- Name also: Common Thistle, Bull Thistle (USA), Scots, Scottish Thistle, Scotch Thistle (see Cotton Thistle)
- Family: Daisy Family – Compositae, subfamily Carduoideae
(formerly Aster Family – Asteraceae)
- Growing form: Biennial herb.
- Height: 30–120 cm (12–50 in.). Stem winged, strong-spined, woolly, greyish green.
- Flower: Single flower-like 2–4 cm (0.8–1.6 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, woolly, involucral bracts linearly lanceolate, with spiny tip, grey, lowest branched. Capitula usually solitary, sometimes several together.
- Leaves: Alternate, stalked–stalkless, decurrent, stalks winged, rosette leaf-stalk wings with spiny edges. Blade elliptic–lanceolate, sharp-tipped, margin deeply pinnately lobed or toothed (upper leaves’ lobes further pinnately lobed), strong-spined, top dense with short, yellow spines, underside grey-tomentose.
- Fruit: Elliptic, flattish, blunt-tipped, 3.5–5 mm (0.14–0.2 in.) long achene, tip with thickened ring and feathery hairs.
- Habitat: Roadsides, meadows, pastures, yards, ditches, wasteland, railway yards, sometimes logging sites and stony sea-shores.
- Flowering time: July–September(–October).
Spear thistle’s form of defence is the most aggressive in Finland: strong, needle-sharp spines on both the leaf-blade margins and along the winged stem, the capitula’s involucral bracts end with a sting and even the surface of the leaves is covered in delicate spines. Most grazing animals leave it in peace, at least after they have received a painful warning on their nose, and it can often be seen growing alone on grazing land that has otherwise been nibbled bare. Although the plant’s spines repel predators, it could not survive without them on overgrown meadows. The decline of grazing has led to a clear, rapid decline in spear thistle. Nowadays the best place to look for the species is on waste ground, demolition sites, fallow fields, beside roadside noise-reduction walls and other at least semi-open places that have been recently created and are affected by people. Like several other plants that can be found around inhabited areas, it also grows sometimes by the sea-shore, where there is a lot of nutritious ground and not much competition.
Finns have never formed a particularly close bond with spear thistle, which folk names such as devil’s spike reveal. In old stories the devil himself was said to have sown thistles on the land to bother people. The plant’s spines pierce the skin and stepping barefoot on a leaf rosette is not something that is forgotten in a hurry. As a biennial, however, it is unable to form large stands that would have a detrimental effect on agriculture.
Many Cirsium thistles are reminiscent of genus Carduus thistles. They can be told apart by the fact that Cirsium thistles have feather-like flying hairs while Carduus thistles have down that is straight and unbranched. Spear thistle stands out from its relatives in that, apart from spines along the edge of the leaf blade and capitulum involucre, they are also on the winged stem and upper surface of the leaf blade.