Performance of Sedum Species on an Urban Green Roof in Colorado
Extensive green roofs are an increasingly common type of man-made ecosystem, and it is important to know how environmental conditions affect plant growth. Higher growth increases transpiration and the cooling effect on the roof and enhances aesthetic value. The green roof on the Student Success Building at Metropolitan State University in Denver, CO is a challenging environment with high summer temperatures and intense solar radiation from a glass wall that reflects onto the site from the northwest. Aerial photography reveals differences in vegetation texture across the surface relative to the reflective wall. Previous research has shown that this difference is not due to spatial sorting of species. There are 10 varieties and species of Sedum and they are evenly dispersed across the site. We hypothesize that variations in individual performance are producing larger plants of the same species near the reflective northwest wall. Plant performance was evaluated by measuring height and diameter for Sedum species with dome-shaped growth forms. Plant location was measured as distance to both the reflective northwest wall and a less reflective brick wall, with smaller dispersed windows, on the northeast.
Results/Conclusions
Sedum plants with a dome-shaped growth form are larger when in close proximity to any enclosing wall. A t-test comparing plants that were within 5m of the nearest wall to those more than 5m from the nearest wall showed a clear increase in plant height near the wall, with an average height of 19.26 cm and 13.07 cm respectively (n=283, p=7.23*10-23, t=11.06). Regression analyses indicate a threshold relationship between distance and plant height. Similar tests run on the distance from each wall individually had similar results, with the all-glass wall having a larger effect than the brick wall. Taken together these results suggest that chemical input of ammonia from cleaning products is fertilizing the Sedum plants growing within 5 m of either wall. Unintended consequences of design decisions can have substantial impacts on the growth and sustainability of green roof ecosystems. Understanding how growing conditions influence the performance of different species in a green roof setting can inform how these facilities are built and maintained.