- Latin synonym: Cirsium helenioides
- Family: Daisy Family – Compositae, subfamily Carduoideae
(formerly Aster Family – Asteraceae)
- Growing form: Perennial herb. Rhizomatous, forms stands.
- Height: 40–120 cm (15–50 in.). Stem usually branchless (sometimes short branches terminated by capitula), wingless, spineless, grooved, woolly.
- Flower: Single flower-like 3.5–5 cm (1.4–2 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets purple (occasionally white), tubular. Stamens 5. Pistil of 2 fused carpels. Involucre hemispherical, involucral bracts erect, linearly lanceolate, reddish–brownish, almost spineless, tip short and soft. Capitula 1–4, usually solitary, stalked.
- Leaves: Alternate, basal leaves stalked, stalk broadly winged, stem leaves amplexicaul, not decurrent. Blade lanceolate–ovate, long-tapered, entire–pinnately lobed, soft, margin usually densely small-toothed, teeth thin-spined, green on top, glabrous, underside densely tomentose.
- Fruit: Flattish, blunt, 3–5 mm (0.12–0.2 in.) long achene, tip with thickened ring and feathery hairs.
- Habitat: Rich forests, damp broad-leaved forests, swamps, fens, damp meadows, stream banks, springs, roadside ditches, banks.
- Flowering time: July–August.
Melancholy thistle has been looked upon more kindly throughout the ages in comparison with its spinier brethren, which even in the Bible have been metaphors for destruction, mayhem and evil. Melancholy thistle is uncommonly gentle for a thistle: the tips of the tiny spines on the leaves are short, soft and fragile and are unable to protect it from hungry mammals such as elk. Even its sturdier spines are useless against smaller predators: the world’s most widespread day butterfly, the painted lady, visits Finland every summer and its grubs eat thistles, as well as genus Carduus plants and nettles. Melancholy thistle is only interested in adult butterflies that will feed on its nectar, however, and the plant uses its impressive capitula to attract them and other pollinators to spread its pollen.
People are also attracted to its large, eye-catching capitula. The degree to which it is lobed changes dramatically, and forms with entire and lobed margins have at one time been seen as different species. The degree to which the leaves are lobed does not seem to be a result of the conditions of its habitat, but it has been suggested that plants with more entire margins could have bisexual flowers, while the lobed form could be unisexual pistillate plants. Melancholy thistle plants that grow in the shade of trees in particular often fail to flower, although they can cover a large area with their leaf rosettes. On cleared land melancholy thistle flowers when the amount of light increases, and the stands can be very large and most impressive. It also grows in open areas such as ditch banks and grazing land.