Thistles are herbaceous biennials or perennials with alternate, lobed, spiny leaves and (sometimes) spiny stems. As part of the aster family, thistles have composite flowers with each flowerhead made up of many tiny florets. Flowers are purple to pink and range in size from 0.5” -3”. In spring, thistles grow as basal rosettes of spiny, lobed leaves. Seeds are brown with long tufts of white hairs to aid wind dispersal.
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There are several native thistles in Wisconsin, with similar stature, leaves, and flowers to invasive thistles. Native and invasive thistles can be differentiated based on leaf characteristics (color, hairiness, etc.), flower characteristics (size and number of flowers, size and color of bracts at base of flowerhead), and habitat.
Spotted knapweed (Centaurea stoebe) and similar knapweeds can have similar quantity, size, and color flowerheads to Canada thistle. However, these knapweeds do not have spiny leaves.
|Classification||Common Name||Other Common Names||Scientific Name||Factsheets||Videos||Statewide Distribution Map||National Distribution Map||Look-Alike Species|
|Not Regulated||Bull thistle||Cirsium vulgare||Factsheet 1||State Map||National Map||PDF 1
|Restricted||Canada thistle||Cirsium arvense||Factsheet 1|
|Video||State Map||National Map|
|Prohibited/Restricted||Marsh thistle||European marsh thistle||Cirsium palustre||Factsheet 1|
|State Map||National Map|
|Restricted||Musk thistle||Nodding thistle||Carduus nutans||Factsheet 1||State Map||National Map||PDF 1
|Restricted||Plumeless thistle||Carduus acanthoides||Factsheet 1||Video||State Map||National Map||PDF 1
Species Summary Table
|Scientific Name||Common Name||Stem||Leaves||Flowers|
|Carduus acanthoides||Plumeless thistle||Densely covered with leafy, spine-tipped “wings”; 3-6’||Deeply lobed, wavy around edges, very spiny; hairy on underside of leaf, esp. on midrib||1” purple (sometimes white) flowerheads; solitary or in clusters of 2-5; bracts narrow, spreading, short spiny tips; July – Oct.|
|Carduus nutans||Musk (nodding) thistle||Not spiny directly below flowerhead; spiny “wings” where leaf bases extend down stem; 2-7’||Deeply lobed, spines along wavy edges, may have white cast on edges||2-3” purple to pinkish flowerheads, often nodding, solitary; bracts large, triangular, showy; June – Oct.|
|Cirsium arvense||Canada thistle||Slightly ridged, may have scattered hairs, not usually spiny; 3-5’||Deeply lobed, up to 6” long, yellow spines scattered along wavy edges, may be hairy on underside||¾”, lavender to pale pink (sometimes white), solitary or in small clusters at ends of branching stems; bracts flat except for pointed purplish tip that peels away from flowerhead; June – Oct.|
|Cirsium palustre||European marsh thistle||Very sharp spiny “wings” where leaf bases extend down stem; densely to moderately hairy; 2-7’||Deeply lobed with sharp spine at tip of each lobe, wavy edges; upper side variably hairy, underside densely hairy esp. along veins||¾” lavender to pinkish-purple flowerheads in tight clusters at top of stem and ends of long branches; green to purple-tinged bracts flat except for darker tip that peels away from flowerhead, fringed with hairs around edges; June – Aug.|
|Cirsium vulgare||Bull thistle||Hairy, covered with small, leafy, spine-tipped “wings”; 2-6’||Lobed with 3+ narrow lobes, each with ½” sharp, yellowish spine at tip; hairy on upper surface; woolly underside||1 ½ - 2” reddish-purple flowerhead, solitary or a few clustered at ends of branching stems; bracts narrow, spreading, tipped with short spines; June – Oct.|