2013 ASHS Annual Conference

The primary purpose of green roofs is to reduce stormwater run-off. Studies have shown that an ideal media for extensive green roofs consists of 80% to 90% inorganic material combined with a maximum of 10% to 20% organic material, such as sphagnum peat moss. Sedum species tolerate the harsh growing conditions on rooftops better than most of plant species that have been studied.

Green roofs are more common in metropolitan areas where ordinances and incentives have been implemented, whereas small municipalities and rural areas typically have fewer green roofs. Therefore, supplies of green roof materials are often more difficult to obtain without incurring huge shipping costs. To encourage building owners in rural areas and smaller municipalities to implement green roofs, locally available materials were trialed on a demonstration site for efficacy in a green roof system. Two media substrates were trialed: one using locally available trap rock [(hadite) 80%], sand (12.5%), peat moss (5%), and compost (2.5%); and the other using reclaimed lava rock (red) in place of the trap rock. Two species of sedum (S. reflexum, S. spurium) were planted after the bags and flats were in place on the roof. To further simplify the installation process, plants were installed on the roof in one of two containers: 1) specially-designed landscape fabric bags or 2) plastic flats such as those used for bedding plant production. Media substrates and containers were combined in four possible combinations. They were arranged in a completely randomized design on a rubber-membrane covered roof having a 2/12 pitch (16.67% slope). Plant growth was evaluated a year after plants were established on the roof, and at the end of the second growing season. Results indicated that plants grew better in lava rock based media as compared to trap rock. Plants also grew better in flats than landscape fabric bags. Whereas some treatments had a score of zero, indicating no plants had survived, others had scores as high as ten, on a scale of 1–10. In spite of severe drought and above-normal temperatures, plants looked better overall in September than they had in March.