Work

A Trial of Ground‐Based Methods for Controlling Osmunda regalis L. Invasion of Waikato Wetlands, New Zealand

Year: 2026

Type: article

Abstract: Over 90% of palustrine wetlands in New Zealand have been lost, and those remaining face ongoing threats such as plant invasions. In the northern North Island, there is concern about widespread invasion of wetlands by royal fern ( Osmunda regalis ), a fast‐growing exotic fern introduced to New Zealand in 1890. We compared the effectiveness of mechanical and cut‐stem herbicide treatments (gel formulations) for controlling O. regalis invasion at three Waikato wetland sites, comprising two mānuka–willow swamps and a Machaerina ‐dominant fen. At each site, five plots were randomly assigned to one of five treatments: Glyphosate (high‐strength glyphosate), picloram, and metsulfuron‐methyl, cutting only without herbicide and control. Plots were revisited the following summer to record survival of O. regalis . Survival of mānuka ( Leptospermum scoparium ) was monitored to assess potential herbicide effects on non‐target species. Glyphosate 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 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 suggest that site conditions and associated growth‐form variation of O. regalis influence treatment efficacy. Our cut‐stem herbicide 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, glyphosate proved the most consistently effective treatment and no treatment caused significant non‐target effects. Management strategies for O. regalis should therefore be tailored to site‐specific conditions and growth form of O. regalis . Future studies are needed to enable a more complete understanding of O. regalis invasion and optimal methods of control.

Source: New Zealand Journal of Botany

DOI: 10.1002/nzb2.70174

Authors: Charlotte G. Leicester, Susan Emmitt, Christopher H. Lusk

Institutions: University of Waikato, Department of Conservation

Language: English

Topic: Weed Control and Herbicide Applications

Subfield: Plant Science

Field: Agricultural and Biological Sciences

Domain: Life Sciences

SDG: Life on land

Open Access status: hybrid

Funder: University of Waikato