Pruning stormwater biofilter vegetation influences water quality improvement differently in Carex appressa and Ficinia nodosa

The maintenance of stormwater biofilter vegetation is conducted under local guidelines, which often include seasonal pruning. However, the effects that pruning has on water quality improvement remain unknown. This study used experimental columns to investigate the effects of pruning on effluent concentrations of nitrogen, phosphorus, and metals when planted with two common biofilter plant species, Carex appressa and Ficinia nodosa. Effluent was monitored in pruned, non-pruned, and unplanted control columns during a 70-day regrowth period, with monthly composite water sampling encompassing the flushed saturated zone water and effluent of each column to best represent a biofilter during a storm event. Differences between pruning treatments and the control were often species-specific and varied with nutrient type. No significant differences between treatments were found for total phosphorus, but pruning treatments affected nitrogen oxide removal in later sampling dates for F. nodosa, but not C. appressa. Total N and P removal ranged from 77 to 88% and 66–93 %, respectively, by both pruned and non-pruned plants. The overall amount of N and P removed in the pruned biomass was 2.1–3.5 times more than the estimated amount removed from the influent by the regrowth of the pruned columns alone during the regrowth period. Consequently, the amount of nutrients removed via pruning may significantly impact long-term removal. Cadmium, copper, lead, and zinc effluent concentrations were similar between treatments with removal efficiencies over 95 %. Overall, pruning appears to affect water quality improvement, but optimal pruning practices that may enhance long-term removal should be investigated further.