We’ve been planting different species on the green roof for the past month, which we will use to determine what grows well and provides the most benefits for green roofs in our climate. The species we chose to plant are limited not only by heat and rainfall but by the thinness of the substrate we’re using. Using just a few inches of substrate is an important part of the experiment because many people who are want a green roof cannot afford the higher cost of using deeper soil, and most buildings cannot handle the added weight. Unfortunately this limits the number of species that will grow on the roof. One benefit that a green roof can provide is increased habitat for native plants and animals, and while I was planting I began wondering how that benefit can be preserved in the thin, dry soil of an extensive green roof high up on top of a skyscraper.
I decided to read some articles on the issue. First I read an article on green roof habitats in Sweden that recommended using local natural soil to encourage arthropods to live there, but this causes a problem because native topsoil can be too heavy and have poor drainage. The article went on to say that, even with the deep substrates found in intensive green roofs, most local animals can’t survive on a roof because it is too hot and dry. It specifically mentioned earthworms, who bury themselves deep in the soil when it is hot out, but cannot bury themselves deep enough in a green roof. Other animals simply have trouble getting up that high, even if they could live up there easily. It was an interesting article, and stressed that although green roofs form an important habitat that would otherwise not be there, there is still room for improvement.
Next I found a study similar to the one we are going to do on our green roof, but in Berlin instead of College Station. They also used different substrate compositions to see how it affected plant vigor. Berlin is much cooler than Texas, but it doesn’t rain much, so drought is a problem for both Texas and Berlin green roofs. Since the substrate was only 10 cm deep, it is similar to the depth of substrate we are using. The study found that certain species grew better than others, and that all species grew significantly better when the substrate was mixed with superabsorbent polymers (SAP). The polymers retain water much better than expanded slate mixed with organic matter, and the plants had water access even after months without rain. From this, I determined that the plant diversity of a green roof could be expanded if a little SAP was added, because plants that ordinarily could not withstand the dryness of an unirrigated green roof might be able to if they had longer access to water during periods without rainfall. However, I don’t know how much it costs to add SAP. It might be too expensive, but on the other hand it might not be too expensive for everyone, and better plant growth keeps the roof prettier, which is an important consideration.
Finally I found an extensive retrofitted green roof in the UK built with special consideration for endangered local fauna. Despite being 160m tall and using crushed bricks as substrate, it quickly became home to grasshoppers and ladybugs, and hopefully in the future ground nesting birds will live there, too. It’s no nature reserve, but it’s still a very inspiring tale.