Plants for Extensive Green Roofs
Selecting Vegetation for Shallow Substrates and Harsh Conditions
Apr 29, 2009
Green roofs are building coverings that include vegetation in their engineered design. Extensive green roofs are an economical and light-weight option for rooftops not intended for public access. These systems use a minimal amount of growing medium and require little or no irrigation. However, the plants used in these systems must be able to handle the harsh conditions on green roofs.
Growing Conditions on Extensive Green Roofs
In their book, Planting Green Roofs and Walls, Nigel Dunnet and Noel Kingsbury (Timber Press, 2004) identify three green roof characteristics that affect plant choice:
- Drought – Extensive green roofs have shallow substrates with growing medium designed to drain excessive water. As a result, green roofs have frequent periods of drought and require drought-resistant plants. However, the plants must also survive periods of heavy rain and, depending on the climate, extreme cold.
- Elevated temperature – Green roofs generally receive more direct sunlight with less shade from trees and shrubs. In addition, the roofing materials tend to absorb heat during the day and radiate that heat during the night. These factors combine to increase the overall temperature that plants must tolerate.
- High winds – Green roofs are subjected to high winds and eddies, which can cause damage to plants and disperse the growing medium. Low-growing plants that spread to cover and protect the substrate and preferable.
In addition, the shallow medium on extensive green roofs requires that plants have shallow roots. Since the roots are unable to grow deep into the substrate where temperatures are lower, the roots must also be able to tolerate the heat and sun. In their book, Green Roof Plants, Edmund Snodgrass and Lucie Snodgrass (Timber Press, 2006) state that "… the most successful green roof plants are low-growing, shallow-rooted perennial plants that are heat, cold, sun, wind, drought, salt, insect, and disease tolerant." (p. 48) They also indicate that in North America, these plants must also be able to tolerate some wet roots.
Common Plant Types For Extensive Green Roofs
Hardy Succulents – The most commonly used plants for extensive green roofs, succulents are able to withstand periods of drought, survive windy conditions, store water in their leaves for extended periods of time, and conserve water through a metabolic process called Crassulacean acid metabolism. They also have a shallow root system and can survive in extreme temperatures, both hot and cold. Hardy Succulents include Sedum, Sempervivium, Talinum, and Delosperma.
Grasses and Similar Plants – Although they have deeper roots than succulents, grasses and similar plants can be used in moderation on green roofs. They add motion and texture and provide bird and insect habitats. However, grasses have a dormant period in which they dry out and can pose a fire hazard. Grasses also require some maintenance including pruning. Some good grass options include Andropogon, Bouteloua, Carex, Sesleria, and Sporobolus.
Herbaceous Perennials – Although these plants offer color, texture, and seasonal variability, they also require deeper substrates (greater than 4") and have limited drought tolerance. Some choices for North American green roofs include Petrorhagia, Campanula rotundifolia, and Phlox.
Although green roof microclimates offer unique challenges for plants, with careful consideration of the growing conditions and environmental factors in specific locations, green roof plants can be effective, varied, and resilient.
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