Green Roofs, Green Walls: The Urban Gardens Of The Future

Whether they are chic sustainability, fashion fads or Babylonian myth, gardens on walls and roofs are popping up everywhere in our cities.

In March this year at the Melbourne International Flower and Garden Show there was a highly popular pop-up garden display by the Melbourne School of Land and Environment highlighted by a freight container with a most impressive green roof.

Writing in The Conversation to coincide with the event Senior Lecturer in Urban Horticulture at University of Melbourne John Rayner said green roofs include a range of different types.

“They can be small, container beds two metres deep supporting trees and shrubs, or lightweight configurations of only a few centimetres in depth and more than a hectare in size covered in succulent and herbaceous vegetation.”

Green walls, on the other hand, fall into two very different types. 

“Living walls support plants through irrigated, vertical containers or felt-based structures fixed to a wall surface. Green facades use climbing plants to provide green coverage over a wall, either directly on the building surface or more commonly using a steel trellis or cable system, with plants grown in-ground or in containers that are supported across the building facade.”

CSR’s Scott Clarkson says green roofs, walls and facades are appealing as a way of adding green infrastructure to a city because they can be included on new buildings or retrofitted onto existing buildings and require little, if any, space at ground level.

“Vertical gardens and green walls are not just a stunning addition to an office. Plants have been shown to lift people’s mood, increase productivity and remove toxins from indoor spaces,” Clarkson says.

The advantages of green walls are numerous including helping to reduce the heat island effect in cities and urbanised areas. Research in Canada suggests a reduction in the ambient air temperature of 2°C is possible if green roofs are implemented across the city. A short stroll on a hot day through Sydney’s Botanical Gardens adjacent to Sydney’s CBD confirms this as it can often feel up to 5 degrees cooler.

Clarkson says green roofs may be constructed for many various reasons – as recreational spaces, architectural features or to achieve particular benefits such as:

  • Stormwater capture and retention
  • Improved species diversity
  • Insulation of a building against heat gain or loss.

“However green walls don’t always reduce the heat load on the building as many of their proponents may claim. Savings in energy use through heating and cooling efficiencies are often predicted; however the thermal behaviour varies with moisture content, spread of foliage, seasonal conditions and orientation. Green walls and roofs are therefore considered transient and not an integral part of the building fabric,” Clarkson warns.

“As such the specification of the building envelope is required to comply with building codes – in Australia Section J. Insulation and glazing specifications must therefore assume there is minimal thermal benefit or shading provided by a green wall or roof, since many of the benefits won’t be realised until many years after completion when plants have matured.”

Maintenance programs also affect how buildings perform and it’s no different with greenery.

“The ongoing maintenance required to keep the garden looking fresh is critical and when that garden is hanging 100 metres above the streets it’s not a simple matter of putting garden shears in your pocket and trimming a few stems after work,” Clarkson says.

At the Melbourne Garden and Flower Show, Dr Claire Farrell from the Melbourne School of Land and Environment said green roofs are commonly planted with succulent plants, because of their drought tolerance.

“They require very little water, which makes them great survivors of drought, but not very good at reducing stormwater runoff.

“When we tested different green roof plants, some of the rocky outcrop plants used twenty times more water than the succulents, when water was available, but could still survive long periods of drought by essentially shutting down.”

“What you really need, are plants that are both drought tolerant and high water users like the ones that grow on rocky outcrops in Victoria. It may seem improbable that a plant that uses a lot of water could also be drought tolerant. But these ensure optimal performance and resilience on green roofs,” Dr Farrell said.

John Rayner agrees green roofs and walls need to be planned and designed properly to ensure they are successful.v

”The most significant issue to consider is weight loading. A roof or wall must have the structural capacity for the mass of a green roof, wall or facade installation – both at construction and over time,” he wrote in The Conversation.

Scott Clarkson says failures due to weight loading overseas have often been because of a retrofit where the wall structure was never designed at any stage to take into account the actual loading of the green wall.

He agrees caution is therefore required when designing a building with a green wall or a green roof.

“Another key risk is how the building will perform if and when the plants begin to die and or the support structure degrades. The building’s design and the mechanical services rely upon the building fabric to perform in a predictable manner.

“Insulation materials need stability in performance while glazing systems need to provide natural daylight while resisting the weather. If all the windows are overgrown the internal spaces within the building are likely to appear dark and enclosed with little natural heat gain during colder months.”

Clarkson says he loves the look that a well-designed, constructed and maintained green roof and green wall can bring to a building and its surroundings.

“We just need to ensure they are not treated carelessly as ‘the great green hope’ for the future of our cities – we’ve all seen what’s happened to the many roundabout gardens in our suburban streets, without that proper design and maintenance.”

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