I've always heard that when you're potting plants, you should put a layer of gravel, rocks, crushed pottery, etc. at the bottom of a container garden to help drainage. And at face value, it would seem to make sense. But this is  an old-time gardening myth. 

When dealing with potting mix, a classic analogy is to think of it like a sponge. If you saturate a rectangular sponge and hold it so it's longer than it is tall, the sponge holds a certain amount of water. If you rotate it so it's taller than it is long, you'll see some more water drip out. Potting mix is the same way: The more you fill the pot, the better it drains. By adding a layer of gravel or such at the bottom, you're actually reducing the drainage a bit. 

That said, one scenario where a layer of gravel at the bottom of a container garden can help if you do the pot within a pot system, where you have a grower's pot sitting inside a more decorative pot that does not have drainage. The gravel will keep the smaller pot up and out of standing water that may accumulate in the bottom of the larger pot. 

Written by Justin Hancock