Sedum, the staple and often first choice green roofing material, is being investigated by researchers for its cooling effectiveness against another plant treatment.
RHS principal horticultural advisor Leigh Hunt told a PlantNetwork conference on 12 April he was working with Reading University to examine green-roof covers. His team was not convinced sedum was the best for cooling.
The insulation effect of Stachys byzantia was much greater and the cooling effect much longer, he said.
"We have all been planting sedum roofs but are they better? Is Stachys the next big thing when it comes to making a real environmental difference in cities?"
The RHS was also doing a four-year science project on whether native or non-native plants were better at supporting wildlife. Non-natives often "got a bad press from the natives' crowd", he told the conference in the new Sainsbury Laboratory.
The team analysed UK native, near native and exotic plants. Biodiversity was rich in all groups and it was too early to say definitively. But it appeared some types of invertebrates preferred plants across the board.
"This is where planting mixes come in. Native plants are very useful, but don't write off others. Often we get pushed into thinking we must plant more natives but it's not always the case."
Finally, Hunt told the conference that reports showed ivy climbers trapped air like roof insulation and were a good, cheaper version of a living wall. A modest Victorian house with little insulation but ivy-clad walls could cut heating bills by a third