Resource Management

Bull thistle - Cirsium vulgare

Description: Bull thistle is a biennial with prickly, winged stems. The leaves are coarse and spiny above with wooly white hairs below. A purple "brush" of flowers emerges from a spiny green ovoid base.

European marsh thistle is also invasive in Wisconsin and looks similar to bull thistle. Also note that several native thistles exist in prairies that may be confused with exotic species. Verify identification before initiating control work. The native species tend to have leaves that are white and wooly beneath or the flower heads are gummy/sticky. The non-native thistles tend to have very prickly stems and leaves.

Distribution and Habitat: Bull thistle was introduced from southern Europe and western Asia. It occurs throughout Wisconsin in disturbed areas such as pastures, roadsides, waste areas, and ditch banks and is a problem in prairies, old fields, and hay fields.

Life History: Bull thistle is a biennial and grows from a flat rosette of leaves the first year. Seedlings emerge from early spring to late fall, and the length of time to flowering can vary from 4 months to 22 months. A single taproot is formed. Reproduction is by seed.

Stem elongation takes place in early May. Blooming starts with the terminal head in early June and continues until mid-August with the lower branches. Seeds mature and may disperse within 7 to 10 days of flowering. The bulk of seed is produced on the upper branches, and germination may run as high as 95%. Wind dispersal allows for movement over long distances. Seeds can remain viable in the soil for over 10 years. Each plant is capable of producing up to 10,000 seeds, which are dispersed by wind.

Ecological Threat: Bull thistle has been known to invade native and restored grasslands despite the presence of dense, native prairie vegetation. Glade communities are also likely areas for thistle establishment. These species are very aggressive in disturbed areas, and can pose a major problem in buffer and restoration areas. Control of these thistles is important before beginning a prairie restoration.

Controlling Exotic Thistles: Control efforts will be long-term since seeds remain viable in the soil for over 10 years.

Mechanical Control: Eliminating seed production is the most effective mechanical control technique. Thistles mowed in bud or early bloom stage will produce new branches from buds in the axils of the basal leaves. However, close mowing or cutting twice per season will usually prevent seed production. This can be done at any time during the growing season, although cutting is easier when the thistles are smaller. Mowing once flowering has begun may result in the spread of viable seeds with the mower. For effective selective control, plants should be cut with a sharp shovel at 1" to 2" below the soil surface before flowering. Competition with native vegetation decreases seedling establishment.

Biological Control: Two exotic weevils, the flower head weevil (Rhinocyllus conicus) and the rosette weevil (Trichosirocalus horridus) have been introduced in several states, and appear to be effective biological control agents that limit populations of musk thistle. However, these insects are not in use in Wisconsin due to the risks presented to rare native thistles.
Chemical Control: Herbicides are not recommended for use on exotic thistles in high quality natural areas. Chemical control is most effective when plants are in the rosette stage and least effective when thistles are flowering. Dicamba, 2,4-D, and clopyralid herbicides are all effective.

References:

WDNR. Wisconsin Department of Natural Resources web page. Available at: http://dnr.wi.gov/invasives/fact/thistles_bull.htm
Lym, Rodney G. 1996. The thistles of North Dakota. NDSU Extension Service.





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