Getting green in the ground – and on the roofs and walls

While consulting arborists and tree growers are finding their industry is expanding, Stephen Fitzgerald, Director of Arboriculture Pty Ltd, says there is still some way to go in terms of planning, architecture and even engineering treating trees as a normal part of the built environment.

Fitzgerald says it’s not unusual to find there are architects, landscapers and councils that are not aware there are Australian standards for retaining trees on a development site, for example. These standards have been in place since 2009. There is also an Australian standard for pruning trees that has been in place since 2006.

“It is not unusual for me to become involved with a project [as a consulting arborist] at a later stage when they are ready to go to DA, and I look at the plans and say, ‘That’s not going to work,’ because that tree [the plans include removing] has to be retained,” he says.

He says there is “ignorance in the community about trees”, and that a large part of the problem is the need for education about the need for trees.

There is also a degree of fear of trees for some people where the trees are close to people and buildings, or concerns around them dropping leaves and branches causing mess, or damage to property.

Fitzgerald says that, actually, the number of people killed by falling branches in urban areas including parks is so few, that is what has made those incidents so memorable for people.

Proper species selection is part of the solution, he says. River red gums, for example, will suddenly drop branches on a still, hot, summer day. So they shouldn’t be planted where people gather.

In terms of risks of damage to property from trees, that requires engineering solutions that plan for having trees near buildings. The same holds true for the way tree roots may interact with light infrastructure such as pavements, or subsurface infrastructure such as water pipes.

“It goes back to engineers making standards that allow for trees as a normal part of the urban environment,” he says.

Fitzgerald says in Chicago, for example, research showed that having trees near roads and carparks extended the life of asphalt with their shading. They also absorb the volatile organic compounds and other fumes emitted by parked vehicles. So planting trees in carparks has been recognised as a smart move.

The hard-working life of city trees

For the trees, the urban environment is not always an easy one. Fitzgerald says the horticulture industry calls the soil beneath the streets in Melbourne, for example, “Garbic” soil.

“It’s terrible,” he says. It contains fill, old bits of pipe, old bits of asphalt, concrete, road base, silt and all manner of other undesirables.

“There are tree species that will handle that,” he says.

These robust species will grow so long as there is soil, and air and water getting to the root system, he says.

In return, trees will absorb much of the water from rainfall that infiltrates the soil and then transpire it out through their leaves, reducing runoff at ground level and adding cooling to the air around them.

“We need more designs that take that into consideration,” he says.

Water Sensitive Urban Design principles and passive irrigation projects are both elements that can be incorporated into planning, whether it is a plan for a city street, or a plan for a development site.

In terms of greenfield developments, the trend of creating tracts of housing where there has been no holistic planning to allow for trees does not make for sustainable homes for people living there, Fitzgerald says. The lack of trees means greater expense for heating and cooling, for a start.

He says developers should get consultant arborists involved at the early stages, to work out what species will tolerate the local conditions best and work within the site’s constraints and the architectural vision.

Getting the green on buildings

Mark Paul, founder of The Greenwall Company, says that where a site has a ground surface, planting trees and climbers in the ground is the simplest, most economic way of adding green to the urban environment. Where there is no ground space available, green roofs and walls are the way to go.

There are lightweight options, he says, as an alternative to some of the podium-style plantings seen in places like Singapore. There is extra cost in terms of concrete and steel to ensure the necessary strength in the structure to support the weight of a sky garden or podium landscape.

Paul says the big challenge in Australia is scale in terms of vertical or roof greening.

“Why don’t we talk 50 hectares?” he says. “Getting that sort of scale would make it cheaper.”

He said that where housing is dense, there should be green spaces between them.

“It needs to be part of the architecture of the building, and integrated right at the start [of the design],” Paul says.

It’s a case of needing to think in terms of simple ecological systems even for detached houses – rain falls across the roof, that rainwater should be running across and into a green roof.

“You start at the house scale, then [look at] the site scale, then the urban scale, including rain gardens and wetlands,” he says.

Paul says that if you look at high-rise buildings in many places, you will see pot plants in windows and on balconies. Design and development needs to be accommodating that urge people have to live with plants.

“I have done so many balcony green walls for gardeners who like plants but go away a lot, so a green wall is easier for them to maintain than plants in pots,” he says.

Greening on a common wall that runs between balconies and apartments could become a green spine down an apartment building.

That’s the scale we need to look at, he says, and it needs to be included in planning for infrastructure as part of water recycling and stripping nutrients from runoff to protect water quality.

Green walls can also replace numerous other products that would otherwise be needed for cladding on that part of the building, he says.

They can also reduce fire risk, as the growing medium – which often includes lightweight polystyrene waste – when it is wet and bound up in plant roots, is completely inflammable. Some species are also natural fire retardants, for example succulents like pigface.

Green walls also improve noise reduction between apartments and inside them, as they have an acoustic damping effect. They also insulate, holding warmth in winter, and shading in summer.

A green roof can replace materials otherwise needed for roofing, such as ballast to protect waterproofing membranes.

Another aspect, Paul says, is that in some council areas, including eight in Sydney, green roofs and walls are recognised as green space. That means a developer can maximise floorspace, instead of needing to set aside ground area for green space where it is required. There is a dollar value for every metre of floor that can be developed instead of allocated to open space.

Green walls also meet the Green Star requirements for points for indoor plants, and adds a high level of amenity to a building both internally, and where occupants look out across other buildings with facade greening.

“Singapore makes money out of its greening,” Paul says. For example, all the properties surrounding the Botanic Gardens have quadrupled in price. The government is earning around $20 billion a year overall, he says, from property investors’ rates and taxes.

Wellington City Council in New Zealand is also undertaking green wall projects on three laneways and the library to create greater amenity near the foreshore. In Auckland, Paul says his company is working to create a “language” of green for the city using green walls above building awnings. Another city he works for, San Francisco, is investing billions in green infrastructure including green walls.

In Sydney, one of the most distinctive features of the One Central Park development is the facade greening. A case study for the 202020 Vision says the dramatic vegetation has benefits including aesthetics, cooling and reducing the burden of pollution.

In cities like Melbourne, Paul says green walls and roofs need to be incentivised. At the moment, he says, it still has no specific and communicated financial value.

CSIRO research scientist Dr Brenda Lin says urban greening has an important and not yet fully understood role in improving the resilience, sustainability and health of cities.

“It’s important not to discount that role just because we don’t have a full economic accounting of its benefits,” she says.

Dr Lin says research investigations are currently underway into whether some types of urban greening are more effective than others. It is research many local governments really want.

Green infrastructure also needs to be planned for the environmental conditions of the specific city location, she says. “Many local councils are challenged with choosing the right trees for now and for future conditions.”

Changes in precipitation, temperature and wind strength are among the factors that can affect the long-term success of urban greenery, as are changes in the urban form, drainage and run-off.

“Research that started to breakdown the types of contribution of different types of green infrastructure is highly needed to help councils maximise their investment in green infrastructure,” Dr Lin says.

“Different types of vegetation structure will have different types of benefits to the environment and to the community. Identifying what the council wants to achieve and understanding what type of GI will help them achieve that goal is of utmost importance.”