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Canada Thistle
Canada thistle control requires diligence and commitment on the part of the grower. Tillage over an extended period of time can significantly reduce the root reserves of Canada thistle. Tillage should be conducted whenever the thistle produces 4 to 6 inches of top growth and must be continued throughout the growing season. Root segments of 1/2 inch or longer are capable of producing new plants. Tillage should not be used in fields having light, scattered infestations of the weed since root sections can be spread to uninfected parts of the field.
Systemic herbicides move with the sugars in the plant. Sugar flow to the roots is high in the fall. Systemic herbicides should be applied at these times. Ten to 14 days should elapse between herbicide application and any tillage operation. The time delay allows for adequate movement of the herbicide in the plant to take place. A Canada thistle control scheme for cropland should utilize a low rate of herbicide applied in the crop to prevent flower formation and suppress shoot production. The surviving Canada thistle should be allowed to achieve 8 to 12 inches of top growth after crop harvest before treating with a higher rate of herbicide in the fall. 2,4-D, Banvel, Curtail or Roundup Ultra RT (Glyphosate) can be applied in the fall for thistle control any time prior to frost. Banvel rotational and Curtail restrictions should be observed.
Translocation of the herbicide to the root system is no guarantee of 100 percent control since a large number of the root buds or the root system are dormant and will not absorb appreciable quantities of herbicide. Good growing conditions are necessary for full herbicide applications to be effective.
Roundup Ultra and Roundup Ultra RT are cleared for preharvest applications to certain small grains not being saved for seed. This application can give good control of Canada thistle when environmental conditions are suitable for application. See label for details.
Curtail is effective in crop Canada thistle herbicide and is also labeled for use in fallow.
For more information see http://www.ext.nodak.edu/extpubs/plantsci/weeds/w799w.htm
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