Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 115

or shoots until the following autumn. However, differences between plant species
transpiration rates can be less than the differences across a roof due to variation
in exposure to solar radiation (MacIvor and Lundholm 2011) or differences in rel-
ative humidity from day to day (Voyde et al. 2010b).
To date, succulent species seem to be the best suited, while sedums are the
most cost-effective plants to perform the sequence described above, if plant sur-
vival and stormwater runoff reduction are design objectives. Other hardy (non-
sedum) succulent species including Taluim , Sempervivium and Delosperma that
are generally also suitable for many non-irrigated roofs (Snodgrass and Snodgrass
2006). In Sydney and Auckland, bromeliads and other tree canopy and rock
outcrop dwelling plants have been successfully established on living roofs. The
issue will be what the client and user perceive to be a year-round healthy (or oth-
erwise acceptable) appearance of a visual roof. This is much less of a complex
goal to achieve if visual accessibility is not a design goal. In the case of a visible
roof, and in the context of climatic extremes, some artiicial irrigation may be
required to maintain a perceived year-round healthy appearance for roofs with
constrained media depth (limited water storage). Similarly, in areas with limited
regional experience of living roofs, the ability to irrigate during peak moisture
deicits reduces the risk of plant selection. A conservative designer concerned
with reliability of moisture supply, maintenance or exposure might restrict the
plant palette to very tolerant plants. Another approach is to use likely suitable
sedums as a “foundation”, and include a variety of other species. Trials over
three years by Butler and Orians (2011) indicate sedums may be used as “nurse
plants” to expand the palette of living roof species, partly by decreasing soil
temperatures.
4.4 GENERAL PLANTING DESIGN GUIDELINES
The following general guidelines for planting design are worth considering:
• Theroofshouldbecoveredwithaselectionofplantsbeingproductive/active
during the rainy season. In extreme climates, extended dormancy and/or other
climate effects should be considered by the local horticulturalist.
• Matchplantactivity(growingseason)toperiodsoffrequentrainfall.Dormant
periods of most plants chosen should match the duration of stress periods
(i.e., as short as possible to maximize aesthetics and stormwater mitigation).
• Aggressiveplantsshouldbeavoided,astheymayoutcompetetheintended
vegetation, use more water and increase maintenance (removal of undesired
species on the roof).
• Low,perennialgroundcoversshouldbethedominantfeatureastheyhave
proven to be the most rapid, reliable and cost-effective spread on the roof
(Gedge et al. 2012; Greenroofguide.co.uk; Snodgrass and Snodgrass 2006).
• Designersshouldbalancefast-growingplantsfor“instantaneous”short-term
beneits while cultivating a long-term plant cover which adapts to the local