The effect of Sedum species on stormwater retention by greenroofs

Background/Question/Methods

Greenroofs are analogous to thin rooftop gardens that are used to retain stormwater.  As installation of greenroofs becomes more commonplace, the need to relate design structures to ecosystem services increases.  For example, the importance of plants for stormwater retention has been debated.  Furthermore, few studies have compared the commonly used greenroof Sedum species to each other.  The primary purpose of this study was to compare stormwater retention from unplanted experimental greenroof platforms to those planted in Sedum album, Sedum kamptschaticum, Sedum sexangulare.  To this end, sixteen experimental greenroof platforms, four replicates of each of the aforementioned treatments, were installed in College Park, MD in 2010.  Stormwater inputs, volumetric moisture content, and runoff have been monitored continuously since at least February of 2011 on each platform using a wireless data sensor network.  Root biomass and plant coverage are also being monitored seasonally for each planted platform.  Environmental variables, i.e. air temperature, relative humidity, wind speed, solar radiation, and photosynthetic flux density are being collected at the study site on a 5-minute basis by the on-site weather station.

Results/Conclusions

Between March and November 2011, 985mm (38 inches) of rain fell on our experiments.  Runoff totals were 851, 791, 817, and 922L for the 1.3 square meter platforms planted in Sedum album, kampschaticum, sexangulare, and unplanted platforms respectively.  For storms during which >50% of the water was retained by greenroofs, ANCOVA analysis revealed a treatment effect (P<0.10) while accounting for variability in the data associated with storm size.  Platforms planted in S. kamptschaticum stored significantly more water compared to unplanted platforms (Tukey, alpha<0.10), with storage for the other treatments falling intermediate of these two.  Root and coverage data for each planted treatment did not vary significantly except in the spring whereby the S. kamptschaticum had less (ANOVA P<0.01) biomass compared to other species.  Future efforts will add two more seasons of data and explore relationships between platform treatments and different storm parameters, such as duration and intensity, as well as associated effects on stormwater retention.  Only with a clearer understanding of how much rainwater green roofs can retain with varying storm characteristics, environmental conditions, and planting scenarios, will urban stormwater planners and managers be able to consider or refine policies regarding permitting and incentives for this type of roof construction.