In 1988, the City of Melbourne had about 40,000 inhabitants and 20 per cent green cover. In 2009, this was measured again and it was determined that the city’s population had increased to 100,000 people with green coverage reduced to a mere 13.6 per cent, according to Gail Hall, a primary author on The Growing Green Guide.

This dramatic loss of green space is still having severe implications for Australia’s most liveable city.

Hall notes that most of the lost green space was originally situated on private property. She adds that it’s critical that we look at ways to vegetate Melbourne’s built environment to combat the heat island effect – it can be six degrees hotter in Melbourne than it can be in the surrounding suburbs. Hall attributes this temperature issue not only to the loss of greenery, but also to a growing population of people, heat generation from cars, buildings and the sun absorbing into concrete.

“Other problems with density have seen this reduction in green cover and also removing biophilia– a human need for nature,” she said. “The logical progression is looking at horizontal and vertical surfaces – roofs and walls.”

Urban areas are looking to roofs and walls to bring greenery back into cities

The opportunities for green infrastructure are extensively detailed in the Growing Green Guide, Australia’s first guide to green roofs and walls.

Chair of the City of Melbourne’s environmental portfolio, Councillor Arron Wood, said the guide was produced to assist the private sector (planners, designers, developers and homeowners) in adding to the city’s green infrastructure and to encourage innovation in urban greening.

The guide mentions some of the key pressure points the city is experiencing where green roofs and walls could assist:

• Energy use: air conditioners in offices, industry and homes increase greenhouse gas emissions, while greenery cools buildings naturally
• Water quality: urban runoff collects pollution; green roofs and walls trap and use this runoff
• Green space: urban sprawl has threatened food production and reduces green space for recreation and biodiversity
• Air quality: dust and vehicular pollution are trapped air in urban canyons but can be “cleaned” using green roofs and walls
• Temperature: hotter cities can lead to heat-related illness and death, plus poor liveability. Once again, greenery has cooling qualities
• Flooding: extreme rainfall and fewer permeable spaces clogs drains and leads to localised flooding, which can also be mitigated through greenery

Despite the benefits of green roofs, a stigma remains that they are too hard to implement. Concerns include perceptions of high installation costs, maintenance and doubts over their effectiveness.

A series of Melbourne experts have offered their thoughts on exactly how the city can be greened in a viable and effective way. This discussion of green infrastructure will be aligned with activity in the City of Melbourne, including its CBD and surrounding suburbs such as South Melbourne, West Melbourne and others.

1. Extensive vs. Intensive

There are two types of green roofs according to the Growing Green Guide.

Extensive green roofs (also known as ecoroofs/brown roofs) are lightweight with a shallow layer of growing substrate of less than 200 millimetres deep, requiring minimal maintenance. They generally have lower water requirements and use small, low-growing plant species, particularly succulents.

In contrast, intensive green roofs (roof gardens, podium roofs) are generally heavier, with a deeper layer of growing substrate and support a wider variety of plant types. Because they can support a heavier weight, they are readily accessed by people and do require more irrigation and maintenance than extensive roofs.

The substrate depth is detailed in the chart below:

2. Amenity and purpose

Hall, who also holds a coordinator green infrastructure, urban landscapes and city design role at the City of Melbourne, says green roofs should be purposefully located.

 “A green wall or a green roof should be placed in a space where there is a clear need for those systems, one that can be met,” she said.

She referred to a catchment in Elizabeth Street, a natural low point in Melbourne’s CBD which is subject to flooding.

“If you have green roofs that can maximise retention in stormwater in areas where they can reduce flooding downstream like in Elizabeth St, it would maximise the value of the roof whilst also reducing flood risk,” she said.

A recent article by Warwick Savvas also explored this benefit of green roofs and spoke of the opportunity to offset the cost of contaminated water.

“A green roof will manage storm water without taking up valuable developable areas. It will also provide amenity and other financial benefits that include energy savings and protection from fire,” he wrote.

In the residential sector, Hall advises that not all apartments have ample balcony space for greenery, so a green rooftop can be a community amenity for residents as well as cooling the building.

Green roofs can also play a significant role in fostering biodiversity.

