Survival of Osmunda regalis and Leptospermum scoparium on plots subjected to mechanical and chemical methods of controlling O. regalis

We compared the effectiveness of mechanical and herbicide treatments (gel formulations) for controlling Osmunda regalis invasion at three wetland sites in Waikato (New Zealand), comprising two mānuka–willow swamps and a Machaerina-dominant fen. At each site, five plots were randomly assigned one of the following treatments: Glimax (glyphosate) , picloram, and metsulfuron-methyl, mechanical control (cutting only), and no treatment. Plots were revisited the following summer to record mortality of O. regalis. Survival of mānuka (Leptospermum scoparium) was monitored as a non-target species to assess potential impacts of herbicide treatments. Glimax was the most effective treatment overall, killing 96–100% of O. regalis at the two swamp sites but only 87 % at the fen site. Metsulfuron - methyl and picloram gels were also highly effective at the swamp sites but substantially less so in the fen. Mechanical control alone was effective only at the driest site and completely ineffective in the fen. None of the treatments had significant negative effects on L. scoparium survival. These results demonstrate that site conditions and associated growth form of O. regalis strongly influence treatment efficacy. Herbicide-based cut-and-paste treatments were most effective where O. regalis formed a trunk, while all methods were least successful at the fen site where plants lacked an arborescent form. Overall, Glimax proved the most consistently effective treatment with minimal observed non-target effects.