I just interviewed a farmer in North Dakota who says the best way
to greatly reduce Canada thistle is to let it grow to the purple
bud stage, then cut everything off with a Noble plow set with three
7 inch sweeps. Continue to cut it off every 21 days until freeze.
This will produce about 95 percent eradication.
This comes from Terry Jacobson, who has been farming organically
for 25 years.