Can aerial herbicide application control Grey Willow (Salix cinerea L.) and stimulate native plant recovery in New Zealand wetlands?

Abstract

SummaryGrey Willow (Salix cinerea L.) poses a significant threat to wetland ecosystems in New Zealand. To manage the ecological impacts and to control further spread, cost‐effective large‐scale control methods are needed. We investigated the response of Grey Willow and dominant wetland plant groups to the aerial boom‐spray application of glyphosate at 9 L/ha and triclopyr (amine) at 18 L/ha at three New Zealand wetlands. We found glyphosate substantially reduced the dominance of tall (>2 m) Grey Willow with commensurate increases in the dominance of most native plant groups. Triclopyr (amine) application resulted in poor Grey Willow control, was not associated with increased native plant group dominance, and some native plant groups declined where triclopyr (amine) was applied. We conclude that the aerial application of glyphosate is an effective large‐scale Grey Willow control tool and could be used to initiate the restoration of native plant communities in wetlands dominated by Grey Willow. But, evidence of Grey Willow recovery after control suggests that increases in native plant dominance will be reversed as Grey Willow re‐establish. Further research is needed to determine how to maintain and enhance native plant dominance after control, and to determine how to manage Grey Willow in fen areas where the Grey Willow canopy is discontinuous and nontarget herbicide impacts can occur. The aerial boom‐spray application of triclopyr (amine) for large‐scale Grey Willow control should be discontinued as it does not provide effective control and results in negative ecological outcomes.