Nature’s AC (Image: Ray Tang/Rex Features)

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Would you decorate your roof with lamb’s or elephant ears?

It’s not an unusual form of taxidermy – these are plants, and some of the best suited to coat our roofs and walls and so make cooler, greener cities.


Green roofs help to reduce the heat island effect in towns and cities because plants absorb less heat than concrete and can also cool the air via the process of evapotranspiration. This can save energy by cutting the need for air conditioning on hot days. What’s more, they reduce the risk of flooding by absorbing water and, of course, they absorb carbon dioxide.

Most existing green roofs use various species of Sedum, because the plants can survive without rain for long periods, meaning they require little maintenance.

But Tijana Blanusa, a Royal Horticultural Society researcher based at the University of Reading in the UK, wanted to know if Sedum really is the best plant for the job.

The ears have it

She compared a variety of plants, including a Sedum mix, lamb’s ear and elephant ear, to see if differences in leaf shape and structure would make a difference to the temperature of the air above them.

She found that lamb’s ear, a silvery, hairy-leafed plant, had the consistently coolest leaves over a two-year period. “Even when it is really stressed, and the leaves of other plants get a few degrees warmer than when they are watered, the lamb’s ear manages to keep its leaves cooler than those that don’t have hairs,” she says.

What’s more, when she measured the air temperature 20 centimetres above each plant, she found that on the hottest summer afternoons the air above lamb’s ear was also cooler than above the other plants.

She will present her findings at the World Green Roof Congress in London tomorrow.

Flood failure

In cities like Austin, Texas, that are hot and prone to flash floods, Sedum is a poor performer, says Mark Simmons, an ecologist at the University of Texas at Austin.

When Simmons compared six green roofs in Austin planted with a range of species, he found that the city’s mix of long periods of heavy rain followed by stretches of drought was too much for the Sedum, causing it to rot away.

All the plants he tested tended to cool the air around them, but grasses such as big bluestem and maize performed best at absorbing rainwater, he says.

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