Annoying thistle in my garden?

I have annoying thistle which I swear grows before my eyes growing in my garden which is planted with perennials and alreayd mulched. We put down Preen before and after the mulch and continue to put it down but the thistle will not go away. I spray Roundup if I can but it's also growing up in the middle of my plants. I can't use Roundup on that without killing my plants. Any suggestions?