Q. Are you supposed to put gravel or clay shards in the bottom of containers to improve drainage?

A. This is a common piece of gardening advice, but it's true only for pots that don't have holes. If you're placing a container plant inside a larger, hole-less container, putting coarse material in the bottom of the outer container helps keep the plant's roots out of excess water. But if you're planting directly in the larger container, having gravel in the bottom is only a partial help to ensuring your plant's roots don't rot. 

But the key point to remember: gravel in the bottom of a pot with holes does absolutely no good in ensuring good drainage. That's because water naturally flows toward finer material, not away from it, so the large air spaces between the pieces of gravel don't "pull" the water into them. So, at most, the gravel or clay shards simply prevent bits of soil from exiting through the holes.

The best way to ensure good drainage is to use a good-quality potting soil. Never use garden soil, because it's too dense for potting.