Click to enlarge

3. New vs. Old Buildings

Hall acknowledges that it’s usually easier to place a green roof or wall on a new building as it’s included in the design and specs from the outset.

“It’s also designed with ‘appropriateness’ that way,” she said. “With an old building, you’re limited by what construction materials that building was made of so you actually have to go back and look at what that building can take.

“Generally, we’ve found (working with the City of Melbourne) that there is a reduced number of old buildings that can already take the weight. However, we’ve also got many roofs that can take a shallow or lightweight green roof.”

Hall adds that intensive green roofs with deep substrate can’t really happen to old buildings without structural changes.

4. Plant Selection

Claire Farrell a lecturer in green infrastructure at the Burnley Campus of The University of Melbourne has spent the last five years researching the best plants for green roofs and Australian conditions.

“Succulents are what you would plant if you want plants that will both survive and use water when it’s on the roof – this will get the best stormwater benefit,” she said. “You want plants that can adjust their water use so they can absorb water and be drought resistant if required.

“Dianella and Leandra are a good option as well but you might need to irrigate them if conditions get dry.”

The Growing Green Guide also details the research Farrell is involved in and the plants that have survived the University of Melbourne’s Burnley Campus Demonstration Green Roof, which has a shallow substrate.

“Experiments on roofs and in controlled environments have quantified the performance of 56 different plants, including both Australian native and exotic species,” the guide states. “For survival in the absence of irrigation, plants with low water use and high leaf succulence, such as Sedum pachyphyllum and S. xrubrotinctum (both Mexican plants) proved to be the most successful.”

Hall notes special care must be taken when it comes to trees and safety at windy vertical heights.

“Initially you need to be able to anchor the tree into the green roof so it doesn’t potentially fly off the building. It’s certainly possible but needs to be sought off and designed,” she said. “If you are to provide a tree, you have to provide enough soil/volume so it can survive and grow to its extent.”

5. Installation and Maintenance Costs

“Some costs are real and some are perceived, and yes it can be costly,” Hall acknowledged.

In the industry, current estimates indicate that a vertical garden costs 20 per cent of the installation cost to maintain a year, but Hall believes the price of a green roof is subject to its design. Considered design can help ensure it is cost effective for the developer/planner and the client.

“The installation and the design/maintenance costs are higher in Australia than say in the US and Canada where more people have green roofs and walls,” she said. “It’s the law of economics, the more things you’re selling, they cheaper they are to install.”

Hall added that maintenance is a general requirement whether it be for a pool, sport amenity or surrounding garden in the built environment.

“I think it’s important that people consider maintenance; if you’re not doing it yourself, you have to be willing to pay for it,” she said. “There are going to be a lot of times that you’ll need to potentially replace plants.”

Maintenance can range from standard gardeners of volunteers to – in the case of some green walls – using a cherry picker or crane, or having someone abseil down the side of the building to maintain it.

“There’s no such thing as a maintenance free green roof,” Farrell added. “They’re growing, living systems so if anything perhaps pick plants that don’t grow too fast and don’t need cutting back.”

She also suggests planning plants closer together so there is less weeding involved. Generally, a green roof will be lower maintenance than a green wall.

6. Legislation

“At the moment I don’t know of any government across Australia to mandate green roofs,” Hall said.

She notes that along with the Growing Green Guide, Melbourne councils such as City of Yarra, City of Stonnington and City of Port Phillip have put together their own policy guides on urban green infrastructure.

However, it’s not a priority to mandate green infrastructure, according to Hall. The researchers are more interested in understanding the system before developing a policy.

“Legislation is not what we’re concentrating on right now,” she said. “We are providing information via the Growing Green Guide, providing research that indicates the best plants on roofs and best substrate soil to help green roofs perform the best way they can in Australia. We’re working with Melbourne Universities and other universities to obtain this research.”

Wood adds that Australia should get up to speed on the practise.

“Urban greening policies that mandate or incentivise the use of green infrastructure in the public and private realm have been embraced in cities around the world, particularly in Europe and North America,” he said. “The City of Melbourne is currently looking at policy options for encouraging green roofs and walls.”