take a look at the square foot gardening forum. not too much you CANT plant in that space.

Be sure your container has good drainage. If it is in contact with the ground so that moisture wicks, it is similar to a squarefoot garden or raised bed, and you can use heavier mixes. But if it is not in contact with the ground, you will need a lighter mix that doesn't compact and hold perched water.

Posted by Ohiofem 5b/6a OH (My Page) on Sun, Feb 26, 12 at 16:36
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Moisture wicks? But my planter bottom is on the ground of frontyard or backyard, very close.

I meant to say that if your planter has several drainage holes in contact with bare ground, as opposed to a deck or concrete surface, the ground can wick away excess moisture from something like heavy rain. In that case you could grow a lot of different vegetables or flowers.