Skip adding gravel to the bottom of a container. Soil scientists have proven that water doesn’t move easily between the fine particles in potting mix and coarser material, like stones. It actually causes water to drain more slowly and the potting mix can stay saturated — the opposite of the intended effect. Take a look at the illustrations at left to get a better idea of how this happens.

However, it is all right to cover the hole with something that lets water drain but prevents soil from washing out, although you don’t need to. A square cut from a plastic mesh bag is very effective, or shard of broken terra-cotta, piece of landscape fabric or coffee filter will all do the trick.