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Sedum may not be the best choice for green roofs
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It could be time to rethink the species chosen for green roofs, according to the results of new research carried out with funding from the Royal Horticultural Society (RHS) and Fundacao para a Ciencia e a Tecnologia (Portugal). The study investigated the properties of several different plants suitable for green roofs.
Sedum may not be the best choice for green roofs

The most popular current choice is Sedum but the researchers also looked at Stachys byzantina, Hedera hibernica and Bergenia cordifolia.

Enhancing the amount of green space in cities is acknowledged to be helpful in addressing a number of environmental problems associated with built-up areas. As well as flash flooding due to a predominance of impermeable surfaces, air temperatures in urban areas can become 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 coloured surfaces which absorb the suns rays. Increased vegetation can, therefore, help reduce urban temperatures and also reduce the energy needs of buildings through their insulating properties. Even in Northern Europe vegetation is 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 results showed significant differences in the leaf temperatures between the plants. Stachys byzantina, commonly known as Lambs Ear, 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.

"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."

The researchers concluded that the choice of plant species on green roofs should not be entirely dictated by what survives on the shallow substrates of extensive systems, but instead that consideration should be given to supporting those species that provide the greatest eco-system service potential.

This might mean justifying the additional expense associated with providing a deeper substrate (such as a semi-extensive system) or even supplementary irrigation from a sustainable source.

Stachys outperformed the other species under test in terms of leaf surface cooling, cooling the substrate beneath its canopy and even during short intervals over hottest periods the air above the canopy, when soil moisture was not limited.

This had the effect of lowering air temperatures around the building envelope thus potentially reducing cooling demand and lowering energy consumption. The researchers say Stachys is unlikely to be as resilient as Sedum in terms of survival in the most-droughty, extensive, green roofs (e.g. 50 to 100 mm deep), but is a drought-adapted species in its own right, capable of survival and persistence without additional irrigation in semi-extensive (200 mm depth) systems within Northern Europe.

Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer better ‘cooling service'?" by Tijana Blanusa, M. Madalena Vaz Monteirob, Federica Fantozzic, Eleni Vysinib, Yu Lib and Ross W.F. Camerond.

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