We have two kind: the tall kind and the flat ones called Bull Thistles.
They turn my bare feet into porcupines so it's time they go.
What's the best way?
My neighbor and I have nearly eliminated thistles by cutting them off at the root each year. This is tedious but effective. Preventing them from going to seed is the key.
At my place, bull thistles don't seem to re-sprout much -- if cut off below the crown, but Canadian thistles are very rhizomous and re-sprout readily. I don't mind annual thistle weeds, because the seedlings are small and delicate. It's the perennial rhizomes that give me fits.
You can pull a thistle every ten seconds or so, and get a lot done very quickly. We have 6 acres of sheep field that at one point was badly overgrazed. As it recovered the thistles went nuts in the patches of bare soil. We took the puller with us when we went out to do other jobs and over 6 months or so we got it under control.
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We have two of a different model of thistle puller. This particular model is an older style I don't see in the store anymore. I can't say enough good things about the advantages of weed pullers that work from a standing position. They work well and are much kinder on your back. Ours occasionally get loaned to volunteers doing work for my sister's church. They are the only tools anyone's asked to borrow and we have no shortage of basic tools.
Cut them and eat them. Wear gloves and then strip the outer layer of the stalk. They are high in vitamins and taste like a sweet celery. After they realize you actually want them to eat, they will stop regrowing.