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.
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.
We have canadian thistle too. I use roundup, but not save for animals.
Try this homemade recipe I found.
Like roundup it must be sprayed on a sunny day to allow the weed killer to work its way into the roots. Good Luck!
Natural Weed Killer Formula
1 gallon White Vinegar
1 cup Table Salt
1 tablespoon Dish Detergent
Remove about 2 cups of Vinegar from the container, pour in salt and dish liquid, shake well to mix, pour the 2 cups of vinegar back into the container and shake lightly. Fill a spray bottle and use as needed. Spray in full sun. CAUTION: It kills all the plants or weeds it touches, but is harmless to animals
There is supposed to be an insect you can buy that will make the thistle plants infertile, and I haven't tried that. I don't know anyone who has eradicated thistle without chemicals, but I use very very little. First I make sure that none of them go to seed. If you just chop or mow them while they have any purple showing in the buds, they will usually go ahead and mature and make seed. I try to chop them before they get that far, but those I miss, I pick off the purple flowers (get some good rose or welding gloves), bag those up, then chop the plant within a few inches of the ground and spot spray my chemical mix right on the open wound. I use WD40 with dishsoap and water. You have to be vigilant. Let one flower head go to seed and you've got a whole new crop next year! 20 years ago, we had huge patches of thistles threatening to take over our farm, now they are few and far between. I can top and spray all of them on our 86 acres in just a few hours.
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