Green roofs have been used for decades in Europe because of their many benefits, including storm water management, increased biodiversity, and mitigation of the Urban Heat Island (UHI) effect. Recently, green roof installations in the U.S. have increased, and research has begun to focus on functional differences among plant species and resulting effects on ecosystem services. The New York metropolitan area, the largest urban area in North America, could potentially benefit from widespread installation of green roofs, but adoptionrequires regionally-specific information for optimization. Our objective was to identify the species diversity and biomasspotential of green roof substrate. We asked the followingquestions: (a) what plants will naturally colonize green roofsubstrate, prior to planting, and (b) how does substrate depth affect biomass and colonization? To address these questions, we compared six deep versus six shallow green roof plots oneach of eight rooftops distributed across the New York City. Each plot was filled with a light-weight, low organic matter, green roof substrate lined with drainage mat. The plots were constructed in late May 2010. In early September all plants that colonized the plots during the study period were identified to species and collected to estimate above ground biomass.
Sixty seven of eighty five plots (~79%) were colonized by at least one plant species, and deep plots were colonized 34% more often than shallow plots. Thirty-one different plant species were identified including graminoids, woody plants and forbs across all eight rooftops. The species richness per roofranged from 6-22 with a mean of 12.38. Of the speciesobserved, 55% occurred on more than one rooftop, withDigitaria sanguinalis, crabgrass, found on all rooftops. Panicum miliaceum, millet, a grain producing grass was identified on 75% of rooftops. Deep plots had an averagerichness of 3.69 species per plot, whereas shallow plots had an average of 2.63 species per plot. The maximum above groundbiomass in a plot was 630.48 g/m2, which is similar to the productivity of the Great Plains. Deep plots produced, on average 3x more biomass than shallow plots. This study demonstrated substrate depth affects both the number of and thebiomass of green roof colonist plants. Further analysis of thephysiological properties of the plants identified in this studyand other plant species suitable for mid-Atlantic green roofsmay provide insight into increasing ecosystem services relativeto conventional roof coverings and Sedum-based green roofs.