Thistles are probably the most difficult weeds to control in Manitoba. The Canada thistle can grow to over 6 feet tall! Just imagine the root structure that supports something like that. That’s where your problem lies. Thistles reproduce in two ways; through seeds, and through rhizomes. Rhizomes are shallow roots that run close to the surface and eventually sprout new plants. Thus, where you find one thistle, you will likely find a whole family of them. They will spread into your lawn from a neighbour’s unkept flowerbeds as well. Unless the entire family of weeds is wiped out, they will continue to be a constant problem. Treating the few small thistles on your side of the fence will not solve your problem since you will only be poisoning a small segment of the cluster. You may weaken the root mass, but not enough to kill it. It will continue to thrive and eventually new weeds will spread back into your lawn.