5 Jan 2013

Is it Time to Rethink Green Roof Planting?

The Royal Horticultural Society last October sent me the press release below about rethinking green roofs - it sadly came to late for my shed roof.... a couple of years ago I seeded the roof of a small shed with wildflower seeds - see my blog here: http://ruscombegreen.blogspot.co.uk/2010/06/making-green-roof-on-my-shed.html

Well drought had a big impact on the flowering the year before last and it was soon looking more brown than green although there was a late flowering into the Autumn that looked great.....it was then I decided to try sedums - some of which grow very rapidly - so last year I planted a few and very quickly nearly the whole roof is covered....see photo yesterday...it looked great when they were all flowering.....it was then I got the press release below questioning whether sedums are the best for roofs....of course on such a small area it's not significant.

Anyway the green roof has improved the view from my window - much better than just a roof! See also my film of the Eco-Renovation Open Homes workshop in Nailsworth that I filmed:

Is it Time to Rethink Green Roof Planting?

Latest research suggests sedums may not be best

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."

Notes to Editors:

1 "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. The report can be found at - www.sciencedirect.com/science/article/pii/S0360132312002132.

2 Gill SE, Handley JF, Ennos AR, Pauleit S (2007) Adapting cities for climate change: the role of green infrastructure. Built Environment 33; 115-133.

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