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. Edible plants chosen for a rooftop garden can be patio or dwarf varieties that are designed for containers and small spaces. In general, heat-loving plants such as peppers, tomatoes, and basil do well on rooftop gardens. Crops like peas and spinach that require cooler conditions may not do as well. Look for plant varieties that are well-adapted to heat and slow to bolt when the sun is intense.
Special considerations for rooftop gardens include sun exposure, wind, temperature, and access. Rooftop gardens are seldom shaded. This 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. Overhead irrigation on rooftop gardens often blows away, so drip irrigation is a better choice when irrigation is necessary. Soils on rooftops will dry out faster after rainfall than soils on the ground. The soil or growing medium will need to be kept moist because food plants aren’t as drought-tolerant as many non-food plants. Mulching soil can help keep it moist in between rain or irrigation. Also, 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. Consult a structural engineer 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. Wet soil is heavy, so a lightweight growing medium is most practical for rooftop gardens. Never use regular potting soil or ground soil for a rooftop garden. Specially-made growing media that are light in weight are necessary for the unique load-bearing limitations of most roofs.