This invasive weed is common in North America and very difficult to get rid of. This is a guide about getting rid of Canadian thistle.
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Hardiness Zone: 6a
Samantha from Pittsburgh, PA
As you already know, Canada thistle is a really tough weed to eradicate. Unfortunately it competes well in deep, well-aerated soils-the same soils most flowers and vegetables prefer. The biggest challenge is the roots. They can colonize areas 3 to 6 feet in diameter. One of the best methods to get rid of Canada thistle is repeated pulling and mowing just when the flower buds are about to open. Pulling and mowing will eventually work to weaken the roots. When done over the course of three years, this method is said to kill up to 90% of the weeds. I actually like your black plastic idea. Dig up the soil well first, cover it with a good layer of compost and throw the plastic on things for the rest of this season. You'll substantially raise the fertility of your soil with the compost, which Canada thistle doesn't like. Next year you'll probably be home free. Maybe you can experiment with container gardening and try growing your veggies in containers for this one season. Burning in the late spring (May/June) can also work well, but must should also be done over the course of several years to be effective. Biological controls and herbicides are risky if your beds are earmarked for edibles.
To get rid of Canadian thistle, cut them at ground level and pour salt on them. They do come back. I tried it myself because I had lots of salt
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Here are questions related to Getting Rid of Canadian Thistle.
Does anyone know how to get rid of thistle once and for all?
By Stephanie (Guest Post)08/25/2008
If you want a method that does not include the use of harsh chemicals, there is another way. No chemical really ever gets rid of Canadian thistle, anyways. Thistle thrives in less nutrient-rich subsoil, which will be increased by the use of chemicals which keep nutrients out of the soil. Thistle actually doesn't like topsoil! If there are enough other plants and compost contributing to the health of the soil you shouldn't really have a problem with thistle.
It is a pioneer plant which decomposes to create better soil for other plants, and using this information will help you get rid of it. So finally to the method. Cut the thistle down to 6 inches above the soil, BEFORE IT GOES TO SEED, and use the clippings as mulch, then add manure, or compost if you don't have access to manure. Finally add seaweed if you have access to it, because it speeds the process. The thistle will thin out and be gone by the 3rd year. I personally don't consider it to be enough of a problem to use round-up and as long as you keep you soil rich it won't come back.
By Polly Munson06/27/2008
We have 160 acres and had several acres of Canadian thistle to get rid of. I couldn't waste time spraying anything that did not work well and kill the roots. I have sprayed with Curtail by Dow for about three years. It is an excellent product and it kills the tops and roots. I have several areas that were thick with thistle that are totally free of it now. Now I have to search for plants to spray and they may be from seeds that have laid dormant or just blew in. It does not kill grass like Roundup.
By heather0402 (Guest Post)05/28/2005
We found using Weed Be Gone works. You spray it at the base of the thistle bush and wait how ever long it say's to on the bottle and after the time is up you pull the hole thing out root's and all. You will have to keep doing this until all of them are gone but it sure does help we still have them but not quiet as many any more. I sure hope this will help.
By Leona (Guest Post)05/27/2005
Only one thing, ROUNDUP. It does not affect the soil. It will kill anything it touches, so be careful, but it will kill root and all, after 3-4 days, you can till the thistle under or easily pull it out root and all.
Recently we stopped raising horses on our farm. I talked my husband into letting one field go wild to attract wildlife and give them a safe haven. We added a brush pile, have dead trees, a stony area and a fence row. We now have fox, deer, rabbits, chipmunks, owl, squirrel and many species of butterflies and birds. However, it is being overrun with Canadian thistle. It seems every year it is so much worse. I hate to use pesticide as I don't want it around. Any good suggestions on how to get rid of most of the thistle. It is ruining our paradise.
NellieMary from West Virginia
By John Gray (Guest Post)06/28/2006
Cut the thistle off at the ground level and immediately pour salt on the wet stub. If any are blooming cut the blooms off into a paper bag and burn the bag or you will have a million more next year.
By (Guest Post)06/28/2006
Only sure fire way of getting rid of them is grubbing them. I agree you can top them while the thistles haven't opened as the seeds are immature and won't grow. The insect control will not eradicate, just keep the population down. The insects can't survive if they have no thistles to live on. Roundup will kill them, but if you want no chemicals, grubbing will cure the problem. The seeds can survive for years and in drought years where there is cracking or bare ground they can come back. It's an ongoing battle. There are also many types of thistle the worst being the Californian as develops a huge underground root system this needs spraying to control. The tap root type ones are easily grubbed out. I am not sure about the Canadian, we have scotch and nodding thistles a tap root type here in nz.