Green roofing is the practice of turning a rooftop space into a garden. In addition to providing a pleasant space and an environment for birds, butterflies and insects, green roofs are purported to improve air quality, combat the urban heating effect brought on by the presence of extensive tarmac and concrete surfaces, and reduce cooling costs in buildings. Plants of the Sedum genus, known as stonecrop, are commonly used in green roof projects for their hardiness and adaptability.
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There are two common methods of introducing sedum to a green roof environment: direct planting, or via mats. Mats are integrated systems that contain soil and other organic matter into which sedum plants are placed. These mats are introduced as fully formed ground covers.
According to University of Missouri professor Christopher J. Starbuck, the spacing of sedum plugs should be based on the amount of space the plants must cover. Mat-forming ground-cover specimens should be planted every 4 inches in an 11-square-foot space, 6 inches for 25 square feet, 8 inches for 44 square feet, 10 inches for 70 square feet, 12 inches apart for 100 square feet, 18 inches apart for 225 square feet, 24 inches apart for 400 square feet, 36 inches for 900 square feet and 48 inches for 1,600 square feet.
As a general rule, for the direct-planting method, fast-growing ground cover plants like sedum can be planted 1 to 2 feet apart.
Additional Planting Information
Sedum will grow in both soil and rock and can therefore be planted on roofs with a light layer of dirt or stone covering. Because most roofs aren't covered in either dirt or rock, fully-formed mats are commonly used.
A green roof study conducted by the Michigan State University Department of Horticulture found that sedum plugs can be successfully planted at depths of 4, 7 and 15 centimetres, or approximately 1-1/2, 2-3/4 and 4 inches.
Plants of the Sedum genus prefer full-sun exposure, making them well-suited to rooftop gardens -- though they tolerate partial shade. However, stonecrop species only flourish in well-drained environments. Green roof gardeners should have an irrigation system in place for areas with significant rainfall.
About Genus Sedum
Sedum is a genus of succulent plants. Succulent plants are species that retain large amounts of water as a method of survival in arid climates. Species range in size from small mat-forming specimens (those best suited to green-roof gardening) to 2-foot-tall shrubby mounds.
Species of the genus are suited to growth in a wide range of USDA hardiness zones (3 to 11). However, most species fall within zones 5 to 9. Specimens can be propagated via softwood cuttings and seed or plug planting.
Green Roof Sedum Species
Species of stonecrop that the Michigan State University Department of Horticulture used in its study include goldmoss (Sedum acre), white (Sedum album), Russian (Sedum kamtschaticum), widow's cross (Sedum pulchellum), and the 'Dragon's Blood' cultivar (Sedum spurium 'Coccineum').
Christopher J. Starbuck further recommends yellow stonecrop (Sedum reflexum) as a mat-forming species similar in appearance and performance to Russian sedum. Both species bear green foliage and yellow, star-shaped flowers in midsummer.
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