Environmental drivers of seasonal variation in green roof runoff water quality

•Seasonal variation in green roof runoff water quality was much higher than among-event variation.•Most dissolved element concentrations in runoff were high in summer, positively correlated with temperature.•Patterns implicate microbial mineralization or other temperature dependent processes.•Nitrate leaching from the green roof increased several-fold for the growing season following application of an organic N fertilizer.•Large temporal variation in green roof effluent water quality supports the need for long-term studies.