Sourced from the Living Architecture Monitor
Since the theme of this issue of the Living Architecture Monitor is making the business case for green roofs, I chose to write about the genera of plants that are probably growing on more green roofs and has more to do with driving the economic engine of extensive green roofs than any other. That is the much maligned or much loved Sedum spp.
The genus Sedum (family Crassulaceae) are a large group of succulents that are primarily native to the northern hemisphere, but also extend into Africa and South America. Most are found in Eurasia, but S. pulchellum and S. ternatum are both native to the eastern United States. The genus name is derived from the Latin word sedeo meaning ‘to sit’ in reference to the growing habit of many of the sedums (they sit and sprawl over rocks). Thus the common name, stonecrop. They are typically found in their natural habitat growing on rocky or stony areas and do well on dry well drained gravelly soils with limited fertility, on sunny slopes, and exhibit great drought tolerance, especially once established. Most are herbaceous perennials and easily propagate themselves from seed or cuttings. Prostrate stems growing along the ground often root forming new plants. Flowers range from red to pink to yellow to white and are known to attract butterflies. In fact, the San Bruno elfin (Callophrys mossii bayensis), a U.S. federally listed endangered species that inhabits rocky cliffs along the coast of California relies on S. spathulifolium for food.
Sedum is a popular choice for extensive green roofs due to its tolerance for drought, shallow substrate adaptability, and ability to limit transpiration and store water. Many have been identified as exhibiting some form of Crassulacean Acid Metabolism (CAM). CAM plants limit transpirational water loss by keeping their stomata closed during the day. They take up CO2 during the night, store it as an organic acid, and then use it the following day as the source for the normal photosynthetic carbon reduction cycle.