Moss on green roofs

Green roofs are growing up! They have an official organization and accreditation program because there’s more to a green roof than meets the eye. The “green” part, the vegetation layer on top — for which moss is a highly desirable candidate in shady areas — is only the icing on the cake. The “roof” part, or “black arts” as it is called in the trade, comprises several layers of substrate and a growing medium, and requires expertise to specify, install and monitor.

Green roofs are gaining tremendous popularity in the US, which takes its lead from Europe and Canada. Green Roofs for Healthy Cities, the organization that sponsors the training and accreditation program, lists on its website several new laws and government incentives intended to entice or otherwise drag building owners and city planners into the 21st century. (greenroofs.org)

Moss is an especially desirable plant component and has many advantages for green roofs because it:

• Is lightweight
• Retains ten times its weight in water
• Thrives in shade produced by building shadows
• Is drought tolerant once established
• Grows rapidly — three to six months for full coverage

And, since moss has no roots, constructing a green roof using moss also requires less growing medium.
We have found that by grinding two varieties of our mosses (hypnum, or sheet moss, and dicranum, or rock cap moss), mixing them with a water-retention agent and broadcasting the fragments over the expanded aggregate, we can establish moss that not only knits together quickly, but also grows into and amongst the aggregate, locking the moss to the roof.

Stay tuned and stay on top of the green roof movement.