Planning considerations - Living Roofs in Integrated Urban Water Systems - page 67

roofs (see Section 4.3.4 ) can change color dramatically as different species lower
yellow, white, pink or red, and leaf color changes from green to red or purple. 5
Aesthetics also change with the closeness of view, and with the person viewing
the roof.
The types and frequency of maintenance also affects aesthetics of a roof.
Removal of dead lower stalks and leaves can expose underlying fresh growth.
For example, planting plans can reveal successively taller lowers as spring
progresses to late summer. In this way, the senescent lowers of early-lowering
species are hidden by later-lowering plants. 6 Maintenance of planting patterns
established using single species or cultivars requires removal of non-conforming,
adventive plants. More resilient patterns can be based on plant height supported
by variation in moisture supply or depth of growing media.
3.4.2.4 Plant biodiversity
Living roofs today should consider biodiversity objectives in their design concept
phase. Roofs with a diversity of plant species present a varied resume of beneits
to the environment, such as a more favorable climate for the sustenance of
invertebrates and vertebrates (Lee et al. 2014). The biodiversity of interest should
be speciied if possible. In most climatic contexts, it is easier to attract native
insects or honeybees than to cultivate native plants, particularly for thin, highly
drought-stressed living roofs. Certain plant species do well because of their natural
growing environment in dry, shallow media or even on rock outcrops (Farrell et al.
2013). As Farrell et al. (2013) caution, selecting plants solely on their drought-
tolerance may be an attractive solution, but consideration of more diverse plant
selection is recommended. This idea is widely championed (Brenneisen 2006;
Dunnett and Kingsbury 2008; Gedge 2003; Snodgrass and McIntyre 2010) with
Dr. Stephen Brenneisen as one of the pioneers and leading proponents of living
roof biodiversity. His research has focused intensively on invertebrate and plant
communities on living roofs in Basel, Switzerland. Brenneisen's designs (Brenneisen
2006) have been adapted in the United Kingdom by Dusty Gedge in London
(Gedge 2003), Dr. Nigel Dunnett, and others in Shefield to encourage the use of
living roofs as environments for increasing biodiversity (Dunnett and Kingsbury
2008). Their designs have built upon Brenneisen's primary principle of varying the
depths of media, by varying particle size and chemical composition, as well as
proximity to drainage features. Gedge and colleagues at the London Biodiversity
Partnership found local plant and insect biodiversity was enhanced if substrates
were stored at ground level before installation (Gedge 2003). Application of
techniques to enhance biodiversity is demonstrated in many small-scale living
roofs (Dunnett et al. 2011). Common techniques include the use of local plants as
well as devices that trap water, logs, stones or wooden features to increase habitat
diversity, particularly for insects. However, as Snodgrass and McIntyre (2010) warn,
designing habitats on roofs is challenging, as the designers must have a
comprehensive understanding of the wildlife intended to inhabit that space.
Furthermore, each native green roof is unique (2010).