Is it Time to Rethink Green Roof Planting?

      Latest research suggests sedums may not be best

This is the first Guest Blog from Tijana Blanusa,  a  researcher  at the RHS who specialises in Sustainable Gardening.

I recently published the results of my research that sedums may not be the best performers for helping cool air temperatures in the online journal Building and Environment.

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.

Based on the results of this work, I would suggest that choosing which plant to use on a green roof should not be decided entirely on what survives in a shallow substrate. 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.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:

¹ “Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer better ‘cooling service’?” by Tijana Blanusa, M. Madalena Vaz Monteiro, Federica Fantozzi, Eleni Vysini, Yu Li and Ross W.F. Cameron. 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.

About the RHS

The Royal Horticultural Society is the UK’s foremost gardening charity, helping and inspiring millions of people to garden. They operate via gardens and shows and through  scientific research, publications, libraries and our education and community programmes. It is entirely funded by members, visitors and supporters.

Tijana Blanusa, the author of this blog, is pictured here on the Environmental section of the RHS stand at the Chelsea Flower Show.

RHS membership is for anyone with an interest in gardening. Support the RHS and secure a healthy future for gardening. For more information call: 0845 130 4646, or visit www.rhs.org.uk

About Fundacao para a Ciencia e a Tecnologia (Portugal)

 The Foundation for Science and Technology is the Portuguese governmental institution responsible for financing and evaluating scientific and technological knowledge, and aims to improve education, health, environment and the quality of life of the general public. Funding is given, subject to evaluation on merit, to proposals presented by institutions, research teams or individuals, and also through co-operation agreements in partnership with universities and other public