Rooftop gardens are one means of incorporating agriculture into the built environment of cities, where they can help alleviate food insecurity for urban residents, and reduce the economic and environmental costs of transporting food from sometimes thousands of miles away.
Food items commonly grown in rooftop gardens include herbs, gourmet greens, and vine crops like tomatoes and cucumbers.
Growing rooftop crops
Herbs and vegetables are often grown on green roofs in containers, although plants may be grown directly in specialized growing medium or soil. In the latter case, though, the load-bearing capacity of the roof needs to support a deep (greater than 8 inches) soil layer and the additional weight of the water necessary for the plants.
Special considerations for rooftop gardens include sun exposure, wind, temperature, and access. Because a rooftop garden is seldom shaded, excess sun exposure can be a problem for some food plants. The plants may also require extra watering to fight against the damaging effects of constant sun and higher temperatures. Wind speeds double for every 10 stories of building height, so it may also be necessary to stabilize plants and/or containers.
It’s also important to consider how people will safely access the garden to fertilize, water, and harvest, and a structural engineer may need to be consulted to ensure the garden doesn’t exceed the load-bearing capacity of the roof. A garden can exert 80 to 150 pounds per square foot, and any irrigation water stored on the roof will weigh about 8 pounds per gallon. But the soil or growing medium will need to be kept moist, because food plants aren’t as drought-tolerant as many non-food plants. Wet soil is heavy, however, so a lightweight growing medium is most practical for rooftop gardens.
For more information on rooftop agriculture, visit cityfarmer.org.