At a recent gathering of budding companies, we met the owner of these vertical gardens. Needing only 20″ diameter for the pot itself (your plant needs may differ) they can stack up to 62″ high, and grow everything from basil to carrots to lavender. It uses a hydroponic system to water and nourish the plants. A base mounted on ball bearings allows it to turn readily for even light.
We would imagine that our testers would pretty much decimate the lower level.