Warning: this product renders the small space excuse useless. Even if your outdoor space is no bigger than a juliet balcony a wall garden is probably workable.
These Italian-designed units attach to a wall and protrude only 2" (the $59 15-pot size) to 3.5" (the $119 30-). Fill with soil and stack more than one to make a green wall -- or make a low, textured hedge effect with plants growing out of the top as well as the pots.
Available through Lee Valley.