Planning considerations - Living Roofs in Integrated Urban Water Systems - page 69

growing media layer can be installed (i.e., retroit) it is best to simply use the
hardiest species, which to date seems to mean sedums.
This strategy differs from information available in the literature to date.
Dunnett and Kingsbury (2008), for example, suggest two design scenarios for
non-native and native plants. In one scenario they propose, native species are
planted, which in most cases beneits wildlife and biodiversity. The second
strategy restricts the plant palette to introduced species, which have been
cultivated locally for a long time (Dunnett and Kingsbury 2008). This is a viable
alternative, as these plants will have adapted to local climate and soil conditions.
This palette will include plants from the following habitats: mountain, high-
latitude environments, coasts and semi- desert (Dunnett and Kingsbury 2008;
Snodgrass and Snodgrass 2006). Additional information on plant selection is
provided in Chapter 4 .
3.4.3 Supply form: cuttings, plugs, mats and lead time
The installation completion time of the living roof's growing medium will dictate
what planting method can be used, as it also affects establishment (growing)
time. Cuttings (small pieces of a plant, large enough to develop roots) and plugs
(cuttings with an established root system) are the most common and tested
planting methods of extensive living roofs (Snodgrass and Snodgrass 2006). Gen-
erally, the establishment period for plugs is a bit longer (about 1-2 months,
depending on the location) than that of the cuttings (which occurs from the last
frost in spring to late summer). Each species has a unique establishment time,
which is dependent on a number of factors, including locational and regional
climate, as well as the plant's classiication as native or imported. If a precise
planting pattern is prescribed by the landscape architects, plugs can be distrib-
uted more precisely than cuttings, as they are larger.
Cuttings are often used to propagate pre-grown “mats” intended for full cov-
erage and instant aesthetics. Mats are usually made from coconut coir or jute
fabrics, with an embedded plastic mesh and a minimal amount of growing
media. The mat is rolled like a carpet or sod, which enables rapid installation.
Supply may require ~6 months' lead time, but a mat should arrive on-site within
day(s) of installation, as extended rolled storage damages plants. Not all plants
will propagate from cuttings, so plant selection should be thoroughly researched
to ensure a suitable match for the conceptual design intent.
Nursery containers are very rarely speciied, as the root systems of the plants
appropriate for container growth are, in most cases, too deep for the shallow
media depth on an extensive living roof. In some cases, container plants are
grown for living roof installations speciically. These varieties may be appropriate.
They are also sometimes used as accent plants for aesthetic purposes, or in light
forms of urban agriculture, such as culinary plantings.
Mats and containers create extra weight on the roof. They can also introduce
unspeciied and unwanted growing media and unwanted seeds and plants.
Cross-contamination can occur simply by mixing the mat's growing media with