AcesNeights,
About the only thing that will kill thistle, is Curtail M with clopyralid. Or -
Do a search for "cloropyralid" herbicide. It is the straight stuff available under several different names, even from the same manufacturer.
These are NOT to be used where kids will be playing. They are pretty much for professional use, but I was able to purchase some a few years ago,
Myron
