When Trees Are Not an Option: Perennial Vines as a Complementary Strategy for Mitigating the Summer Warming of an Urban Microclimate

This study evaluates the potential of Boston Ivy (Parthenocissus tricuspidata) to reduce building surface temperature in a mid-latitude North American city center where vine use for this purpose is uncommon. Vegetation can regulate city summer temperatures by providing shade and evaporative cooling. While planting trees has been a focus for many urban municipalities, trees require space (above and below ground), access to water, costly planting and maintenance, and may only be desirable to some city residents. To explore viable vegetation alternatives with fewer growth constraints, we deployed temperature loggers on the exterior walls of buildings in the urban core of Toronto, Canada, a large mid-latitude city. Perennial vines shaded some walls, while others were bare. These devices systematically tracked exterior surface temperature fluctuations over six months, including the growing season, with full vine-leaf coverage. During peak solar access periods, average daily temperature differentials between vine-shaded and non-shaded building surfaces ranged from up to 6.5 °C on south-facing building exteriors to 7.0 °C on west-facing walls. Models were developed to estimate daily degree hour difference, a metric integrating the magnitude and duration of the temperature-moderating potential of vines. At ambient temperatures ≥ 23 °C, solar radiation intensity and ambient air temperature were positively correlated with vine effectiveness in mitigating the rise in built surface temperature; relative humidity was negatively associated. Installing vine cover on urban buildings in the form of green façades can complement tree planting as cities become hotter due to climate change, and space for growing trees diminishes with urban densification. Future research into the capacity of green façades to regulate outdoor temperature must establish uniform measurement protocols and undertake evaluations in diverse climatic scenarios.