- Name also: Field Thistle, Canada Thistle, Canadian Thistle, Way Thistle
- 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 branching from top, wingless, glossy, woolly at top, sometimes densely spiny (var. maritimum).
- Flower: Plant mostly dioecious (male and female flowers on different plants). Single flower-like 1.5–2.5 cm (0.6–1 in.) capitula surrounded by involucral bracts. Male plants’ capitula rounder and shorter than female’s. Capitula’s ray-florets lacking; disc florets light purple, tubular. Stamens 5. Pistil of 2 fused carpels. Involucre elliptic, involucral bracts erect, tips with short spines, slightly woolly, often purplish. Capitula solitary terminating stem branches or borne in a lax corymbose cluster.
- Leaves: Alternate, usually stalkless, slightly amplexicaul. Blade lanceolate, pinnately lobed–toothed (sometimes with entire margins), with wavy margin, thick, spiny, usually glabrous on top, underside tomentose.
- Fruit: Flattened, glossy, yellowish brown, 3–4 mm (0.12–0.16 in.) long achene, tip with thickened ring and feathery hairs.
- Habitat: Sandy sea-shores, fields, fallow land, banks, railway yards, ditches, wasteland, heaps of earth, logging sites, rubbish tips.
- Flowering time: July–September(–October).
Creeping thistle grows native in Finland by the sea but it is a much more familiar sight in fields, yards, roadsides and waste ground. When creeping thistle runs wild in a field it reduces the size of the crop and gives farmers grey hairs – it is regarded as one of the most pernicious weeds in the temperate zone because, apart from using powerful chemicals on it, there is not much that can be done. It spreads over a wide area via buds in its rootstock, and even half a centimetre of root is enough to give rise to a new plant. The more the land is disturbed, the more pieces the root is broken into, and the more new shoots appear. The rootstock is 10–50 cm (4–20 in.) deep, affording it good protection against poison and other repellents.
Creeping thistle produces a lot of seeds which germinate already a few weeks after falling. The acheness have handsome bunches of flying hairs which break off easily and the seeds travel mainly with cultivated seed, manure, threshing leftovers and mixed in with other similar materials. The efficient cleaning of seed has slightly curtailed its victory march across cultivated land, however. Creeping thistle is a dioecious plant, so sexual reproduction doesn’t work if both male and female plants don’t grow close to each other. It also grows vigorously, in fallow and unused fields, and is very common around fields and beside roads that cut through fields.
Creeping thistle’s reputation as a weed often overshadows its diversity. The most common form in Finland is var. arvense, which has glossy leaves and is delicately spiny, while rarer var. maritimum is strong-spined and crinkled. Sometimes creeping thistle that grows in Finland can be divided into other, mostly rare or casual species that grow close to human influence.