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Joyce from Cambridge, Id writes

Will Grass Out Max (Clethodim Herbicide) kill Canadian and Bull Thistle? If so, when is the best time to spray?

I have tried many different products, organic and not, and the thistles keep coming back year after year. Even tried digging them out and that just seems to make the thistles multiiply the following year. Recommendations?


Grass Out Max (Clethodim Herbicide) is not labelled for Canadian & Bull Thistle. Tenacity Herbicide is labelled to treat Canadian Thistle, but doe does not list Bull Thistle. Herbicides containing glyphosate like Roundup QuickPro can effectively control bull thistle. Mowing may need to be repeated throughout the season to prevent reflowering

Answer last updated on: 09/10/2018

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