Ideal growing conditions the past couple weeks have helped to mature many of the thistles. In many cases the ideal window for chemical control is past for the best control. However, landowners still have the opportunity to plan a management strategy for controlling both perennial and biennial thistles.
Perennial sow thistle and Canada thistle develop extensive root systems which can make them difficult to control. Seeds can spread by wind, making it important to control the plants prior to seed production. Canada thistle seeds may become viable within 10 days after flowering.
Standard programs include Tordon+2,4-D, 2,4-D (for maintenance programs), Milestone, or ForeFront. Stinger or Transline may be used for Canada thistle around trees, but follow label precautions. These herbicides may be applied from mid-June (pre-bud stage) to Sept. or early Oct. (fall regrowth while the leaves are mostly green). It is preferred to control Canada thistle before seed production in early July.
See the “Herbicide Use Restrictions” section in the 2013 Weed Control – Noxious Weeds publication for more information on herbicide use.
Dense grass may reduce control from fall applications, so consider setting-up the site with mowing or grazing in the summer prior to fall applications. Bio-control insects can be fairly effective on Canada thistle in some circumstances, but success rates are low. Stem mining weevils may be most effective.
Biennial thistles (musk, plumeless, bull, and Scotch) are developing rapidly in areas of the state. These species generally emerge as rosettes in the fall and early spring and bolt during the second year of growth.
Control is most consistent when herbicides are applied at the rosette stage. Tordon or Milestone may provide some short-term residual control for plants that germinate after the herbicide application. At the rosette stage, 2,4-D may be very effective. After bolting occurs, consider using aminopyralid (Milestone or ForeFront) or metsulfuron (Escort or Cimarron).
The musk thistle seed weevil (Rhinocyllus conicus) and the rosette weevil (Trichosirocalus horridus) have been released in many areas of South Dakota and can be found in most musk thistle infestations.
- 2013 Weed Control – Noxious Weeds by SDSU Extension