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Green spaces contribute significantly to a reduction of soil and aerial temperatures during spells of hot weather, so contributing to human wellbeing. In the garden context, there is, however, little information as to what extent various types of plants differ in their cooling potential and how certain planting combinations may maximise cooling under a scenario of low rainfall and minimal water inputs.
From August 2008 to July 2009 we were recording aerial humidity and temperature at least hourly in several locations around RHS Garden Wisley. We monitored changes in: a woodland area; traditional border plants; lawn away from a water surface; lawn by a water surface; and paved area away from vegetation and in the proximity of a building.
From 2009 we started a series of experiments, in glasshouses and outdoors, with various types of plants subjected to water deficits, to characterise their ability to maintain aerial cooling under reduced soil moisture. In 2009 and 2010 we tested Sedum sp. (commonly used on green roofs), Stachys byzantina, Hedera helix and Bergenia cordifolia.
In January 2011 a PhD student joined the project investigating in more detail the role of particular leaf traits (hairiness, succulence, colour) in the regulation of leaf temperature and the leaf-air energy exchange. We extended a range of tested species within this project and included Salvia officinalis, Heuchera and Sempervivum cultivars alongside Sedum and Stachys. This project was completed in December 2014.
We are now working on summarising the data to provide recommendations on plant traits and species/cultivars which gardeners should consider when choosing plants to keep their green spaces cool. We have also secured additional funding (from the Climate-KIC programme) for another PhD student who is investigating means of sustainable irrigation of the plants that showed highest cooling potential.
The project will provide answers to gardeners seeking to maximise the cooling potential of their gardens during spells of hot and dry weather, by choosing the species that maintain maximal evapo-transpiration with minimal water inputs - those that shade the best or have the coolest surface.
We are focusing on plant species that can be grown in the garden, but are also looking for plants that can be suitable for use as extensive and intensive green roofs.
Conclusions to date
Our measurements at RHS Garden Wisley confirmed that vegetated areas had lower summer temperatures than the bare, built up surfaces. We were unable to distinguish consistent differences between the particular vegetation types in this layout, apart from consistently lower daytime temperatures and higher night-time temperatures in the woodland area compared to other surfaces.
In our controlled and outdoor environment experiments we consistently observed lower leaf surface temperatures, soil temperatures and occasionally lower air temperatures immediately above plants, in hairy species Stachys byzantina and Salvia officinalis compared to bare soil and other plant species/cultivars.
Data relating to the measurements around Wisley Garden have been analysed and were published in full in the RHS publication The Plantsman in 2011 (T Blanusa, R. Tanner, 2011, The effect of plants on garden microclimates, The Plantsman, 10 (3), 186–189)
Results of the experiments to understand the importance of various plant traits for cooling were presented at a number of scientific conferences (e.g. including American Green Roof Conference in Nashville in November 2014) and RHS events.
Following papers summarising some of the results of this project might also be of interest:
Blanusa T, Vaz Monteiro M, Fantozzi F, Vysini E, Li Y, Cameron RWF, 2013, Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer better ‘cooling service’? Building and Environment, 59, 99–106
Blanusa T, Vaz Monteiro M, 2015, The cooling effects of green roofs, The Plantsman, 14 (1), 48–51
We are working towards a submission of another scientific paper, providing information about the mechanisms that enable best performing plants to cool, to a peer-reviewed journal in 2015.
The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.