched and quite tall, and its flowers are small (14–17 mm, 0.56–0.68 in.).