Evaluation of lakewide, early season herbicide treatments for controlling invasive curlyleaf pondweed (Potamogeton crispus) in Minnesota lakes

Abstract

Curlyleaf pondweed (Potamogeton crispus) is a nonnative aquatic plant found throughout temperate regions of North America. Its early season growth, propensity to form dense surface mats, and ability to out-compete native aquatic plants allow it to degrade the ecological and recreational quality of many lakes. Consequently, there is great interest in adopting lakewide management strategies that can reduce the negative impacts of curlyleaf and provide long-term control. We collaborated with the Minnesota Department of Natural Resources from 2006 through 2009 to evaluate lakewide, early season herbicide treatments for controlling curlyleaf. Nine curlyleaf-infested lakes were treated with endothall at 0.75–1.00 mg active ingredient per liter (ai/L) or fluridone at 2–4 μg ai/L (only one lake treated with fluridone) for up to 5 consecutive years. Three additional infested lakes were selected to serve as untreated reference lakes. In each year we assessed the frequency and biomass of curlyleaf (May and Jun), documented the production of new curlyleaf turions on standing plants (Jun), and tracked changes in the abundance and viability of turions in lake sediments (Oct). We found that herbicide treatment inhibited turion production and substantially reduced curlyleaf frequency, biomass, and sediment turion abundance in treated lakes. The largest reductions of curlyleaf frequency, biomass, and turion abundance occurred in the initial 2–3 years of treatment, with less substantial reductions in the subsequent years of treatment. Despite these reductions, viable turions remained in the sediments of treated lakes after up to 5 consecutive years of treatment. These results suggest that although lakewide, early season herbicide treatments can effectively control curlyleaf, inhibit turion production, and reduce the abundance of turions in sediments, ongoing management will likely be required to maintain long-term control.