Non-native thistles
Carduus nutans
Cirsium arvense
Cirsium vulgare
Non-native “thistles”include spotted knapweed (Centaurea stoebe ssp. micranthus), musk thistle (Carduus nutans), Canada thistle (Cirsium arvense), bull thistle (Cirsium vulgare), and Scotch thistle (Onopordum acanthium ssp. acanthium), with the first two species being the most widespread in Missouri.
Musk (Carduus nutans) and bull thistle (Cirsium vulgare) are both biennial, herbaceous grassland invaders that form a rosette in their first year and flowering stalks in their second. Both have coarsely toothed, spiny leaves, but bull thistle has a spine on each leaf tip. Like all other non-native thistles, the underside of their leaves are green. Musk thistle flowering stalks typically end in a single, nodding flower, with numerous spine-tipped bracts. Bull thistle flowering stalks can end in either small clumps or single flower heads. Both species can flower May-October. Methods of control for both species in high quality habitat include hand-cutting or mowing in the second year, prior to flowering. Chemical control is most effective in the rosette stage for both species. This U.S. Forest Service fact sheet lists chemical control options for musk thistle; these and all herbicides should only be applied according to label directions. Additional treatment options can also be found on this University of Wisconsin fact sheet.
Note: native thistles bloom in late summer and their leaves are white underneath (except for the native swamp thistle, the leaves of which are green underneath); they are beneficial to many insects and birds.
Identification and fact sheets
- Learn about Non-Native thistles at Invasive.org
- Read more from the Missouri Department of Conservation about identification, threats, and control practices of Non-Native thistles
- Invasive plant resources from Missouri Prairie Journal: “Missouri’s Thistles” article provides detailed drawings and a key to help you identify thistles
Representative photos of Non-Native thistles:
Photos by Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org
Non-Native thistles impact, abundance, and trend in Missouri
