- Name also: Marsh Plume Thistle, European Swamp Thistle
- Family: Daisy Family – Compositae, subfamily Carduoideae
(formerly Aster Family – Asteraceae)
- Growing form: Biennial or once-flowering perennial herb.
- Height: 50–200 cm (20–80 in.). Stem branchless–branching at top, winged, spiny, grooved, sparsely woolly, usually dark reddish.
- Flower: Single flower-like 1–1.5 cm capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets magenta–dark purple (occasionally white or pink), tubular. Stamens 5. Pistil of 2 fused carpels. Involucre hemispherical, involucral bracts erect, tips with short spines, reddish brown. Capitula in dense, racemose clusters of 2–8.
- Leaves: Alternate, stalked–stalkless, decurrent, stalks winged, basal leaf-stalk wings with spiny edges. Blade lanceolate, sharp-tipped, pinnately lobed, rigid, thin-spined, sparsely woolly on top, dark green, underside woolly and straight-haired, light green.
- Fruit: Elliptic, flattish, glossy, shiny, 3–4 mm (0.12–0.16 in.) long achene, tip with thickened ring and feathery hairs.
- Habitat: Damp meadows, pastures, arable land, fallow land, ditch banks, shores, rock-pools, springs, young, dense broad-leaved forest, rich bogs.
- Flowering time: July–September.
Marsh thistle is one of the tallest biennial in Finland (compare giant hogweed): the stem that rises from the centre of its rosette often reaches 2 metres (6.5 feet) in height. They can be spotted from afar, but in fact they are much more abundant on the ground in the form of different-aged leaf rosettes: although in optimum conditions the plant will complete its life cycle in two years, it usually takes at least 5 years in Finland to gather enough strength to flower. Marsh thistle suffers greatly in its youth from the harsh conditions and only a small proportion of them ever flower.
Marsh thistle can be found throughout almost all of Europe. It is common in Finland until around Oulu, north of which it begins to get rarer and is to a certain degree casual in northern Lapland – although a wild stand is known to exist around Enontekiö. In Finland the open areas that suit marsh thistle are spring swamps, stream-side quagmires and waterside meadows. It spreads easily, however, on land that has been affected by people and to damp habitats that have been created, such as roadside ditches, field banks and neglected lawns. Marsh thistle has never really adapted to live on pasture land, however, and it is not spiny enough to avoid being eaten by cows and horses.
Marsh thistle looks vaguely like its stouter relative spear thistle (C. vulgare), although the latter’s leaf blades are much spinier. The clearest difference is in the capitula: spear thistle’s are solitary while marsh thistle’s have several together. Marsh thistle’s intermittently winged stems can also bring to mind genus Carduus thistles: the flying construction of Carduus achenes is comprised of a bunch of unbranched hairs while genus Cirsium plants have more complex, branched, feather-like hairs.