Comparison of stormwater runoff from sedum, native prairie, and vegetable producing green roofs

Stormwater retention is one of the well-studied benefits of green roofs. A roof’s ability to retain stormwater depends on factors such as the intensity and duration of the rain event as well as substrate depth, substrate moisture content at the start of the rain event, and vegetation type, health, density and water use efficiency. Extensive green roofs used for crop production differ from traditional Sedum and prairie-covered extensive green roofs in plant density and water use efficiency, but their impact on stormwater retention has not been well studied. Three vegetation types (unfertilized Sedum and native prairie species mixes, and a fertilized vegetable and herb species mix) were compared for stormwater runoff quantity over three growing seasons and stormwater runoff quality during one growing season. The prairie covered green roofs had the lowest increase in runoff as precipitation increased, almost half that of Sedum or vegetable producing green roof treatments. Vegetation type had no effect on runoff nitrate-nitrogen (NO₃⁻) concentrations, but NO₃⁻concentrations decreased over the course of the growing season. Runoff phosphorus (P) concentrations also decreased over time in the Sedum and prairie treatments, which were lower than P concentrations from the vegetable green roof throughout the growing season. This is likely a result of the difference between amounts of NO₃⁻and P applied to the vegetable green roof and the needs of the crop plants in that treatment. The similarities in water retention and water quality between vegetable producing extensive green roofs and Sedum green roofs suggest that vegetable production with careful nutrient management will not have a negative impact on stormwater retention or runoff water quality.