Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 99

tolerant species are not competitive and diversity reduces. Shallow media are less
able to store water for plant growth than a similar, deeper media, and experience
more extreme temperatures. A moderate to high proportion of ines increases
moisture storage and may beneit plant growth but decreases permeability and
increases weight. Maintaining high permeability of the growing media is impor-
tant to prevent ponding and excess weight.
The organic matter content of an established extensive green roof should stabi-
lize at a level based on normal biomass turnover. In theory, a moderate to low initial
organic content (< 20 percent v/v) combined with low fertility and limited irrigation,
favors plants adapted for the unique growing environment of a low-maintenance
living roof. As organic content is typically not supplemented, over the long-term the
natural life-cycle of the plants should provide the ongoing organic matter and nutri-
ent requirements. Measured periodically over 4-5 years after installation, four exten-
sive living roofs in Auckland showed relatively stable levels of exchangeable
macronutrients considered important for plant growth (Fassman et al. 2013). Total
carbon content and carbon to nitrogen ratios were also relatively stable in three of
ive Auckland living roofs. Two systems demonstrated carbon loss over time, and
were associated with low plant cover due to plant death and disturbance of the
media by cultivation and replanting. The latter loss was consistent with other studies
in Texas and Sweden (Aitkenhead-Peterson et al. 2011; Emilsson and Rolf 2005).
None of the Auckland roofs showed an increase in organic content, contrary to
observations reported in Michigan (Getter et al. 2007), where a sedum roof doubled
in organic matter content over ive years. The pH of all of the Auckland living roofs'
media remained between 5.4 and 6.3 over two, four and ive years after installation
depending on the site. In contrast, European and North American studies have indi-
cated that acidiication may be problematic, induced by sulphuric and nitric acids
from air pollution (i.e., acid rain) (Berghage et al. 2007).
The predominant considerations in growing media design are safety, system
weight, stormwater control (where this is a project objective) and plant viability.
Table 4.1 identiies growing media characteristics relating to the key concerns.

Table 4.1 Speciications to consider in growing medium development

Growing medium characteristic Safety Weight Stormwater

control

Plants

Water-holding capacity (including ield capacity and permanent
wilting point)

   

Weight at ield capacity   - -

Saturated weight   - -

Permeability    

Particle size distribution    

Nutrient content and availability - - - 

pH - - - 

Cation exchange capacity - - - 

Organic matter stability -   