Last year I created a little bog garden in a naturally wet spot in my yard. I just weeded heavily, and I'm happy to report that almost everything I planted has returned! I remember last year I put down a light layer of pine bark mulch, only to wave goodbye after a heavy rain (part of our yard gets very soggy after a moderate shower, and can turn into a mini river with a real soaker.) So...is there anything heavier that would work as a mulch? We've got some unused slate tiles that I was thinking about breaking up, and putting there. What do you think? Thanks!