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Introduction
Scotch thistle is an introduced species of thistle in the aster family (Asteraceae) that was introduced to North America, South America and Australia in the 19th century as an ornamental plant. In North America, it is now found in disturbed sites and roadsides in most US states and in several Canadian provinces (BC, MB, NB, NS, ON, QC) (USDA 2010). In BC, it has been confirmed across the southeast and south-central part of the province, including southeastern Vancouver Island, the Lower Mainland, and the Okanagan. It has been collected as far north as Vernon. It is also reported from a few locations beyond the confirmed range in the Kootenays (Castlegar, Trail, Nelson, and Slocan areas).
This is a biennial (sometimes annual) species that produces a large rosette of spiny leaves and a large tap root (up to 30 cm long) in its first year. In the second year, it is a tall (1.5-3.0 m) coarse spiny plant with large solitary (globe-shaped) pinkish-purple flowerheads. Basal leaves are lacking, and leaves are alternate. Leaves are toothed to deeply pinnately lobed, and spines are 2-5 mm long. It is densely woolly-hairy below and sparsely woolly-hairy above, and is very drought-resistant.
This is the familiar thistle that is the emblem of Scotland.
Species Information
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expanded illustration for this species.
General: Biennial herb from a fibrous root; stems erect, simple, branched, broadly spiny-winged, woolly-hairy, 1.5-3.0 m tall.
Leaves: Basal leaves lacking; stem leaves alternate; lower leaves elliptic to oblanceolate, toothed to deeply pinnately-lobed, 10-60 cm long, 5-10 cm wide, spiny, the spines 2-5 mm long, sparsely woolly-hairy and green above, densely white woolly-hairy below; upper leaves reduced and linear-elliptic to linear-lanceolate.
Flowers: Heads discoid, few to many at the ends of branches; involucres egg-shaped to more or less rounded, 2.5-3.5 cm tall; involucral bracts lanceolate, tapering to a stiff, spreading spine 4-5 mm long, outer ones 2 mm wide at the base, inner ones similar but more flexible, flatter, and with shorter spines; disk flowers reddish-purple; receptacles flat, fleshy, honeycombed, often with short bristles on the partitions.
Fruits: Achenes hairy, transversely wrinkled, greyish-brown, 4-5 mm long; pappus of numerous slender bristles.
Scotch thistle is very similar in appearance to bull thistle (Cirsium vulgare), and the two species can be confused. However, Scotch thistle lacks basal leaves (bull thistle has basal leaves), has larger and fewer flowerheads, and is much more densely woolly, which gives plants a silvery appearance. Bull thistle is strongly spiny-hairy above, glabrous to sparsely woolly below, and plants are green in appearance.. Scotch thistle is less common and more restricted in distribution in the province, while bull thistle is widespread and common across southern BC.
