Two advantages of green roofs are their ability to cool the building below and the air above, and to reduce stormwater runoff volumes, thanks to the transpiration of moisture through their leaves. The most common green roof plant are species of Sedum. Those plant can survive drought conditions in the rooftop environment in part because they transpire relatively little water through their leaves.
To what extent does the characteristic that helps Sedums survive, their low transpiration rate, negate their environmental benefits? Is the performance of a Sedum-planted roof significantly higher than that of a non-planted roof with the same construction and soils? If not, does the Sedum offer other benefits that still make its use worthwhile? If it offers no real benefits, and a higher-benefit roof is not feasible, is the only alternative a conventional roof, or is a plant-free soil-covered roof a realistic alternative?*
*Green roof people discourage the use of the word soil in favor of “growing medium.” In a world of where manufactured soils are more and more the norm for built landscapes, I am not sure there is a meaningful distinction.
Deborah Howe
Toby Wolf
What the heck is manufactured soil? It sounds like a contradiction.
There’s only one green roof in our town that I am aware of. It’s on a little drive-through coffee shop, only one story high, so you can see it easily. I’ll have to stop and get coffee to see what it’s planted with.
Most soil used on medium-to-large landscape projects — those where earth is moved, those where plantings and lawns are created where there used to be none — is now manufactured. This practice is grown as it has become increasingly clear that the use of “real’ soils, often imported from agricultural lands, is too expensive, environmentally irresponsible, and ineffective. (Agricultural soils are often poorly suited for horticultural use.)
Manufactured soils contain a portion of “base soil,” from the site or elsewhere, mixed with sand (and/or other inorganic materials) and compost. It takes a relatively small portion of “real” soil and stretches it. It also yields a soil that is predictable and controllable. It is a custom product that can be designed to fit the site conditions and the plants that it will support.
Oh, and sometimes something extra to introduce or boost soil microorganisms (often an application of compost tea)
A sedum blanket will keep your roof cooler than a simple layer of growing media. In addition, the sedums take up water which means that they reduce stormwater water runoff more than soil alone. A healthy, fully vegetated green roof will additionally prevent soil and wind erosion, and the most sustainable way to have a fully healthy green roof is through use of plants like sedums, which have Crassulacean acid metabolism (CAM) properties to allow them to survive drought conditions.
So, manufactured soil is actually custom-mixed soil. Interesting. Thanks!
Does anyone know about the implications of too much water on sedums? We’ve been known to have the occasional extremely wet summer here in the Northeast; I know that sedums deal well with drought, but wonder how landscape sedums react to too wet of a season. I’ve had houseplant succulents ‘melt out’ from getting the same amount of water as my other houseplants. Is this possible with sedums?
I understand the benefits of rooftop plants; my question has more to do with the level of benefit obtained through the use of plants that are selected for their low rate of transpiration, the next questions being whether in fact anybody has quantified the benefits of a living roof compared to various kinds of non-living roofs. How much more water is transpired by Sedum than by a soggy layer of organic soil? How much less heat is absorbed by a Sedum-covered roof than by a roof covered in white pea stone? Is there a light-covered spongy material that is resistant to wind erosion that would offer advantages similar to Sedum with lower costs and greater predicatability?
It’s not that I dislike Sedum or vegetated roofs, treehugger that I am; I just want to know as much as I can about alternatives and trade-offs.
Good questions. What about juniper species (especially rug and creeping junipers), dune grasses, other grasses, and various herbaceous perennials that deal well with poor growing media and/or shallow soil depths? Sedums are very neat-looking, and make a nice tapestry, but it seems as if plenty of other plants may be as useful.