I went on a LOVELY garden tour this past weekend and one of the homes that we visited, the host had many, many container gardens. I noticed that she had the small, white rocks (the ones that look almost bleached). I commented on that to the gardener and she told me that it is a wonderful way to keep "vermin" from digging in the containers. She did have some lovely gardens with sedum and chicks that had pea gravel as well. It made such a lovely contrast with the plants also.