How will our roof affect runoff?

We have been planting the green roof for over a month now. It is time consuming because each module has a different species composition. We have four different combinations of plants, and they are planted in a different order in each module. Planting the different roof compositions reminded me of a study I read called “Influence of vegetation composition on runoff in two simulated green roof experiments” published in Urban Ecosystems in 2008. The study found that things like plant height and structure influenced the amount of storm runoff on a green roof. Decreasing runoff is one of the many benefits of a green roof because it helps prevent flooding after a heavy rain. The study addressed the question of whether all green roof provide the same benefits, and concluded that they do not, and that taller vegetation with broad leaves are better at retaining runoff. I wondered how our vegetation might perform. Instead of setting up a runoff study, I wrote down if the plants are more “upright” or “creeping” and just hypothesized about how they might do.

Roof 1 has four upright plants and one creeping plant. I think it may be the best at retaining runoff.

Roof 2 has one creeping and two upright plants. Roofs 3 and 4 each have two creeping and four upright plants. Each of these roofs has the same ratio of creeping to upright, so I theorize that they will do about the same, but not as well as roof one, at retaining runoff.

If the plants end up growing sparsely or vigorously it will affect how well they retain runoff, and there is more to plant structure than just upright or creeping stature. I was also unsure if we are using an upright or trailing variety for several species, and that will also affect how accurate my conclusion is. Maybe, in the future, a customer installing a green roof here in Texas could include runoff capabilities and other differences in performance in the factors that help them decide what they want the species composition on the green roof to be.

Teresa McIntyre

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