An Evaluation of Different Plant Taxa on Wall Thermal Properties

Hot cities are increasingly seeking nature-based cooling strategies to combat the rising urban temperature. Cooling the constructed environment begins with incorporating the right plants – taxa that hold the power to cool, on or around the buildings in various forms of greeneries such as vegetated roofs, walls, facades, corridors, and balconies. Although the thermal benefits of these greeneries were widely investigated, very little information is available on wall taxa that can provide efficient cooling to make an informed plant selection. This study uses four scale-model wall panels, to evaluate the thermal properties of a range of landscape taxa and how the changes in the external environment (ambient temperature, wind, and substrate moisture) and contrasting plant traits (canopy thickness and leaf characteristics) can impact taxa’s wall cooling ability. Maximum wall surface cooling of 25 ºC was achieved by Hebe and Hedera when the control wall was 55 ºC. Wall cooling values vary widely among taxa and the external temperature to which they were subjected. No generic leaf traits (leaf colour/size/thickness) can be related to a plant’s cooling capacity except total leaf surface area. However, the additional cooling value of taxa is not proportional to the added leaf area and varies among taxa; i.e. additional cooling values of 5.3 ºC, and 1.8 ºC, were realized in doubling canopy thickness of Hedera and Hydrangea respectively. A subsequent controlled experiment was conducted to investigate how cooling capacity altered as plant water availability changed. While Carex and Hedera retain their cooling ability in short-term desiccated conditions, Alchemilla loses its capacity to cool the wall and leaves. The overall study establishes the individuality of plants in microclimate modifications, but further evaluation of wall plants and their physiological interactions with the external environment is warranted to identify the decisive list of ideal wall taxa.