Think of it as 90-degree gardening: not the temperature but the angle.
As more people move to cities, there is less horizontal space for gardening, but there are thousands of acres of prime vertical space—on schools, homes, restaurants, even apartment buildings—that are ripe for the planting.
As with green roofs, building an extensive vertical garden along a wall isn't for amateurs—although if you're handy, a basic model can be constructed with chicken wire, two-by-fours and black plastic.
But unless you have a solid background in landscaping and/ or construction, for a major undertaking it's advisable to leave the job to the professionals or to buy a kit.
The ready-to-install systems include soil-filled plastic cells—think of them as plant pockets. These are affixed to walls or metal frames, many of which have a waterproof PVC backing and a felt mat to hold in the moisture but prevent it from damaging the building's exterior.
Flowers, herbs and veggies that naturally trail or cling are best bets: impatiens, thyme and grape tomatoes. (And obviously, eggplant, cabbage and watermelon would be worst bets, unless you're interested in your vegetables doubling as weapons.)
French botanist Patrick Blanc builds vertical gardens without soil—hydroponically—so they can be installed in well-lit areas indoors. If you're committed to growing your own on the wall, check out his book, The Vertical Garden: From Nature to the City, or check out Blanc's Web site.