Rooftop Landscaping

Weight

• Rooftop landscaping requires structural considerations, especially regarding weight capacity. Before laying sod or soil, determine your building's load per square foot limit. The weight of soil, plants and moisture add up quickly. For example, the 1-acre or 40,000-square-foot Brooklyn Grange Farm (brooklyngrangefarm.com) in Queens, New York, contains 1.2 million lbs. of soil, or about 30 lbs. per square foot.

Green Roof Systems

• Rooftop landscapes require layers of protection between the rooftop and the garden base. These layers are called "green roof systems" and can be extensive or intensive. Extensive green roof systems weigh less, require less maintenance and contain a shallower growing area. In contrast, intensive roofs weight more and require more irrigation and pruning but are more like a traditional, earth-based garden. Systems may consist of layers of root barriers, waterproof membranes, felt, drainage materials and containers to collect excess rainwater. Your roof's structural capacity, weight load and condition determine the green roof system that's most appropriate.

Growing Media

• Once your green roof system is installed, choose your growing media. The best types are lightweight, drain easily and provide moisture and nutrients to plants. Special blends of soil for rooftop landscapes are commercially available. Many contain blends of lightweight stones, recycled materials, and organic compost. Apply as deep a volume of media as possible within the constraints of your roof's structural capacity. Rooftops experience more wind than the ground, so use a wind blanket around plants until they're well-established.

Selecting Plants

• Whenever possible, choose native plants. Native species are inherently adapted to the conditions in your region, such as rainfall, temperature range, pests and diseases. When selecting species, choose those that tolerate heat, drought, cold and high winds, as all of these conditions are magnified on rooftops. Many vegetable plants grow well on rooftops, such as beans, beets, carrots, fennel, greens, herbs, tomatoes and radishes. If you live in a cooler climate, overwinter the landscape with cover crops such as buckwheat, clover, rye or vetch. Low-growing ornamental species that grow well in rooftop landscapes include blue fescue grass, cacti, ice plants, pine leaf penstemon and sage.