Mechanical and Cultural Control
Mowing alone is not effective unless conducted at one-month intervals over several growing seasons. Combining mowing with cultural and chemical control can be effective. Mowing at hay cutting stimulates new Canada thistle shoots to develop from its root system. In irrigated grass hay meadows, fall herbicide treatments that follow mowing can be an effective management system because more Canada thistle foliage is present after cutting to intercept herbicide. Additionally, root nutrient stores decrease after mowing because the plant draws on them to develop new shoots.
Chemical Control
Chemical control is most effective when combined with cultural or mechanical control. There have been reports of Canada thistle resistance to synthetic auxins in Hugary and Sweden, but no reports of resistance in the United States. For more information of resistance please see
http://www.weedscience.org/Summary/USpeciesCountry.asp?lstWeedID=55&FmCommonName=Go.
For herbicide recommendations for specific grain rotations and weeds in Montana, please see the MSU
Herbicide Chooser Tool.
Examples of herbicides that can be used to manage Canada thistle
Consult herbicide labels for additional rate, application, and safety information. Additional herbicide information can be found at
http://www.greenbook.net.
Herbicide Active Ingredient trade name Mode of Action Product per Acre Application Time or Growth Stage AlfalfaGlyphosateInhibition of EPSP synthase*Roundup Original MAX1.5 - 2 quartsApply prior to crop emergence when Canada thistle is at or beyond bud stage. In fall, apply before killing frost.
Wheat, Barley, and OatsFluroxypyr; clopyralidAction like indole acetic acid (synthetic auxins)*WideMatch1 - 1.3 pintsApply post emergence to actively growing wheat, barley or oats from the 3 leaf stage through flag leaf emergence when thistle is in the rosette to bud stage.
Grass Grown for SeedClopyralid; 2,4-DAction like indole acetic acid (synthetic auxins)*Curtail2 - 4 pintsApply only to established grasses before the boot stage of growth. Do not apply to bentgrass unless injury can be tolerated. For control of late-emerging Canada thistle, a preharvest treatment may be made after grass seed is fully developed. Treatment of Canada thistle in the bud stage and later may result in less consistent control.
FallowClopyralid; 2,4-DAction like indole acetic acid (synthetic auxins)*Curtail2 - 4 pintsApply to young emerged weeds in the rosette to pre-bud stage under conditions that promote active growth. See label for rotation restrictions.
Rangeland and Permanent Grass PasturePicloramAction like indole acetic acid (synthetic auxins)*Tordon 22K1 - 2 pintsPicloram is a restricted use product. Apply when most basal leaves have emerged, but before bud stage, or apply to regrowth in the fall. Apply rates less than 1 1/2 pt/acre only under favorable conditions and in combination with 1 lb ae/acre of 2,4-D. Retreatment may be required.
