From the Winter 2012 copy of Green Building Magazine (volume 22, No3).

Two green roof studies. One, carried out in Berlin, where green roofs had been installed in the 1980’s. They varied the PV arrays to find the most efficient:

• Over 5 years, PV panels over a green roof Vs a bitumen roof has 6% higher yields.
• If over a green roof AND on a swivelling stand that tracks the sun, then 10% higher.

So green roofs boost PV productivity AND as they insulate the roof, they decrease the amount of energy needed for heating and cooling.

The second study looked at planting schemes. The most popular current choice is sedum, du to high resistance to draught. Sedum was compared to Stachys byzantian, Hedera hibernica and Bergenia corifolia.

Stachys byzantina (commonly known as Lamb’s Ear) outperformed the other species in terms of leaf surface cooling, cooling the substrate beneath it’s canopy and even the air above it’s canopy during short intervals over hottest periods, when soil moisture was not limited.

The conclusion was to not choose the best plants for a shallow substrate, but those that provide the best all round environmental performance, including bio diversity. This may involve deeper substrates and some form of irrigation.