snapshot , freehand and preen are all flower bed pre emergents to prevent weeds from germinating. These products will not kill existing weeds. Usually apply them early spring and sometimes in the fall. Snapsot is the better one for thistles. The product is not cheap though. Preen is ok just not as good as the commercial products such as snapshot and freehand. Each spring we apply snapshot to our customers yards that back up to the muirfield golf course. I take no chances with these yards and do all i can to prevent the weeds. It has proven to be about the best out there.