The green wall concept is essentially very simple: instead of growing plants on roofs or on the ground where they take up space, grow them up unused space on walls. Just in terms of aesthetics, green walls are far superior to green roofs as they are visible to a much greater proportion of the population. As for wider environmental benefits such as reducing the heat island eect and surface temperatures (Wilmers, 1988), there is a far greater opportunity for delivering some environmental services with green walls than with green roofs. Köhler (2008) estimated that walls represented double the ground footprint of a building, in urban centres, although this seems likely to be on the conservative side, with Peck et al. (1999) estimating it at between four and twenty times the roof area depending on the height of the building, and much of the space on flat roofs is taken up with other infrastructure, reducing growing opportunities (Cheng et al., 2010). On the negative side, environmental conditions become increasingly harsh for wall vegetation at higher elevations (Peck et al., 1999) and at lower elevations salt spray during winter may cause damage (Whittinghill & Rowe, 2011). Green walls have been known and grown for thousands of years; Köhler (2008) identifies vines grown in palace gardens in the Mediterranean some 2,000 years ago as the earliest deliberately planted examples. Whilst many green walls establish naturally, there is increasingly global interest in incorporating them in building and city design because of their multifunctional benefits (e.g. Köhler, 2008; Loh, 2008;

FIGURE 5.1 In this student accommodation development (The Minories, near Tower Bridge, London) the 191 m2 Biotecture Living Wall has been used as the external rain cladding of the building allowing the costs of the green wall to be oset against using conventional cladding. © Biotecture Ltd

Sheweka & Magdy, 2011) and because retrofitting green walls is relatively straightforward compared with green roofs.