When Patrick Blanc was a boy, he suspended plants from his bedroom wall and ran their roots into a fish tank. The greenery received nourishment from the diluted—ahem—fertilizer and purified the water in return. Forty-five years on, the French botanist's gardens have grown massive in scale. One inside a Portuguese shopping mall is larger than four tennis courts, and there's one in
Blanc uses a kind of techno-trellis as the underlying structure: A plastic-coated aluminum frame is fastened to the wall and covered with synthetic felt into which plant roots can burrow. A custom irrigation system keeps the felt moist with a fertilizer solution modeled after the rainwater that trickles through forest canopies.
But plants for this vertical landscape must be chosen with care. Because the walls are so high, conditions vary widely. The shade at ground level is perfect for rare Asian nettles; on the brighter upper stories, plants that usually cling to windblown cliff faces brave the blustery British breezes.