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green roof systems

Sedums have been a popular choice for manufacturers of green roof systems, but research from the Royal Horticultural Society (RHS) suggests that it may be time to look at other available option.

Researchers in the latest edition of the online journal Building and Environment are suggesting that sedums may not be the best performers for helping cool air temperatures.

The research1, carried out with funding from the Royal Horticultural Society (RHS) and Fundacao para a Ciencia e a Tecnologia (Portugal), looked at the possibility of using different plants for green roofs. The most popular currently used is Sedum but the researchers also looked at Stachys byzantina, Hedera hibernica and Bergenia cordifolia.

Enhancing a city’s green infrastructure is often considered a means to help address a number of environmental problems associated with built-up areas. It is now accepted that air temperatures in urban areas are higher than in surrounding rural areas, a phenomenon called the ‘urban heat island effect’.

This increase in air temperatures is largely due to vegetation being replaced by dark and impervious surfaces. Increased vegetation can, therefore, help reduce urban temperatures and also reduce the energy needs of buildings through their insulating properties. In Northern Europe vegetation is considered vital to reducing air temperatures on a city-wide scale.

The research looked at three key factors:

·                     the effect of water availability on each of the species’ and leaf-surface temperatures;

·                     the ability of each type of plant to reduce air temperatures above the canopy; and

·                     the effect of these plants on ground cooling, and therefore potentially on the cooling of the building.

The research showed significant differences in the leaf temperatures between the plants. Sedum byzantina, for example, had the lowest leaf-surface temperature when exposed to high air temperatures on clear sunny days.

“We would suggest, based on the results of this work, that choosing which plant to use on a green roof should not be decided entirely on what survives in a shallow substrate,” says RHS scientist Tijana Blanusa. “Building designers should give greater consideration to supporting those species that provide the best all-round environmental benefits. This may mean introducing some form of irrigation system and deeper substrates to grow in – which in turn will have an effect on structural-strength decisions.”

Previous research in the UK, based on model predictions, has shown that increasing green space such as parks, gardens and green roofs by 10 percent would reduce summertime air temperatures in the region of four degrees2.

With the climate getting warmer, gardeners and architects will play an even more important part in helping reduce the effects. “Getting planting right in urban spaces, which can be very limited, is particularly important,” says Tijana. “But the advantage is that it not only can have a major effect in helping reduce urban temperatures but will also provide other environmental benefits – such as increased biodiversity and the collection of excess intense rainfall, thus lowering flooding risks.”