Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 109

• Addingwater-retainingmaterialstoagrowingmedium:organicmaterials
(both temporary and long-lived) and inorganic materials. Inorganic materials
include aggregates that hold water in internal pores, such as pumice,
spongelite and, to a lesser extent, scoria and brick. Water-absorbing minerals,
hydrophilic polymers (hydrogels) and silicate-based granules may be used to
enhance short- to medium-term water availability. The effectiveness of various
additives differs depending on the aggregate used in the media and plant
species. Only water that is available for plant uptake will enhance drought tol-
erance (Farrell et al. 2013a).
Choice of moisture retention technique should consider longevity of the material,
particularly organic matter. Coir or jute retain water well, but eventually will bio-
degrade (perhaps over several years), especially if exposed to UV and continuous
wetting and drying or freeze-thaw cycles. Nonetheless, these materials may
present a helpful solution for surplus moisture storage during plant establish-
ment.
Supplemental moisture storage techniques may increase the live load, depend-
ing on the extent of application. Subject to the same environmental conditions, a
coir-based pre-planted sedum mat living roof in Auckland, New Zealand, cap-
tured on average 4.7 mm more rainfall than an adjacent built-in-place system
during most storms over the course of one year (Voyde et al. 2010a). While this
is beneicial to stormwater management, the structural load would have
increased marginally - by approximately 5 kg/m 2 (1 lb/ft 2 ).
4.3 PLANT SELECTION
Plant selection for extensive living roof projects is severely limited compared to
ground-level landscaping projects. The combination of very shallow growing
medium and high ET (which translates to water demand) causes extreme mois-
ture and temperature stress on plants (Snodgrass and Snodgrass 2006). Root
expansion space for plants may be limited to a few centimetres; in such condi-
tions only low-growing plants tolerant of high winds and turbulence will remain
anchored and undamaged.
In the last decade, “extensive living roof” plant selection and establishment
guides have been written by leading experts. Rather than repeat information, this
section attempts a concise and descriptive insight into design speciics of differ-
ent geographic locations for the successful thriving of plants. For further reading
on plants for North America see Edmund and Lucie Snodgrass (2006) and the
publications link from the Michigan State University Green Roof Research Team
website ( www.hrt.msu.edu/greenroof/index.html) , especially Getter and Rowe
(2008). For information on plant selection for the Northern Hemisphere in
general, see Dunnett and Kingsbury (2008) and Koehler (2003). For more infor-
mation on plants appropriate in New Zealand, see Fassman-Beck and Simcock
(2013). For plant screening using identiication of comparable natural habitats,