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Thistles

Canada thistle (Cirsium arvense)
bull thistle (Cirsium vulgare)

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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Look Alikes

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.

Resource Table

ClassificationCommon NameOther Common NamesScientific NameResourcesVideosWeed ID slides ppt (AGR 375)National Distribution MapLook-Alike Species
Not RegulatedBull thistleCirsium vulgareUWEX Factsheet
Cirsium vulgareNational MapPDF 1
PDF 2
RestrictedCanada thistleCirsium arvenseUWEX Factsheet
Link 		VideoCirsium arvenseNational MapPDF
Prohibited/RestrictedMarsh thistleEuropean marsh thistleCirsium palustreUWEX Factsheet 1
UWEX Factsheet 2
Link 		Cirsium palustreNational MapPDF
RestrictedMusk thistleNodding thistleCarduus nutansUWEX Factsheet
Carduus nutansNational MapPDF 1
PDF 2
RestrictedPlumeless thistleCarduus acanthoidesUWEX Factsheet
VideoCarduus acanthoidesNational MapPDF 1
PDF 2

Species Summary Table

Scientific NameCommon NameStemLeavesFlowers
Carduus acanthoidesPlumeless thistleDensely covered with leafy, spine-tipped “wings”; 3-6’Deeply lobed, wavy around edges, very spiny; hairy on underside of leaf, esp. on midrib1” purple (sometimes white) flowerheads; solitary or in clusters of 2-5; bracts narrow, spreading, short spiny tips; July – Oct.
Carduus nutansMusk (nodding) thistleNot 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 edges2-3” purple to pinkish flowerheads, often nodding, solitary; bracts large, triangular, showy; June – Oct.
Cirsium arvenseCanada thistleSlightly 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 underside¾”, 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 palustreEuropean marsh thistleVery 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 veins¾” 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 vulgareBull thistleHairy, 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 underside1 ½ - 2” reddish-purple flowerhead, solitary or a few clustered at ends of branching stems; bracts narrow, spreading, tipped with short spines; June – Oct.
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