Bridging the grand canopy! How does plant canopy width affect the cooling capacity of green walls?

Growing plants on buildings provides a range of ecosystem services. In addition to offering wildlife habitat, green walls insulate buildings and reduce their thermal load in summer. But how does plant choice/design affect building cooling? While some research indicates the importance of foliage width (foliar density), limited information is available on what density optimises cooling. This research aims to determine how plant canopy width impacts wall cooling. Cooling was estimated by comparing wall panel temperatures behind plant foliage against control panels exposed to direct solar irradiance; the plant canopy width varying by the addition of extra plants, i.e. altering Wall Leaf Area Index (WLAI). The value of wall cooling per unit leaf area for six contrasting taxa was estimated, and factors affecting these attributes were identified. The potential to cool a wall increased with additional canopy layers, but was not proportional to the greater leaf area. Greatest cooling was recorded with Heuchera micrantha cv. Palace Purple, with 21 °C of cooling provided by a single canopy layer when control wall temperatures were 45 °C. Adding two more canopy layers increased the cooling differential to 24 °C under these conditions. Shading accounted for much of the cooling, but variations in cooling capacities for taxa with similar leaf areas suggest variable evapotranspiration was also influencing cooling. This study concludes that for those taxa that cool primarily via shading additional canopy layers are beneficial, but for those that cool via evapotranspiration, then a single layer suffices.