I bought a sack of Lava Rock at my local home improvement center. It is very inexpensive, extremely rough (it IS lava, after all), and quite pretty in color, ranging from a deep red through brown to nearly black.

When I plant a container, I cover the dirt with a single layer of lava rock. It keeps the diggers out, I haven’t seen a slug for years and, best of all, it keeps the heavy summer rains from splashing the pot dirt all over my flowers and herbs. The fact that the rocks don’t deteriorate is also a plus. I have washed them off and re-used my lava rocks for a couple of years now, with no obvious detrimental effect. They also allow you to blow off leaves in the fall without disturbing the mulch.