Weeds are survivors, aren't they! Some thistle species have either long taproots (like a carrot) or an extensive network of underground stems that enables them to spread. Either way, the plants have plenty of stored energy to push through the mulch. ------- One solution would be to spray them with a weed killer, although if the weeds are close to the bushes, you could damage the bushes if the weed killer gets onto them. If you want to try weed killer, you could use Round Up, unless your soil has a lot of gravel. You can keep the spray from getting on the bushes by using a big piece of cardboard to block the stray spray. ------- Another option is to rake back the mulch, cut down the thistles, lay a thick layer of newspaper or a sheet of cardboard on the ground, and then cover it with the mulch. The newspaper or cardboard forms stronger barrier than the mulch; some thistle might break through. ----- Another method is to dig up the thistles, getting as much root as you can. Repeat for any that come back. Eventually you will force them to use up all their stored food, and they will die.