"Green roofs" – roofs covered with living plants -- have gained a strong toehold on the West Coast. In Portland, Oregon, for example, city officials have committed to using vegetated roofs for any necessary roof replacement where the technique is feasible. The city offers incentives and grants for private property owners who apply the technique, reports the Portland Oregonian (" Enviros and city of Portland cultivate a movement to top buildings with plants", by Shelby Wood). Oregon's climate is well suited to the technique, but it's a little different in Florida, where a subtropical climate creates several challenges, says University of Florida doctoral student Sylvia Lang. Lang, who is working toward a Ph.D. from the Department of Soil and Water Science, is studying the problem on a green roof installed in 2007 on the Charles R. Perry Construction Yard at the University of Florida's Gainesville campus. "Out there in Portland," says Lang, "they'll have a period of time when it's just light rain continuously, and then another time when it's dry. Here, we can have a drought in the middle of the summer, but then we also get huge storms." The weather patterns affect several characteristics of a green roof design, including depth of growing medium, selection of plant species, and whether the system will need a cistern to capture and store water for irrigation. Lang's Florida roof needed six inches of growing medium (compared to just two inches used in two Virginia buildings she is also studying). And the planting mix included "beach dune habitat plants," she explains — "like this plant called beach dune daisy, and beach sunflower, and firewheel daisy, that can handle drought, but then if it rains really hard they're okay, too." Green roofs offer a variety of environmental benefits. By capturing and holding rainwater — even for a few hours — they reduce the amount of stormwater runoff that enters storm sewers and floods streams. Evening out the stream flows prevents scouring and destruction of habitat, which lessens the environmental impact of extensive urban "hardscapes." On the other hand, green roof runoff, as with runoff from regular lawns, may contain fertilizer nutrients like phosphorus and nitrogen that pollute the streams and groundwater. There's a tradeoff — and that is part of what Lang is studying. After sampling runoff from the University of Florida’s green roof through several seasons and testing it for pollutants, Lang says, "I did not find that the water quality was so great. But the roof is doing a great job with volume control." The roof detains and re-evaporates more than 45% of the rain that falls on it. Lang's system captures and stores the excess in two 1,300-gallon cisterns — enough water to let her irrigate the roof twice a week with one-half inch of water through three dry weeks. When the cisterns run out (which happens rarely), she supplements the irrigation with reclaimed water (treated sewage) from university buildings. But roofs that don't re-use water could present a problem, Lang notes. "That's the only trouble I see with green roofs becoming popular in the South. If people start to put them in for esthetic reasons or because it's the cool thing to do, but then they end up having to irrigate — well, we already have a problem with lawns being irrigated. It uses up water and it puts fertilizer in the runoff. So if green roofs just became a new source of lawns and fertilizer, that wouldn't be so great. But if you're capturing the runoff and re-using the nutrients, then it's fine — and it even keeps the building cool and you keep rainwater from going into the stormwater system." If Lang's research can produce reliable numbers for the reduction in stormwater runoff, she says, builders might reap the benefits of a tradeoff against required stormwater controls. "When you put in a housing development, you have to put in your stormwater treatment. So if you can reduce your runoff by 60% using vegetated roofs, then you don't need as big a footprint for your stormwater ponds. You could fit another unit on the site, or at least just not have to excavate and install the pipes and ponds." In Portland, Oregon, Lang says, building owners get a one-sixth tax reduction for implementing a green roof, because of the reduced load on municipal storm drain systems. "They do different tax breaks," she explains, "but they have better numbers already on what it's doing for stormwater retention. That's my next research step, to see what is the tradeoff if there's water quality degradation. Suppose you have an increase in phosphorus, but there's a savings because you don't have the water quantity leaving, so you don't need as big a BMP ["best management practice" stormwater control] on the ground — what's the tradeoff? I don't have those numbers yet."
