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Managagement Of Canada Thistle
Canada thistle is difficult to control due to the extensive and creeping root system. A new plant can grow from roots as small as ½ inch. Seeds can remain viable in the ground for up to 20 years. The best management strategy is to integrate chemical and cultural or mechanical control.
Mechanical Control
Research has shown that repeated removal of all shoots will provide control of Canada thistle if repeated as soon as new shoots appear. Although new plants can form from tiny shoot segments, repeated, deep cultivation will prevent carbohydrate storage in the root system and, if repeated frequently enough, stress and kill the plant. Occasional hand-pulling is not successful. Mowing throughout the growing season (usually at one month intervals) followed by a fall application of herbicide such as Tordon, Tordon + 2,4-D, clopyralid + 2,4-D, and Banvel or Veteran 720 has been shown to be a successful control program.
Chemical Control
In general, effective chemical control requires multiple applications. Timing and application rate are crucial for successful control. Again, application rate is critical; MORE DOES NOT NECESSARILY MEAN BETTER CONTROL. Always read the label when applying a herbicide. The label will be able to tell you how much to apply per acre. If you have any confusion as to how much to chemical to apply to your tank, please call our office and we will assist you as soon as possible.
Herbicides containing picloram (e.g. Tordon), clopyralid (e.g. Curtail), and dicamba (e.g. Banvel), dicamba + 2,4-D (Veteran 720) effectively control Canada thistle. 2,4-D should be applied at a rate of 1.5 to 2.0 ae/acre before Canada thistle reaches the bud stage. A lower application rate or an application applied later than the bud stage generally prevents flowering. Dicamba can be applied at any time during the growing season to actively growing plants; however the late vegetative to bud stage or in the fall if Canada thistle has 8 to 12 inches of regrowth is perhaps most effective (Sheley and Petroff 1999). Clopyralid is the chemical found in Curtail (+ 2,4-D) and Stinger. Curtail should be applied at 1.2 to 1.8 lb. ae/acre. The higher rate is recommended for dense infestations or applications under poor growing conditions such as drought. These herbicides can be purchased at an agricultural products supplier (please check the yellow pages for specific stores).
Biological Control
No effective agents for Colorado at this time, however, research in this area persists.
Cultural Control
Some research indicates that crop or perennial grass competition alone can suppress Canada thistle. Preplant rototilling will improve perennial grass establishment. Grasses and alfalfa are good competitors and conditions favorable to these species should be maintained.
Research indicates that soil nutrients likely play a role in weed distributions. A study in Nebraska showed that the addition of ammonium nitrate fertilizer (40 lbs./A) increased Canada thistle density and biomass over a 2-year period. Canada thistle was thought to be able to utilize the nitrogen more efficiently than forage grasses. However, when irrigation was increased along with nitrogen rate, Canada thistle did not increase.
Some observational data indicates that adding calcium to the soil assists in Canada thistle control. According to Charles Walters in Weeds: Control Without Poisons, the best soil nutrient levels were (in pounds per acre) calcium, 2,000; phosphate, 400; potassium, 200; sulfate, 200; nitrate nitrogen, 40; ammonical nitrogen, 40; iron, 40.