Alternatives to Sedum on green roofs: Can broad leaf perennial plants offer better 'cooling service'?

► Of all the species tested, leaf surface temperature was lowest in Stachys, even when water was limited. ► On warm days, both Stachys and Sedum cooled the air above the substrate compared to bare soil. ► On several hot afternoons in the glasshouse Stachys provided more aerial cooling than other species. ► In outdoor conditions we recorded one incidence where Stachys provided additional localised aerial cooling. ► On a warm day, temperatures below Stachys and Sedum canopies were 11 °C and 4 °C lower than of bare soil.