Here is a case study in killing thistles that we did a few years back with our NatureZap product line. http://g-neighbor.com/2015/07/14/how-to-properly-use-the-naturezap-to-kill-thistles/ Thistles can be extremely hard to get rid of in your garden. The reason this happens is because they have a massive underground root structure that stretches across your garden. If you kill one here another will pop up over there. In this PDF case study we cover some of the issues as well as how to properly use the NatureZap to kill thistles. The NatureZap is a no chemical way that helps not only kill the top of the plant but break down the actual root structure of the thistle. So if your having some trouble with thistles check out the Article. Thanks!