The subject of roof gardens is very topical at the
moment so I thought I would look at them in some detail. The concept of
a roof garden is not new – those of you with European ancestry will know
that the roofs of huts were covered by our ancestors with turf thousands
of years ago! In Europe too, roof leaks were plugged with sempervivums
hence their common name of houseleeks! Roof gardens are now being hailed
as a way to reduce the urban heat island effect, absorb water and
pollutants and provide an aesthetically appealing place for recreation.
Even looking at a roof garden can ease the stress of working in a built
environment.
My interest in roof gardens was sparked over a decade ago when I saw a
picture of a rural letterbox (as big as a fridge) in New Zealand with
its roof covered in growing plants. I immediately wanted to do that to
my letterbox but at the same realising that none of the plants on the NZ
letterbox would grow on a letterbox in my neck of the woods! I would
have to use something entirely different. I experimented with a range of
small succulents placed into a shallow punnet tray full of regular
potting mix and over the last 12 months to 2 years I have found that
some succulents are hardier than others.
This is exactly the type of work Dr Nick Williams, John Rayner and their
associates have been doing at the Burnley Campus of the School of Land
and Environment at Melbourne University.
They have been and continue to
carry out research on roof gardens that work in our climate. You might
have seen the installation of a roof garden at Burnley featured on
Gardening Australia last year. The team have been trying to find out the
best method of creating a roof garden for extensive green roofs. An
extensive green roof has a shallow substrate (soil) – between 5 and 20cm
whereas an intensive green roof has a deep substrate – between 20cm and
2m in depth.
The long and the short of the experiment at Burnley is that the
composition of the substrate and the choice of the plant species are
crucial to its success. You can’t just put any old soil or potting mix
on a roof in Australia and expect it to support plant life indefinitely
on only what falls out of the sky. The Burnley team has discovered that
the substrate has to be carefully designed and must contain mineral,
organic and synthetic components in order to work. For instance sand
works really well in a substrate to help with drainage but it is too
heavy. Perlite helps with drainage too, but is so light it simply blows
away. The team have discovered that fly ash (a by-product of the steel
industry) actually works very well to help drainage and is not too heavy
and not too light.
Like me the team has also discovered that there are drought tolerant
plants and then there are drought tolerant plants! Most of the plantings
at Burnley failed after the scorching temperatures of last summer – only
the wallaby grasses (Austrodanthonia spp.) and some of the succulents
recovered after the first autumn rains: notably the jellybean succulents
(Sedum spp.) and a little known succulent called Oscularia deltoides. In
my little experimental plantings in a tray at home it was also the
jellybean sedums that came through. Sedum acre is traditionally used in
this situation and on rock walls in Europe yet it failed miserably.
Future work may involve looking at Australian native succulents which
sounds like a very logical and exciting step. It is discoveries at
Burnley that are paving the way for a flourishing roof garden industry
in Australia.
While in San Francisco last year I visited the newly re-opened
California Academy of Sciences specifically to check out its roof
garden. The Americans seem to be well ahead of us when it comes to roof
gardens and this garden was 15,000 square metres of well researched and
well constructed roof garden of the extensive type. The plants were
growing in a (secret) substrate recipe in shallow trays made from
coconut fibre. Only four species were planted with a further 5 species
added as seeds. It seemed to be working very well but I had to remind
myself that their summer heat is mitigated by the daily blanket of fog
that waters the plants almost effectively as a fall of rain. The same
plants would not work in Melbourne.
Another experiment on a roof garden in the centre of Melbourne saw
phormiums (New Zealand flax), echeverias, senecios and sedums used. Can
you guess which species failed first? Yes it was the phormiums. This
genus grows in swamps in their native environment and although they are
almost indestructible in many situations, a Melbourne rooftop is not one
of them. The echeverias failed too and although the senecios and sedums
looked very sad in late summer, again the first of the autumn rains
helped to revive some of them.
So roof gardens are a great idea but more Australian based research is
needed before this type of gardening becomes a sustainable cost
effective option. Meanwhile don’t let that stop you from carrying out
your own experiments on your own letterboxes!
Related info:
Check out the information in this issue on a
Melbourne CBD balcony
garden for employees
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S.F. Roof Gdn X section: A
diagram showing a cross section of the roof garden at the
California Academy of Sciences showing there is a lot more to
this roof than bunging a few plants on concrete! |
S.F. Trial Roof Trays: The coconut coir trays in which they grew
the plants for the California Academy of Sciences. |
My Roof Gdn Experiment: This experimental tray
of mine holds only a couple of centimetres of potting mix. After
several years the jellybean sedums are still doing okay as is a haworthia, a corpuscularia, a tiny bit of a senecio and an
echeveria. The tray started out with over 10 different species
of succulents. |
Links:
http://www.abc.net.au/gardening/stories/s2400152.htm
http://www.calacademy.org/academy/building/the_living_roof/