By Jorg Breuning
A recent British study in Building and Environment found that broad-leafed plants, such as Stachys byzantina, Bergenia cordifolia and Hedera hibernica, which can thrive in partial shade, outperformed the traditional Sedum in cooling the substrate and surrounding air. The study concluded by suggesting, “…the choice of plant species on green roofs should not be entirely dictated by what survives on the shallow substrates of extensive systems, but consideration should be given to supporting those species providing the greatest eco-system service potential.”1
This is certainly one of the many worthless studies on the web that shows us that no horticultural understanding or common sense was ever part of the research study. A green roof is all about the vegetation and if we are not entirely dictated to by what survives, we have simply no plants, no vegetation, and therefore no green roof. It is also not new that more leaf mass provides cooler temperatures – standing in summer in the forest is cooler than on a corn field. I always thought this was common knowledge.
The broad-leafed perennials that were tested (less than 2 year tested) typically require deeper soil profiles. More green roof growing media (soil) increases the costs of the green roof, the structure and the maintenance dramatically. Substantially increasing green roof costs decreases the likelihood of having a green roof to start with. The study also did not mention that the climate conditions in Great Britain are unique and are generously supported with moderate temperatures by the golf stream all year around. This effect allows higher varieties of plants to start with but won’t guarantee a higher survivability of these plants.
Comparing Sedums with the mentioned type of perennials is like comparing wild strawberries with apples. The survivability of Sedums under extreme conditions on a shallow layer of soil with low nutrients makes them to the ideal, perennial and stabilizing groundcover for extensive green roofs.
There is no doubt that we use other annual or perennial plants as temporary fostering or nursing plants or to promote the growth of Sedums or reduce the pressure of unwanted plants.
In modern green roof technology there are multiple choices of herbaceous perennial plants that supplement the fundamental Sedum carpet.