If you are dealing with Canadian Thistles, they will be back, don.t try to dig them out because you cannot get all the root and then they will sprout and have more than ever. Make sure at least you cut the buds off before they flower, 24d and round up will knock them down but it will be back. These bith will kill them but you will have to spray them a minium on 2 times a year and also if you have deer in these pens the dried up leaves will still have prickers and will still get in their jaws. You can find more on getting rid of them by googleing Canadian Thistle. Here is what is working for me. Cut them off at ground level and pour a !/4 cup of rock salt on the root. The salt works thru the root system and will kill them. The salt will not harm the deer either. This is time consuming, but is cheaper than chemicals, the plants are out of the pens, no buds to go to seed. I burn them after they lay out after a week or so. When you google them and read the studies and reports on erradicating them you will find it take time and alot of chemical expense to rid them. Plowing them under don't work either.