Effects on Natural Communities
Canada thistle is an alien exotic species capable of crowding out and replacing native grasses and forbs. It is detrimental to natural communities where it occurs, particularly non-forested communities, and it can change the natural structure and species composition where it becomes well established.
Prairies, barrens, savannas and glades are susceptible, particularly those sites that have been disturbed and are reverting naturally to native species, as well as those undergoing manipulative restoration management.
Canada thistle is a noxious weed under Missouri law. As such, all landowners are required to control the plant if it is growing on their property. Control is considered to be prevention of seed production. County prosecuting attorneys are required to notify offending landowners in a prescribed manner before leveeing penalties.
Thistle-control Practices for Natural Communities of High Quality
Prescribed fire can be effective in controlling this species and is a preferred treatment. Late spring burns, between May and June, are most detrimental to this noxious weed and should be used when possible. Prescribed burns to control this plant should not be conducted early in the spring, as early spring burns can increase sprouting and reproduction of this species. During the first three years of control efforts, burns should be conducted annually.
Maintenance of Native Species
Management practices that maintain and encourage the development of healthy stands of native species will help prevent establishment of Canada thistle or help shade and weaken plants on sites already infested.
Cutting or Pulling Thistles
Repeated and frequent pulling or hand-cutting of individual plants will eventually starve underground stems. Cutting or pulling should be at least three times each season, in June, August, and September. This treatment is feasible for light and moderate infestations, but may be relatively time consuming in heavy infestations.
Spot application of the amine formulation of 2,4-D according to label instructions can control this plant. Individual plants of Canada thistle should be treated with a wick applicator or hand sprayer. The herbicide 2,4-D amine is selective for broadleaf plants. To reduce vapor drift, use an amine formulation of 2,4-D rather than an ester formulation.
Precautions should be taken to avoid contacting non-target plants with the solution. Do not spray so heavily that herbicide drips off the target species. The herbicide should be applied while backing away from the areas to avoid walking through the wet herbicide. By law, herbicides may only be applied as per label instructions.
Thistle-control Practices for Lands Other Than High-Quality Natural Communities
Control procedures recommended above for high quality natural communities are also applicable to buffer and severely disturbed sites. Additional control measures are as follows.
On large sites (old fields, ditch banks, roadsides) with heavy infestations, thistles should be mowed when in full bloom, and as close to the ground as possible. Cut flower-heads should be removed to prevent scattering seeds on site. Repeated mowing may be needed for several years to obtain adequate control.
Use of Roundup
A foliar application of a 1- to 2-percent solution of Roundup (a formulation of glyphosate) applied in spring when plants are 6 to 10 inches (15.2 -25.4 cm) tall is an effective herbicide treatment (Note: some products containing glyphosate or another herbicide may be pre-diluted, so be sure to read product labels to understand herbicide concentration levels). Individual plants should be spot-treated with a wick applicator. Roundup normally kills the entire plant, including the roots, when applied in this manner.
Roundup is a nonselective herbicide and precautions should be taken to avoid contacting non-target plants with the solution. Do not spray so heavily that herbicide drips off the target species. As with 2,4-D amine, Roundup should be applied while backing away from the areas to avoid walking through the wet herbicide. Roundup should not be used in high-quality natural areas during the growing season because of the possibility of harming non-target plants.
Plow and Sow the Area with Cover Crop
On severely disturbed sites with heavy infestations, such as cropland or abandoned cropland, the site could be plowed and sowed to a cover crop (wheat, alfalfa, rye), if practical and desirable. The following May, the cover crop should be plowed under and desired native species should be seeded. Caution is advised with this practice as soil disturbing practices are often ideal for re-invasion by thistle or other exotic species.
Failed or Ineffective Practices
Fire early in the growing season can increase sprouting and reproduction. Prescribed burns in late spring are effective, as discussed above.
Tillage disturbance of soil may provide ideal conditions for re-invasion and for introduction of other exotics.
Grazing is not an effective control measure as the prickles prevent livestock from grazing near Canada thistle.