Vertical gardens are growing in popularity. One at a shopping center in Milan is said to be the largest in the world at nearly 13,595 square feet with 44,000 plants. New York is considering installing one along its West Side Highway, and some people see the gardens as necessities instead of luxuries. If installing them along walls and fences, make sure the structure is stable and add built-in irrigation systems. If installing indoors, provide adequate lighting and ventilation.