Roof Gardens

case-roof-gardens2It’s not an accident that Darling Park was so named, given that at the centre of this Sydney waterfront development lies a jewel of a roof garden. Created with some of the best talent around – Eric Kuhne & Associates, Site Image Landscape Architects, the Royal Botanic Gardens Sydney and The Lend Lease Design Group – from concept through to creation, these roof gardens were ambitious. Roughly ten years on, they remain a wonderful example of what can be achieved with the right expertise and a willingness to push the boundaries. In fact, many of the boundaries pushed have become today’s benchmarks.

Ross Shepherd recalls the brief – to develop a multi-dimensioned corporate roof garden with many functions. “It was to provide an outlook for the tenants; areas for lunchtime seating; spaces for use during functions.”

All this was achieved by creating a parterre which, when viewed from above, reveals itself as an astonishing flower. Each section within the flower gives Park users that chance to find seclusion, while a stroll around the perimeter takes you through a series of rooms, each distinguished by flora from the continents of South America, Europe, Africa, Oceania, Asia and North America. Winding its way through all flows a creek, broken here and there by falls which muffle noise from outside the Park and help build a sense of distance from the city beyond and the traffic below.

Probably the most challenging aspect of the project was the slab over which the roof garden was constructed. “The beams within the slab created planting pockets, the soil depths varying from 600 to 1200mm deep. Our task was to design a landscape without apparent interruption (by the beams) to the design of the garden.”

Also part of the team were Ian Innes and Bruce Rann from Sydney’s Royal Botanic Gardens. Ian recalls, “It was a site with a lot of constraints – quite hostile – but that was offset by the fact that we were working with Lend Lease who were open to our suggested solutions to the technical aspects. This was a project where the landscape was not a minor afterthought. It was the key element. The result is a long way from the space marked in green on the initial master plan.”

Bruce and Ian’s main task was to support Eric Kuhne’s plant designs with knowledge of how plants would likely behave in that particular harbourside site. But what any good horticulturalist knows, success comes from the soil up, and apart from the plant knowledge, it was Ian’s self-confessed interest in soils that proved invaluable.

“The garden is essentially a large planter box, sitting over a car park and Sydney’s Western Distributor. Our first task was to ensure good drainage, not only to reduce the potential load on the slab but to avoid anaerobic soil conditions. We then spent some time getting a specialised planting mixture concocted – light-weight, free draining, without too much organic matter to produce subsidence later. We explained to Lend Lease that it was important to spend time and effort getting the soil right – you don’t want to have to take it out.”

The final recipe included a high inert quotient (volcanic ash), pine bark nuggets for their ability to break down slowly, ground dolerite for good cation exchange – and TerraCottem. “Not knowing what the future maintenance would be, we specified TerraCottem, not only because of the hydrogel content and additional boost to cation exchange, but because of the slow-release fertilisers.”

The mix proved to be a performer – “Plant growth was a great success from the outset”, says Ian while Ross describes it as “a very high level of success of both the garden and the planting.” However it wasn’t an easy task getting it or the trees into position.

Ross explains. “All materials were brought in on conveyor belts, and the trees, which had been grown specifically in crates only 600mm deep, were planted by crane.”

“Post planting some trees failed, though this was well within the accepted range, and while we don’t have two Darling Parks to make a comparison, TerraCottem reduced requirements for fertilisers and watering — it was an integral part of the solution to on-slab planting.”

From Ian’s perspective, success came through paying attention on two fronts; appropriate plant species and a good soil mixture. Ten years ago, TerraCottem was chosen to be part of the mix, and it looks as though the decision was a good one.

To read more about the use of TerraCottem as a soil conditioner in roof gardens, click here.