Pamela, placing rocks at the bottom of a pot inreases the amount of water in the potting mix so that you end up with less, but wetter, potting mix. A pot should have the same texture of mix throughout the whole pot for best drainage. This happens because water does not cross easily from a fine texture to a coarse textured material, instead it only drains when the potting mix has become so wet it cannot hold any more water.
