You can get rid of Canada Thistle, but it usually takes time and attention. You also have to prevent new seeds from landing on your property to start new plants, and never let the plants you have go to seed.

Once established, you have to kill the roots which herbicides can speed up, but otherwise you have to starve them out. Keep all green sprouts chopped out--even the first couple leaves poking through the ground. If it can't get any greenery up to manufacture food, the roots can't be fed and will gradually die.

Glyphosate (Roundup) will work well, but it has to translocate throughout the root system for complete control. A common practice is to spray the new growth in the spring somewhere between 6" high and bud stage to weaken the plants, then keep all regrowth sprayed or removed until about a month before expected first frost and let the plants form rosettes of leaves. These are busily manufacturing food to sustain the roots through the winter and translocating it down to the roots. Just before first frost, spray again with glyphosate. This time, the glyphosate will be translocated well into the roots with the food the plant is producing and do considerable damage there, plus the weakened roots will end up suffering some winter kill. While it usually takes a couple years, you will see noticeable improvement each year.

In addition, the glyphosate will be more effective if you add a small amount of surfactant and some nitrogen to your sprayer along with the glyphosate. Many of the Roundup labels do not call for these additions and I've been told that the Roundup already contains the extra ingredients in the bottle, but I've always found better activity when I add my own. Surfactants can be found with the herbicides in many garden supply stores, otherwise, a small amount of dish or laundry soap will work. For nitrogen, I use ammonium sulphate crystals, which also helps adjust the high pH of my water downwards so it doesn't degrade the glyphosate as quickly, but you could also use a water soluble (high nitrogen-low everything else) plant food mixed as if you were watering your houseplants.

Sometimes a single fall treatment will eliminate the thistle entirely, but usually you have to repeat this process of starving them out all spring and summer, then hammering them in the fall for a couple years before they are eliminated. But--once you get your first frost below 30-28 degrees, spraying is not likely to have much effect. The plants remain green and look good, but they just won't take the glyphosate down to the roots and you will be wasting your time and spray.

There are a few commercial pesticides that will work a little better (not much, just fewer applications), but they also remain active in the soil for a period and will prevent anything but grass from growing during this time. These are generally restricted to commercial use and licensed applicators due to the environmental dangers they present if not...

Wow, that sounds like quite the production! Unfortunately I can't prevent the thistle from seeding, there is a field of it near the garden beds and I have no control over it. I guess a pre-emergent would be a good idea, too.

So if I understood you correctly, I can remove all the greenery until fall when I can let the plant form a rosette or even flower stalk, then spray with Roundup (to which I can add a surfacant and high-nitrogen fertilizer). This will weaken the roots (eventually) and kill the plant.

But, I have read on other sites that when you remove the leaves and even some of the roots it only makes the plant more aggressive. Isn't this true?

Thanks for any advice regarding thistle, it's a real nuisance.

Seedlings are really pretty easy to control because they don't have a root system yet. It is when they have gotten established that the difficulty begins. It may not result in anything, but you could speak to the owner of the nearby patch about controlling the seed production, and most Weed and Pest districts have quarantines in place to prevent the removal of crops from property where noxious weeds are allowed to go to seed. Some even will require weed control to prevent seed production.

Keeping the weeds cut back doesn't make them come back more aggressive, however if you are dealing with a weed that spreads by roots, chopping and mixing the roots will result in tons of new plants where there were only a few before. The key is to keep the roots from building up energy reserves, and then in the fall when they try, spike the flow of energy with glyphosate to spread it throughout the roots. and get better kill. In addition, people will often only treat once, then give up. Tilling/chopping/mowing thistle once results in new growth that seems like is is stronger than ever, but it really is less strong that what was knocked down, though it is quite vigorous trying to make up for the lost vegetative matter. It still needs repeated removal, though

There was a really "nice" CT growing in the lot of the empty home next to me this past spring---It gave up! I soaked it with "Weed B Gone Max" then while it was still wet, went over it 4 times with the Scotts weed and feed. CT all gone, hasn't shown any signs of growth. Over kill, yes, but its gone and the grass will come back, eventually.