- Name also: Curly Plumeless Thistle (USA), (also Welted Thistle, see Carduus acanthoides)
- Family: Daisy Family – Compositae, subfamily Carduoideae
(formerly Aster Family – Asteraceae)
- Growing form: Biennial herb.
- Height: 50–180 cm (20–70 in.). Stem branching, broadly winged, spiny, woolly, greyish. Spines max. 2.5 mm long, quite delicate.
- Flower: Single flower-like 1.5–2.5 cm (0.6–1 in.) capitula surrounded by involucral bracts. Capitula’s ray-florets lacking; disc florets purple, tubular. Stamens 5. Pistil of 2 fused carpels. Involucre hemispherical, involucral bracts overlapping in many rows, linearly lanceolate, with spiny tip. Pedicels spineless, densely haired. Capitula in dense clusters of 3–5.
- Leaves: Alternate, lowest short-stalked, upper stalkless, edged of stalks winged. Blade elliptic, pinnately lobed (sometimes only toothed margin), edges spiny, underside usually white-haired, top sparsely haired.
- Fruit: Roundish, slightly flattened, glabrous, glossy, shiny, yellowish brown, 3–4 mm (0.12–0.16 in.) long achene, tip with 8–12 mm (0.32–0.48 in.) long unbranched hairs.
- Habitat: Yards, fields, pastures, logging sites, railway yards, banks, heaps of earth, wasteland, fortress and garrison areas, around inhabited areas. Nitrophile.
- Flowering time: July–September.
Curled thistle grows in almost all of Finland but it is most common on one hand in the densely populated areas of southern Finland and on the other hand on cultivated land in southern Lapland and Kuusamo. It has established itself in southern parts of south and south-west Finland with people in ancient times, growing against walls and on banks. It arrived in northern villages from the White Sea area in the 18th and 19th centuries with grain seed and ended up as a weed on arable land. Nowadays it is no longer found on fields but it is still common around rural areas. It is especially fond of nitrogenous soil, so it favours waste ground, gardens and land next to walls. As it is large and spiny people do not usually allow it to grow in their yards. Thistles are treated as weeds but are good nectar plants and will attract e.g. butterflies if they are left in peace. Birds also like the seeds as winter food, and the scientific name of the goldfinch – Carduelis – is a reference to this.
Genus Carduus stems are usually strongly winged while, of the members of genus Cirsium, only spear thistle (C. vulgare) fits this description. Curled thistle’s denser hairs make it look clearly more greyish, and its spines are quite delicate compared to spear thistle’s strong ones. The genera also differ with regards to the flying hairs on their cypselas: those of genus Carduus have unbranched or at most hooked, unbranched hairs while on genus Cirsium they are strongly branched and feather-like. Also, compared to its rare relative spiny plumeless thistle (C. acanthoides), curled thistle’s spines are quite tame, thin and short: one can for instance hold the leaf in one’s hand without being jagged. Its involucral bracts are also soft-tipped, while spiny plumeless thistle has a spine. Spiny plumeless thistle is also a brighter green; curled thistle is greyish.