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

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/