Plant cover and biomass change on extensive green roofs over a decade and ten lessons learned.

Green roofs are well studied for the environmental, social, and economic services these provide. As a result, green roofs are widespread and within the common vernacular of city residents. Green roof bylaws and construction standards are present in many cities in North America, rooting the presence of this green infrastructure within urban landscapes. Although examples of green roofs constructed decades ago exist, rarely are green roofs monitored over such long periods, and in ways that allow for experimentation, analysis, and conclusions about performance or function. In this study we present findings on plant cover and biomass from a green roof testing facility in Toronto, Canada that was monitored for over a decade. We examine the contributions of growing media, planting, and irrigation in the first seven years (2011-2021) of the eleven-year monitoring period. We found that during this maintenance phase period (2011-2017), plant cover and biomass was highest in modules planted with Sedum, included organic media, and were irrigated, whereas non-irrigated modules planted with forbs and grasses had the poorest performance regardless of media type. Following the stoppage of irrigation, and the post maintenance phase (2017-2021), modules initially planted with Sedum continued to sustain cover and biomass whereas planted forbs and grasses mostly disappeared, and these treatments were overtaken by Sedum. Our findings demonstrate that with irrigation, plantings of forbs and grasses can sustain plant cover and biomass. However, Sedum buffers against major changes to environmental conditions or abrupt changes to maintenance, adding insurance against failure of extensive green roofs.