Planning considerations - Living Roofs in Integrated Urban Water Systems - page 68

Taller, green herbaceous species may be seen in general as being more
biologically diverse and simultaneously most successful in providing a visual
reprieve from urban environments (Lee et al. 2014). As Lee et al. (2014) point
out, the association of height and green with biodiversity is a misconception. If a
primary function of a living roof is to provide both biodiversity as well as an urban
escape, it is possible to select species that are similar in appearance but
biologically diverse. The study implies that because categorizing certain types of
vegetation can be misleading, plant selection must be carefully researched and
an ecologist consulted, especially if biodiversity is a project goal.
The value placed on using local soils has been tempered with the dificulty in
ensuring such soils meet speciied drainage, stormwater retention and weight
characteristics, as well as the increased need for intensive early maintenance to
remove unwanted weed species. Hence, local soils are more typically used on
small, private, non-commercial living roofs or as part of a pre-vegetated sod. This
is another case where, given that speciic biodiversity is an objective, the
landscape architect or team lead should engage an appropriately-skilled
ecologist.
3.4.2.5 Native and exotic (non-native) plants
Due to constantly intensifying globalization, plant species are increasingly moved
around the world, creating a challenge for any living roof planning team to select
the appropriate species for the roof. Even more challenging in this situation is the
ability to discern between indigenous and non-indigenous species. The authors
propose consideration of several ideas.
Non-native, aka exotic, plant species may provide little biodiversity and habitat
creation. However, native plants do not necessarily mean they are better at
providing biodiversity, especially if they do not thrive in the thin growing medium
on the roof. Plant selection should depend on plant design objectives of the living
roof and assurance of a relevant maintenance regime. However, it is more
important to consider a plant species' adaptability and suitability for the harsh
growing conditions of the roof environment.
In the end, the availability of water and rooting depth are strong indicators of
whether any particular plant will survive. On experimental living roofs in
Auckland, New Zealand, where retroit conditions limited assembly design, the
shallow (50-70 mm) non-irrigated growing media sustained only one out of ten
native species originally planted in terms of number of plants and extent of
coverage (Fassman et al. 2010). On the other hand, at the same time and on the
same systems, a variety of sedum species produced greater than 85 percent cover
after only two years. Plant succession is inevitable without external intervention.
A seven- year study in Michigan, united States, found that the diversity of
succulents declined from 25 initially planted varieties to only seven varieties.
Succession was more rapid in 25 mm and 50 mm depth growing media than in
75 mm depth growing media (Rowe et al. 2012). For projects where a regular
maintenance regimen is not possible or feasible, and/or where only a very thin