Planning considerations - Living Roofs in Integrated Urban Water Systems - page 65

3.4.2.2 Tolerance of plants to climate (temperature, precipitation and water
demand)
The location of a living roof within a certain climatic zone as well as the
conditions induced by a building's microclimate has a strong impact on the
vegetation's growth, survival and continual thriving.
Dry hot climates (as in Arizona, for example) need plants with a high water-
holding capacity in their biomass and low transpiration rates (e.g., cacti), if the
living roof is not irrigated. Semi-cold arid climates like Spain with medium water-
holding capacity and medium transpiration rates need xeriscaping (e.g., various
orchid and herbaceous species). Plants need to be tolerant to frost and/or freeze-
thaw and extreme temperatures of roof microclimates. Water logging and root
rot is generally avoided in wet climates. This is because the growing medium is
designed to be extremely porous so that the weight of the living roof assembly is
kept at a minimum. Different climatic zones likely need different roof construction
assemblies, including varying growing media depths, different water-holding
capacity growing medias and speciic plant species (ASTM 2008; Roehr and Kong
2010; Stovin et al. 2013).
The climatic and contextual factors need to be considered as early as the plan-
ning stage and more closely investigated in the design stage, as they largely
control aesthetic appearance, physical functionality and effectiveness in stormwa-
ter mitigation (ANSI/SPRI 2010). Interventions to overcome the effects of adverse
climates may be associated with increased maintenance (irrigation, fertilization,
plant replacement) and decreased resilience. 3
MICROCLIMATE CONTExT
Plants on extensive living roofs have to withstand more extreme weather/climate
conditions than plants at ground level on natural soils (Getter and Rowe 2008;
Roehr and Primeau 2010). These include rapid luctuations in temperature (hot
and cold), radiation and wind exposure, drought, and in maritime or oceanic cli-
mates, salt spray. Climatic conditions are most extreme and most variable on tall
buildings in city centers, as large amounts of concrete and hard surfaces increase
temperature, decrease humidity, delect and concentrate winds (Blake et al.
2011; Fertig 2010; Fifth Creek Studio 2012). Microclimates in cities can change
dramatically over very short distances depending on sun and rain shadows and
relected radiation. The wind-tunnel phenomenon caused by medium- to high-
rise buildings increases the ET rate of plants by reducing boundary layers over the
plant canopy (Theodosiou 2009).
Annuals and taller herbaceous perennial and biennial plants are not generally
recommended as a dominant component of low-maintenance living roofs or
roofs where a large irrigation supply (either natural or artiicial) is unfeasible
(Snodgrass and Snodgrass 2006). This is partly because these plants have a
higher moisture demand and greater rooting depth requirement than hardy suc-
culents. They also experience dormant periods, and therefore provide little aes-
thetic beneit during winter/early spring. Dunnett and Kingsbury (2008)