Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 113

3 Control biomass production. Manage nutrient content of the growing media
through restricting the type and amount of organic matter. Allow drought
stress to limit biomass production via growing media depth and limiting irriga-
tion. Overall, this approach means the aesthetics of a roof may be reduced
(because lowers may also be smaller) - so select plants that retain aesthetics
under low fertility conditions.
Plants that produce substantial above-ground biomass and that use a lot of water
should generally be avoided (Snodgrass and Snodgrass 2006). Because extensive
living roofs typically hold only a small volume of water relative to ground-level
landscaping, high water users will rapidly exhaust available water and hence be
more vulnerable to drought stress and plant death. Literature identiies different
plant species that have different water use strategies, and access water from dif-
ferent depths within even a shallow substrate. Using a range of plant species that
have different root architectures as well as water use and drought tolerance strat-
egies should optimize both the resilience and performance of living roofs (Farrell
et al. 2013b). For example, sedums appear to be particularly sensitive to declining
water availability (e.g., a drying growing media) and reduce water use accord-
ingly (Voyde et al. 2010b). Mixing sedums with drought tolerant plants that have
high water use increases contribution to stormwater attenuation, while the latter
are transpiring. This approach, trading off between stormwater mitigation and
plant survival, has been proposed by MacIvor and Lundholm (2011).
4.3.3 Plants and nutrient loads
The philosophy adopted in this topic is to promote living roof design irst and fore-
most as a stormwater control measure (SCM). In addition to establishing continu-
ous cover, consistent with ground-level SCMs such as bioretention or swales,
passive operation includes minimizing (or eliminating entirely) external inputs such
as fertilizer application or irrigation - both for cost and good SCM functionality.
Plants that require a high amount of nutrients, or conversely, creating condi-
tions where plants receive high amounts of nutrients (through excessive organic
matter in the growing media [ Section 4.1.1 ] or added fertilizer), increases the risk
of three negative outcomes. First, high nutrient supply increases the likelihood of
nutrients being discharged in runoff. The impact can be mitigated by passing
runoff through a ground-level SCM such as a rain garden or landscape irrigation,
thus avoiding direct discharge to piped stormwater system. Second, high nutrient
availability typically produces more biomass, increasing potential ire risk unless
mitigated by a speciic maintenance regime. Third, high nutrient-supply typically
produces a more lush, “soft” growth and larger leaves, and more growth of
leaves than roots. This predisposes plants to drought stress and wind damage.
Where drought stress is avoided by irrigation, the potential amount of nutrients
washed from the roof increases. High nutrient supply also accelerates decompo-
sition of carbon (growing media organic content). Where plant biomass is
removed, or plant carbon inputs to the media are lower than carbon