Islands in the sky used as Noah’s ark for threatened plants

The fate of some plants is up in the air

By Alice Klein

It’s a case of lofty living meeting Noah’s ark. Gardens atop city buildings can act as refuges for threatened species and help plants colonise the surrounding landscape.

For the last six years, a team of Australian conservationists has been growing critically endangered native plants on the roofs of buildings in Melbourne. The plants form unique communities on the volcanic plains of Victoria, but are in severe decline because of agriculture.

One of the major benefits of roof gardens is that threatened species don’t have to compete with plants found at ground level, says project leader Nicholas Williams at the University of Melbourne. Moreover, there are no snails or slugs to eat them.

Elevation is another advantage, because the seeds of the endangered plants can drift off in the wind and take root in the wider landscape, Williams says.

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Watered by the rain

It was in 2010 that the team began growing two native grass and 27 wildflower species from the volcanic plains in four 18-square-metre plots on top of Melbourne’s Pixel building.

Instead of soil, a lightweight substrate of volcanic rock was used to improve drainage. Initially the plants were watered to help them establish, but subsequently they survived purely on rainwater.

Some of the species thrived and spread across the gardens, while others survived but did not spread. However, there were plants – particularly the smaller species – that did not establish themselves.

This suggests that some plants are better suited than others to the urban roof environment, says Williams, who will present the findings at the Australasian Plant Conservation Conference next week.

The next step will be to determine how well the roofs perform as seed distributors. The team will test this using a threatened native fireweed, Senecio macrocarpus, recently planted on a roof at the University of Melbourne.

Bouncing back: Noah’s ark island is saving vulnerable species in Australia

Conservation efforts should still primarily focus on maintaining plants in their original environment, but green roofs provide useful back-up, says Williams.

They are also a cost-effective way of improving storm-water absorption, small-scale carbon capture, and providing insulation that reduces the air conditioning needs of rooms below, he says.

Andrew Lowe at the University of Adelaide, Australia, says that using green roofs to conserve threatened plants is a great idea, although not all species will be able to survive the harsh wind and sunlight on tall buildings.

“The other thing is that rooftops are increasingly being used for solar panels, so there might be a bit of a conflict between conservation and renewable energy,” he says. “It’s becoming pretty hot real estate up there.”