I've been using rocks in my planters this year too because the squirrels and, especially, chipmunks are out of control with their digging. I'm using the "river rock" and "egg rock" from Lowe's. I tried some large, flat pieces of slate or shale that I already had on hand but I didn't have good luck with those because the vile, obsessive diggers just flipped them over and out of the pots, or dug pockets under them. They are really bad this year!
