We had a bad infestation of thistle in a long bed of creeping rug juniper. We were so tired of hand-pulling! Our son mowed over it in frustration one day and voila! The bed looks lovely and the rug junipers were unfazed. I wouldn't suggest this solution for anything less hardy than those junipers, however!

I professionally managed an urban prairie restoration for a couple of years that was initially overrun with canada thistle. There are a few herbicides that specifically target things in the asteraceae and fabaceae familes that did a great job on thistles. However, for those without the license to buy these chemicals, RoundUp (especially the concentrated powder forms) work wonders. Mix it up a little stronger than recommended and dribble it into the actively growing tips of the plants. It will keep them from flowering and if they don't die, a second application often does the trick. Like others have said, it will take a few years for every last root to die, but the worst thing you can do is to try and pull them out. They have a rhizome that sends up thistle plants every few inces, pulling the plants can break the rhizome and essentially creates more independent plants. A friend of mine recommends applying herbicide with 'The Hand of Death'. He takes a chemically resistant glove and puts a worn-out winter cotton glove over the top. He dips his gloved hand into the herbicide and grabs the plants he wants to kill. He says it works like a charm. And NEVER use wildlife as an excuse to grow invasive weeds. Native animals prefer native plants.

Thistle much prefer to grow in compacted soils, so after pulling all you can work on making that soil less compacted and the thistle will stop growing there.

We got Canadian thistle under control glyphosate (studies that do not have friends of Monsanto involved or other conflicts of interest show it as 'carcinogenic' or 'probably carcinogenic' - it just wasn't worth the risk to my loved ones and neighbours). We moved into our house 5 years ago - 100' x 50', so the thistle seemed to take over our little yard. It was a nightmare. They grew in hardpan here, so pulling up WITH their roots is literally impossible without heavy equipment (the roots supposedly can grow up to 20' down). Here's what we tried: 1) On advice from experts we tried cutting them back when they were still young. They kept growing back, and more - probably from the roots getting stronger. 2) On advice from more experts we tried cutting them down just before flowering - the theory being that all their energy goes into flowering and they will eventually die this way. They kept growing back. Options 1 and 2 were very time-consuming and only a band-aid to the problem, so I tried number 3: 3) I did extensive research and found that the USDA did a study in 2002 and found that 20% vinegar (acetic acid) killed 80-100% of Canada thistle. So we sprayed them - young/small and old/large. I even tracked some that I sprayed to see if they regrew from the same roots, and those did not. For the really large ones (3-ish ft tall), I cut them back to 6" or less, then sprayed. Needed to hit them a few times because their root systems are probably very strong. I even sprayed a 3' tall one through the fence, in the neighbour's yard, and it worked - that one surprised me! For the first 2 years, we had to spray all summer because there were so many. Now we only need to spray weedlings occasionally - mostly in the hottest part of summer. Most of our neighbours still have a problem with them - they either don't spray with acetic vinegar or only spray after they have a lot of mature thistles. Canada thistle is now my least problematic weed. Still haven't gotten those dandelions under control, and oh, the horsetail weed! I know this is an old thread, but hope this helps anyone who finds it in search of help for this tenacious weed.