I use smartpots outdoors and keep them elevated about 4in. Off the ground. Rocks would not do any good at all. I harvested recently and today I pulled the root systems out of the Smartpot and out of some plastic pots with rocks in the bottom I also used. The root structures were very different. The smartpots were a mass of fine, dense roots, with lots of "feeders" whereas the plastic pots roots were fatter with an irregular structure, they also looked like a little slime was trying to form on some of the rocks. Ill never use any plastic or clay pot again.