Off-the-shelf sedum (Stonecrop family) blankets may be a convenient, light weight, easy to install green roof requirement, but there are hidden problems that are important to understand before purchasing.
Consider the following:
1. Sedum blankets provide a fraction of the ecosystem service benefits compared to biodiverse roofs. This is primarily owing to their shallow soil depth, and thereby reduced water-storage capacity along with limited cooling and insulation.
2. Many London boroughs will not accept sedum blanket roofs, due to their limited ecological value. They will expect specifications to follow best practice guidance, namely The Green Roof Code. There is also a Green Roof Guide for developers. The guidance states that a green roof should at least have a minimum depth of 60mm of substrate under a sedum blanket.
3. Some sedums are invasive. Dragon’s blood or Caucasian stonecrop sedum spurium is invasive in mainland Europe and is banned from sale in Germany, Poland and Austria. It is currently still available in the UK, forming a component to sedum blankets, but should be avoided. In addition, sedum kamtschaticum and sedum spectabile are currently listed as low threat invasives in the UK. Read more.
It is not that sedum’s are wholly bad plants. Several are native to the UK (such as sedum acre, fosterianum, reflexum, telaphium), but they occur typically as a small component to semi-natural habitats, providing nectar to only a selection of invertebrates. They are not suitable for an entire habitat at roof level. To ensure long-term benefits, the Ecology Consultancy recommend obtaining specialist advice when specifying a green roof, to ensure optimum conditions are created to provide for high biodiversity.
If you already have a sedum roof that is not performing to your satisfaction, then there are ways to retro-fit features and floristic diversity to improve the ecology of these spaces.
Please get in touch. Our sister company, The Green Roof Consultancy have experience of undertaking such work and will inspect existing structures. They recently inspected one sedum blanket roof that developed problems after a few years down the line. Read their Blog.
And finally, new guidelines are on the way as an updated GRO Code (with contributions from The Green Roof Consultancy and Living Roofs) is to be launched this autumn.