Are All Green Roofs Created Equal?

The following was published as part of my regular “Growing Greener” column in Public Garden magazine, Vol. 24 No. 3 (2009). Public Garden is the flagship publication of the American Public Gardens Association. In “Growing Greener” I answer sustainability-related questions from public garden staff.

Q: There are so many different green roof systems. Are they all effective?

A: It’s easy for us plant enthusiasts to be seduced by the idea of verdant rooftops. Aesthetically, living roofs are a major improvement over typical asphalt or tar roofs, which are about as hospitable to humans and most other life forms as Death Valley on a mid-summer day. But before falling head over heels for green roofs it’s worth asking whether they really, as touted, help insulate buildings and thus save energy that would otherwise be consumed for heating or cooling, counteract the urban heat island effect, remove particulates from polluted air, detain and cleanse storm water, and more.

Green roofs certainly seemed too good to be true to Steve Windhager, director of landscape restoration at the Lady Bird Johnson Wildflower Center, and his colleagues, so they compared the performance of six extensive green roof systems from six different manufacturers to each other as well as to traditional non-reflective blacktop and somewhat cooler reflective white roofs. The study was conducted on hot tub-size mini roofs, each 30 square feet. Each of the test green roofs was planted with the same 18 native plants chosen for their wide tolerance of both drought periods and saturation after rainstorms, and the plants were provided with the same amount of irrigation when necessary.

Steve and his colleagues were surprised to find that there was a wide variation in performance among the different green roofs. They were much better at preventing the temperature of the air below from spiking on warm days, compared to both the conventional and reflective roofs. But while some of the roofs were able to capture a lot of storm water, others weren’t significantly better in this respect than the white or blacktop roofs. And while some of the roofs had nearly no adverse effect on water quality, others were worse than the typical suburban lawn—the more fertilizer in the planting medium, the worse the water quality, although after the first growing season, water quality dramatically improved. In short, no one system excelled at providing all the benefits conventional wisdom says green roofs are supposed to provide.

The Wildflower Center’s research is important in part because although a host of studies have been done on green roofs in temperate environments, Austin’s climate is significantly drier, hotter, and more prone to flash flooding than other study locations. To what extent does the Center’s research apply to other regions and climates? “I think that the storm water retention numbers will be pretty uniform no matter where you go,” says Steve. He adds that the study of green roof performance in Austin’s subtropical climate is useful “particularly if we are seeing a warming trend in our more temperate climates.”

When asked what advice he can give other public gardens considering green roofs, based on the Wildflower Center’s research, Steve pointed out that it’s important to determine why you want a green roof,  then make sure that a green roof is the most efficient way to achieve your goals. Once you have decided that you’re going to pursue a living roof, make sure the manufacturer you’re working with is aware of your goals. “Green roofs can certainly be designed to capture storm water, have clean runoff, provide energy savings, and provide valuable additional gardening space,” Steve says, “but these expectations need to be made explicit at the outset of the project or it will have an unfortunately high chance of not achieving all of these goals.”

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