There are two basic types of green roofs. Extensive roofs cover a large area, incorporating small plants with shallow root systems. An intensive roof design may take the shape of an urban garden, in addition to serving as an oasis for owners. Intensive roofs allow for deeper planting and can support larger plants like shrubs and even trees. Extensive roofs tend to cost less, while intensive roofs provide more functional space, but also require greater maintenance and more construction.
Some types of green roofs incorporate a growing media instead of actual soil. Called a soil substitute — also known as a substrate composition — it contains nutrients and components such as shale or clay. A filter fabric keeps the growing media in place.
The University of Oklahoma’s Urban Design Studio notes that specific plant choice is based on the location of the home or building and the needs of the owner. Factors influencing plant choice include the elevation and climate, the amount and intensity of light, rainfall levels, appearance and the cost of the project.
Plants for extensive roofs are generally low-growing, with a spreading habit. Plants with fibrous root systems are less likely to break through membranes meant to retain water and plant materials. The plants need to have wind, temperature and drought tolerance. Low-maintenance options include evergreen and deciduous plants.
Green roof plant material comes in vegetated mat, potted plant, plug, cutting and seed form. The mat form installs with ease and speed. Pots and plugs allow for intricate design creations. Cuttings have the advantage of faster rooting. Seeds provide a lower-cost option for large area planting.
For a green roof without additional irrigation, Michigan State University’s Department of Horticulture finds all nine species of sedum among the best plants to use. A mixture of sedum varieties would additionally prove a viable choice. The Organic Gardening website suggests sedum, noting the contrast of Sedum spurium ‘Voodoo’ and S. spurium var. album for creating patterns or designs on a green roof space.
The Organic Gardening website chose Allium schoenoprasum, or chives, among its top green roof plants. The plants have the appearance of grass, which may hold more appeal for some home or business owners. Michigan State University’s Department of Horticulture also listed A. cernuum among the best.
The North Carolina Cooperative Extension suggests succulents for extensive roof systems, noting the cactus family relatives as capable of surviving on non-irrigated roofs. The extension further notes that such plants may not be native to all areas, but when used as green roof material, the plants pose little danger to native plants and should not become invasive.
Perhaps surprisingly, wildflowers serve well as green roof material. A Michigan State University Department of Horticulture study found that Coreopsis flowers (C. lanceolata) and spiderwort (T. ohiensis) are well-suited. The Organic Gardening website lists Heuchera — commonly known as coral bells — as an excellent choice. The plants adapt well to partial shade, for buildings that do not receive full sun.
Resources and References:
Michigan State University Department of Horticulture: Green Roof Research Program
Organic Gardening: Great Green Roof Plants
Green Roof Plants, Emory Knoll Farms: Frequently Asked Questions
University of Oklahoma Urban Design Studio: Green Roof Plant Trial Array May 2009
University of Wisconsin, Great Lakes Water Institute: Great Lakes Water Institute Green Roof Project,
Green Roof Installation
North Carolina Cooperative Extension Service: Urban Waterways — Permeable Pavements, Green Roofs, and Cisterns
Green Roof Plants: Green Roof Links
Green Roof Plan: 6 Tips for Choosing Your Green Roof Plants
University of Minnesota Extension: Cacti and Succulents