Rooftop gardens are gaining popularity in the United States, although they have been successfully utilized in Europe for decades. The type of plants chosen for the rooftop garden will depend in large part on the design of the building. For instance, a drought-resistant plant will do better near the top of a sloped roof because it has better drainage. Another consideration is the additional weight of the plants and having enough soil, or substrate, to grow them.
Roofs having shallow substrate, 1-1/2 to 4 inches, can be planted with sedums, sempervivum and delosperma. All three are hardy plants that will require little maintenance. The shallow substrate will sustain the non-woody roots and keep the self-sustaining plants looking good for most of the year. Drought tolerant and able to endure the sunny conditions of the roof, the plants are also able to withstand hot and cold temperatures as well as most stormy weather conditions.
For roofs that can handle a substrate over 4 inches, alyssum, dianthus and thymus can be grown to add more color to the rooftop garden, in addition to the previously mentioned sedums, sempervivum and delosperma. All are low-maintenance and hardy plants that will tolerate the full sun of the rooftop. Substrates between 4 and 6 inches can also handle a variety of grasses and sedges.
Roof meadows can be created on flat roofs that can accommodate a substrate of up to 7 inches. Grasses, sedums and wildflowers that can withstand the heat, wind, sun and weather conditions of the roof can all be planted in the roof garden with the soil and moisture to sustain them. A roof meadow can have a weight load of 15 lbs. per square foot, so a structural engineer should be consulted before this type of garden is created.
Native plants are being studied to be used as ideal plants for rooftop gardens. They already have adapted to survive local climates and are able to withstand attacks from local insect pests and diseases. Proponents also point out that native plants will replace the local wildlife habitat that the buildings have removed from the landscape. Local university extension offices will be able to offer guidance on using native plants on a rooftop garden. Root structure, moisture requirements and invasive qualities of the individual native plants, as well as adaptation to constant exposure to the elements, have to be taken into consideration when considering plants for green roof gardens.