Sedums May Not Be the Best Green Roof Choice, Scientists Say

Green-roof plants other than sedums may help cool air temperatures more effectively, according to researchers in the latest edition of the online journal Building and Environment.

The study, carried out with funding from the UK’s Royal Horticultural Society and Portugal’s Fundacao para a Ciencia e a Tecnologia, looked at the possibility of using different plants — sedum, Stachys byzantina, Hedera hibernica and Bergenia cordifolia — for green roofs. Sedum is currently the most popular, according to RHS.

Green roofs can help combat the urban heat island effect, in which air temperatures in built-up urban areas are higher than in surrounding rural areas. This increase in air temperatures is largely due to vegetation being replaced by dark and impervious surfaces. Increased vegetation such as green roofs can help reduce urban temperatures and also reduce the energy needs of buildings through their insulating properties.

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 cooling 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.

According to RHS scientist Tijana Blanusa, the study shows building designers should not choose plants for a green roof based only on what survives in a shallow substrate. Instead, they should build roofs that support plant species with the best all-around environmental benefits. This may mean roofs with some form of irrigation system and deeper substrates in which the plants can grow, Blanusa said.

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 reduces summertime air temperatures in the region of four degrees, according to RHS.

In April, the Hilton New York – Manhattan’s largest hotel – installed a green roof and cogeneration system, Environmental Leader reported.

The 16,000-square-foot green roof, which plays host to locally grown plants harvested from an upstate New York farm, was installed by Xero Flor America and is located on the hotel’s fifth floor rooftop setback.

Photo Credit: Green Roof Blocks