There is an identifiable need to provide additional guidance to green roof designers in the selection of growing media depth and composition for extensive green roof systems. Green Roofs for Healthy Cities has established a technical Growing Media Committee to begin the work of establishing guidelines for growing media which reflect the wide variety of climates that exist throughout North America.
When selecting a growing media for a green roof system, it should be recognized that the performance of the plant material is directly affected by the microclimate. Media designed for sedums and other succulents for the green roof are limited to the species of plants that can tolerate the temperature conditions. Plants that are not drought tolerant cannot survive the extreme heat in some types of extensive green roof media. Rooftop microclimates are very different than what is observed at ground level. Media designed for placement above structures may have a weight requirement, and lightweight mixtures are recommended for planting where it is desirable to reduce the media weight.
Learning from other guidelines, such as, the German FLL (Landscape Research, Development and Construction Society) green roof guidelines, has given us a platform for growing media guidelines based on the highly variable microclimate conditions around North America. We are discovering that it is not just about water, high temperatures in shallow media systems and high nighttime temperatures over 70 degrees F that are causing even established sedums and some other species to go into dormancy or die. Sometimes, all it takes to improve the conditions is increasing the growing media depth to provide a cooler root zone to improve the conditions.
To create a specification to just meet the weight limitations of a roof may not be the best scenario for a vegetative green roof. What green roof professionals should hope to achieve is to provide an aesthetically-pleasing product that meets environmental and economic requirements as well as enhances the community.
The Green Roof Growing Medium Committee has come to agreement that their first order of business is to develop regional guidelines for growing media (depth and composition) related to a standard extensive green roof utilizing a standard sedum mix of plants, rather than a single “one size fits all” approach. By dividing the U.S. and Canada into regions based on climate, we can then research and apply methods for improving growing media performance.