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Green roofs are often planted with mixtures of plant species. Studies of green roof plant community composition over time show that species diversity often declines, in part due to competition between plant species. The incorporation of substrate depth heterogeneity into green roof designs is expected to increase habitat heterogeneity, and could reduce interspecific competition among plants, but this has not been tested empirically. Two species with contrasting responses to substrate depth and water availability, Festuca rubra and Sedum acre, were planted in a rooftop substrate depth heterogeneity experiment. There were a total of four treatments: three homogeneous treatments (substrate depth 5 cm, 10 cm, and 15 cm) and one heterogeneous treatment (patches with substrate depths of 5 cm and 15 cm), with the same substrate volume and average depth as the 10 cm homogeneous treatment. By the end of the …