We hear you. Did you know Canadian thistles can grow to over 6 feet tall?!
But to answer your question — the problem lies in the root structure. 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. If you find one, you find a whole family of them. They can spread into your lawn from a neighbour’s unkept flower beds 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.
