Quantifying Performance of Succulents and Novel Substrates for Extensive Green Roof Applications in the US Pacific Northwest

Abstract:

This study aims to improve the performance of extensive green rooftops using native plant species under minimal maintenance and irrigation regimens. Methods involved developing three new soil profiles specifically for the installation sites on two urban rooftops in Portland, Oregon, in order to provide the necessary drainage in a climate averaging 66 cm of October-to-April liquid-water-equivalent precipitation, yet moisture retention through summers that usually include several months with little to no rainfall. For each rooftop, we built an array of 15 containers, each containing 18 liters of soil, and planted a total of 60 succulents native to the US Pacific Northwest. Species tested included Dudleya lanceolata, Sedum album, S. laxum, S. oreganum, S. ‘Silver Moon’, and S. spathulifolium. The three substrate types we designed included varying mixes of organic compost, sand, vermiculite, perlite, and pea gravel, arranged in distinct profiles, and instrumented with soil moisture sensors. Remote automated weather stations also measured air temperature and dew points at multiple heights above each rooftop, along with precipitation, wind speed, wind direction, incoming photosynthetically available radiation, broad-spectrum solar radiation, and leaf wetness. Other measurements included periodic infrared and visible-wavelength photography of plant health from stands we designed for this study, along with traditional and novel digital transect surveys of plant performance. The study sites varied in aspect, exposure, and winds, along with proximity to nearby trees, water bodies, and urban heat sources, which produced corresponding differences in plant growth rates. Results show relative and absolute performance of these succulents and soil types in different green-roof settings in the Pacific Northwest, and quantify their effects on building energy budgets. Potential applications include reducing heating and cooling costs, reducing the effects of impervious rooftop surfaces on local hydrologic systems, and promotion of native species in heavily impacted urban environments.