How does plant taxonomic choice affect building wall panel cooling?

Vegetation is increasingly being valued for its contribution to urban cooling, with implications for mitigation of urban heat islands, building energy efficiency and human thermal comfort. Relatively little information exists, however, as to how plant taxonomic choice affects micro-climate cooling. This research used a building wall scenario to determine how plant type affected material cooling during summer in a temperate climate (UK). Thermocouples recorded wall panel temperatures behind 24 different plant taxa compared against a control panel unshielded by plants. When the control wall panel temperatures were 25 °C, 35 °C, 45 °C and 55 °C, the maximum cooling observed on panels screened by vegetation was 5.1 °C, 13.5 °C, 20.1 °C and 25.7 °C respectively. For each of these temperature scenarios, maximum cooling was attributed to Hebe, Lonicera, Hebe and Hedera, respectively, i.e. the optimum taxa choice could vary with the ambient conditions being experienced. Data was assessed, subsequently to determine if cooling potential could be associated to a certain plant attribute. Cooling was maximised by increasing the overall leaf surface area between the wall panels and the incoming solar irradiance, i.e. larger plants with dense canopies promoted cooling, but otherwise there was no overall consistent pattern linking cooling to key plant traits, such as leaf colour. The research confirms that plants are effective thermal regulators within the urban environment, but further wider evaluations of taxa are required before the landscape sector has a definitive list of optimum genotypes that it can recommend.