In 2008 Proserpinaca palustris (Marsh Mermaid Weed) was found in Lake Fairlee. Its presence was recorded, but was not considered when our application to use the herbicide triclopyr in the lake was considered and granted in 2010.Early this year we proposed to treat a much smaller area of the lake with the same product. In March we were advised that the presence of the rare species Proserpinaca palustris would “trigger further review”.
The concern was that the mermaid weed might be susceptible to the triclopyr, as it is in the class of plants (dicots) that are affected by this class of chemical. The other plant in this category commonly found in the lake is the pond lily. These were somewhat affected by the 2010 application (link), but survived and have completely recovered. Scientists from SePRO, the company that manufactures the herbicide we use, had no experience with mermaid weed and their product. They had never been asked, probably because P. Palustris is widespread and abundant in other states.
In April the Department of Environmental Conservation made it clear that our chemical treatment was conditioned on somehow ensuring that the mermaid weed would not be harmed, and detailed how we might do this. In mid May scientists from Lycott and from the State visited the lake and mapped the locations where milfoil and mermaid weed are growing (link).
Since then we have been gathering information and determining how to proceed. In addition to the cost of the herbicide and its application, we now have to fabricate two custom 400 foot sections of plastic barrier to isolate the area where the mermaid weed is growing. They will have to be installed the day before the herbicide, and then removed two days later. In part because of how quickly we need them, they will be very pricey. As we added up the costs it became apparent that protecting the mermaid weed would almost double the original cost of killing off a relatively small area of milfoil.
Therefore we have decided that we will not use chemicals in the lake this year. We will pay instead for divers to hand pull the milfoil, and probably to deploy our bottom barriers in areas where the plants are growing densely. Herbicide treatment remains a possibility for future years. This year we will measure the efficacy of mechanical control, and will have more data about our ability to control the milfoil by non-chemical means.
At the same time we plan to work on two fronts to open the possibility to treating without special protection for the mermaid weed next summer. Now that we know where and how P. palustris likes to grow, we can look in other Vermont lakes, and invite residents of other lakes to join us. If we can demonstrate that it is not as “rare” as previously thought it will not require special treatment. Also we will gather data on how tolerant the mermaid weed is to triclopyr. We will engage SePRO to conduct controlled experiments with P. palustris in measured concentrations of the herbicide.
It is our belief that the careful use of triclopyr is safe, and that is is a cost effective way to control dense growths of Eurasian milfoil. It is likely that in future years we will employ it again.