Research Matters... Other species for green roofs

The main reason for growing on roofs is that the plants will absorb solar radiation and use much of that absorbed energy to evaporate water. This has the benefit of cooling the roof, which reduces the cost of air-conditioning within the building and improves the local environment around it.

Sedum species are frequently recommended for green roofs but most grow slowly and their colours are limited. In the research detailed below, ten differ- ent plant species were assessed for their ability to grow in just 10cm of substrate (soil, leaf mould and vermiculite) within containers sited on the flat roof of a building in Kobe, Japan.

The containers were watered every day for one month and then every other day, unless it rained. The plants were pinched regularly to promote branching and to keep shoots shorter than 20cm. Evolvulus pilosus, Fragaria ananassa, Petunia x hybrida and Thymus serpyllum all grew well during summer and increased in area by more than 70 per cent.

Evolvulus performed particularly well at high temperatures but may have to be treated as an annual in the UK. The surface temperature of the soil mix was 6-8 degsC lower than that of the concrete roof even by night.

Evaluation of Growth and Green Coverage of Ten Ornamental Species for Planting as Urban Rooftop Greening by Sendo, Kanechi, Uno and Inagaki (2010). Journal of the Japanese Society for Horticultural Science 79 (1): 69-76. The authors' abstract is available online at from where the whole article can be downloaded as a PDF.

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