Gardening Australia

Series 34 | Episode 02 - Broadcast Fri 17 Feb 2023 at 7:30pm

We're in Barangaroo, in Sydney's CBD. It's hard to think of a location more central than this, at the city's western, waterfront edge surrounded by gleaming skyscrapers, shopping malls, crammed streets and a casino.

We’re here to visit the roof of Daramu house, a 7-story office building that’s been built with sustainability in mind (it’s the tallest wooden building in the southern hemisphere). As is increasingly common in new buildings, there’s a large array of solar panels on the roof. What’s less common is these solar panels sit on top of a green roof garden.

Dr Peter Irga is a researcher at the University of Technology Sydney and has been leading a research project attempting to quantify the various benefits the green roof offers to the building and the surrounding environment. Importantly, there was a very similar building adjacent to Daramu house without a green roof, allowing for explicit comparison. "For the first time we had the opportunity to compare these two buildings against each other”

“We wanted to quantify 5 different things, to help know how to use plants in urban areas. They were urban biodiversity, air pollution, insulation, stormwater retention and stormwater pollution and impacts on renewable energy”.

Solar Panels

The comparison building has an equivalent solar panel system, minus the plants underneath.

The study compared how much energy the two solar systems produced over an eight-month period. What they found was that the "green roof" improved performance by as much as 20 per cent at peak times. This excess can then be sold back into the grid as renewable energy. This translates to generating an additional 9.5 MWh or $2,595 worth of renewable energy over the 8 months.

“It might not sound like a lot but with solar panels at the moment to even get a one or two per cent increase is a big deal. People will upgrade entire systems to gain an increase of only a few percentages."

Peter says that solar panels work better when they weren’t facing high temperatures common over summer; “Temperatures above 25 degrees make photovoltaic panels less efficient”. Sun is good, but there it can be too much of a good thing!

The plants work to cool the surroundings of the solar panels by releasing water vapour as they photosynthesise. By acting as a layer between the sun and the structure of the building, the plants also lower heat absorption into the structure of the building itself.

But the benefits of the garden don’t end with the efficiency gains for the solar panels. The garden has also brought in a range of other benefits Peter and his team were able to measure.

Biodiversity

The team wanted track just how much biodiversity the green roof was attracting versus the empty control, so set up pitfall traps for insects and wildlife cameras for birds. The resulting data showed a nine-fold increase in insect species diversity, as well as a four-fold increase in bird diversity.

Pollution and Stormwater

Samples taken from water running off the roof show a reduction in heavy metals like nickel, lead and cadmium compared to water that didn’t pass through the garden, suggesting the roof is filtering and absorbing these pollutants before they make their way out. “Ordinarily these would have gone straight into the catchment, into the harbour…it’s a solution, a use for plants for high levels of pollution”.

Another benefit of having all the plants up there is water flows are slowed and reduced, as the water passes through roots and substrate instead of sheeting off flat concrete. This in turn lessens the strain on urban drains, helping to lessen flooding events. “You get a slow release like a sponge. We were looking at 700 litres per second, like a once in 5-year storm. The green roof gets the runoff down to 7 litres per second”.

Plants

The garden’s done all this with a mixture of local native species, and some exotic succulents (chosen for their ability to flower in winter and complete photosynthesis at night). As Clarence would know, plant selection had to be adaptable, as there’s only 12cm of substrate to grow in up here, and they’re exposed to the harshness of the urban environment. Standouts include:

• Goodenia ovata-Adaptable woody shrub covered in small yellow flowers, flowering for close to 8 months out of the year. Magnet for invertebrates.
• Myoporum parvifolium-Woody groundcover that’s a favourite with gardeners for its fast-spreading nature and reasonably tough temperament.
• Dianella caerulea-A hard to kill clumping perennial plant, this would be suitable as it’s unlikely to get over a metre tall and therefore won’t obscure the panels. It has sprays of small blue flowers in spring and summer (likely attracting the bees), followed by blue berries (may be attracting fruit eating birds like currawongs.)