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The Garden Plot for August 03, 2007
3:55 PM - 4:00 PM
[Program Website]
Today's Highlight: “Green Roofs”
As all gardeners know, plants buffer our cold and hot summer climate in Montana. Plants can be grown on roofs for the same purpose. Green roofs are planted for many reasons: To add the aesthetic pleasure only a garden can provide, for cooling and heating, storm water runoff management, water filtration, habitat restoration, food production, and energy saving.
There are challenges to growing plants on roofs. Most importantly, the substrate on a roof is much more shallow) than the substrate, or soil, in our gardens. In fact the substrate on a green roof may be 6” or less. A green roof requires several layers. Layer one is the roof deck engineered to withstand the added load of growing plants. Concrete and metal decks have more structural strength than the more common plywood roof. The deck layer must include a waterproofing layer and an insulation layer. Other required layers are a root barrier (so roots can’t grow into the house), a drainage layer (so roots don’t suffocate from excessive water), a filter layer, and a substrate layer in which plants will grow.
The substrate, or growing media, for a green roof must be lightweight and porous, but also able to hold oxygen, water, and plant nutrients. An ideal roof substrate should consist of 75 to 90% inorganic, weed-free media, such as expanded slate or shale or pumice. The other 10 to 25% should be a compost with large particle size, such as composted wood bark with added composted manure.
The best green roof plants for our area are low-growing, shallow-rooted, heat, cold, and drought tolerant perennials. Green roof plants should also not rely on heavy fertilizer use. Hardy succulents, such as our native Sedum lanceolatum are good plants for green roofs. In a 3-year Michigan State University study, nine non-native Sedum species out-performed eighteen Michigan natives. Good Sedum species for our area that have been successful on green roofs include Sedum spurium ‘Red Carpet”, Sedum rupestre, Sedum sexangulare, Sedum album “Coral Carpet”, and sedum divergens. Harbells (Campanula rotundifolia) is a Montana native that has been successful as a green roof plant. Grasses may also be important additions to green roofs. We have had some success in Montana with our short native grass Blue Grama (Bouteloua gracilus).
In our area green roofs will need some maintenance, including irrigation for establishment and during dry months as well as pruning once a year when plants go dormant. As in most gardens, weeding may be necessary, especially during the 12 to 18 months it will take for the green roof to become established.
Green roofs are relatively new in the U.S., but since 2000, there has been a lot of interest and experimentation with them. For more information on green roofs, you can visit greenroofs.com.
Helen Atthowe's new short program of gardening tips
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