Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 97

Organic content in the growing medium decomposes over time, releasing
nitrogen used by the plants to produce more biomass. Unlike ground-level land-
scaping projects, the organic content is not usually artiicially “topped up”
because of challenges in access, weight and the potential deleterious effects on
drainage and nutrient leaching. In keeping with the concept of the dynamic life-
cycle presented in the Introduction, a properly speciied plant palate will supply
its own ongoing needs for organic input once established, through the plants'
natural growth and death cycles. Without external input, an out-of-balance
system resulting from an initially high input of organic matter, particularly if an
unstable organic matter (e.g., incompletely composted material), may produce
unsustainable vegetative growth (excessive biomass), result in a net loss of media
depth over time (as organic matter decomposes) and/or compromise drainage
from reduced permeability (as media may self-compact). It may also increase the
resistance of media to root penetration (Nagase and Dunnett 2011), resulting in
limited root exploration (impacting nutrient and plant uptake).
To minimize nutrient leaching, not only must the level of organic matter be
able to be sustained by plant growth, but the majority of the organic matter used
must be relatively stable both physically and chemically, i.e., lasting until plants
have fully established. Relatively stable organic matter includes composted bark
ines, coir and suficiently composted and aged leaf or arborist mulch. A high
proportion of young, incompletely composted materials is usually unstable and
breaks down too quickly. This may generate, and leach, excessive nitrogen and/or
phosphorus. However, small volumes of fully-composted, nutrient-rich organic
matter are beneicial to boost plant establishment, e.g., mushroom or green-
waste compost, as plants do require nutrients to grow and establish vegetative
cover. Composts containing rapid-release fertilizers designed for ornamental or
vegetable gardening are also likely to leach excess nitrogen and phosphorus, and
are not appropriate for stormwater living roof installations. Low levels of slow-
release fertilizers designed to release nutrients when plants are actively growing
are most suitable (i.e., products where release is inluenced more by water availa-
bility than temperature). Peat and coconut coir boost water-holding capacity,
however this must be balanced against weight limitations. Very ine organic
matter, e.g., ine-milled peat, may be washed through coarse living roof aggre-
gates, and growing mediums using only peat as an organic source are likely to
require pH buffering to reduce acidity.
Living roof growing media are substantially dissimilar to garden mixes used in
ground-level landscaping applications. Extensive living roof growing media typically
contain no natural soil, such as topsoil or garden mixes. Typical topsoils are heavy
(silty-loam to sandy loam has a moist bulk density of ~1,450-1,600 kg/m 3 that
further increases with sand content (nrcs.usda.gov); Auckland Region in-situ topsoil
typically has a dry bulk density from 950-1,100 kg/m 3 ), have inadequate drainage
and aeration when placed at extensive living roof depths (i.e., demonstrate a
propensity for water-logging), are prone to compaction, and have high variability -
all undesirable characteristics for a living roof growing media. Humidity at the