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Ontario | As others have mentioned best way to kill a thistle is to spray it at flowering. As the plant is flowering there's a shift and it starts moving nutrients to the roots to store so it can come back again next season/life cycle. As it does this it will translocate the herbicide down into the roots killing it completely (as long as you use a systematic herbicide). This is especially true with Canada Thistle as its a rhizome so it has an inconveniently large root network especially if its a larger well established plant.
I'm not sure there's a best time to cut them other than all the time, I've had luck cutting frequently to kill them as eventually they'll run out of root reserves to come back and compete with other plants around them though. | |
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Canadian thistles
