Integrating stormwater performance and architectural design - Living Roofs in Integrated Urban Water Systems - page 110

diversity of growth form and drought tolerance strategies see Lundholm (2006)
and McIvor and Lunholm (2011). Farrell et al. (2013b) combine physiological
traits and habitat template to identify candidate plants for Melbourne, Australia,
mainly from rock outcrops. A rigorous glasshouse stress-test methodology is used
to inform optimization of ecosystem services. ASTM E2400-06 Standard Guide
for Selection, Installation, and Maintenance of Plants for Green Roof Systems is a
useful, short overview and starting point when identifying matters related to
vegetation.
Most clients would like to see a year-round and dynamic aesthetic in the veg-
etation on an extensive living roof if it is visible from surrounding buildings and/
or at-grade. An ever-changing appearance may be achieved through plants' sea-
sonal color change, the structure of stems and leaves, variation of the plants'
height and width, and plant species' succession (if not actively managed). Sea-
sonal color change occurs with fruiting and lowering, as with ground-level
projects, but living roofs also typically change color with seasonal stress; green
spring and autumn growth lushes are typically separated by red, purple, brown
and yellows leaf colors during summer drought stress or winter dormancy (in
colder regions). The vegetation cover is a “living” building material, different
from the inert natural stone cladding of a façade. As a “living” material, vegeta-
tion weathers differently to hard landscape. Hard landscape materials weather by
changing their surface color. Lichens and mosses may stain a textured surface
(depending on the material itself) over a certain period of time. This kind of
weathering on a hard landscape can be removed by physical (chemical) cleaning.
By comparison, plants need a maintenance regimen corresponding to plant selec-
tion, in order to create dense, long-term plant coverage, and to protect speciied
plants from being overgrown by unwanted, invasive species. Such a maintenance
program was proposed in the concept design of the DaimlerChrysler Project at
the Potsdamer Platz, Berlin.
4.3.1 Plant site concerns
A “site” is where a living roof is located geographically and climatically. The geo-
graphic location, e.g., coast, prairie, rainforest, desert or mountains, together
with the plant hardiness zone deined by climatic conditions, acts as a guideline
for plant selection. Plant hardiness zones describe areas in which speciic plants
are capable of growing, including its ability to withstand the local temperatures
extremes (minimum and maximum). This can dictate the opportunities and limita-
tions of plants' contributions to the success of living roof project objectives. The
distinction between urban and rural contexts further deines location within the
different geographic regions. Even iner grain site contexts will inluence plant
performance, such as the elevation on a building. For example, plants will
perform differently if located on the irst versus the ifteenth loor, and are fully
or partially exposed to sun, wind and precipitation. Within a single roof, plants
near the corners and ridgelines (for a sloping roof) are typically exposed to
highest winds and greatest stress.