Mermaid Weed in our Lake – a Mixed Blessing

Last November we reported that there is a dense patch of E. milfoil growing near the mouth of Middle Brook, which our consultant (Lycott, Inc.) is recommending that we treat with a local application of triclopyr [LINK].  We are planning such a treatment, and the State has indicated its approval.  The affected area covers about eight acres, and is located in shallow water just offshore.

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But wait!  In late June of 2008 Leslie Matthews, a State biologist, took a group of us on a “field trip” in that location as part of a workshop to teach us how to recognize invasive plants (and others).  One of the participants pointed out a plant that Leslie could not name.  So she took a sample for later identification. She soon emailed that “it is a native plant called “marsh mermaidweed” – Proserpinaca palustris.  This plant is in the same family as the milfoils (Haloragaceae) but it is not in the “water milfoil” genus (Myriophyllum).”

Prior to the issuance of our herbicide permit in 2010 Lycott conducted an extensive audit of plant species in the lake.  The mermaid weed was not found.  Nor was it noticed in subsequent inspections by Lycott and by the State.  This might have been because it is growing in such shallow water that it is hard to get to even with a kayak.  Or its small incidence just escaped notice.

But P. palustris has “S1” status in Vermont, meaning that it is “very rare, generally 1 to 5 occurrences believed to be extant and/or some factor(s) making it especially vulnerable to extirpation from the state.”  So in the course of a routine review of our proposal the Department of Fish and Wildlife noted the presence of the rare mermaid weed in the same area.  To protect the rare weed from the effects of the herbicide we will be required to install a barrier between it and the treatment area.  And we will pay special attention to this species in the post application plant survey.

Just how this will play out is unknown.  The water is less than a foot deep in the area where the barrier will be placed, which poses problems.  And while the mermaid weed has this rare plant status, the State botanist reported:

“Overall, likely thousands of genets, but hard to tell number of individuals. Very vigorous growth of plants; nearly all in fruit. Two areas, one 10×10 feet and another 15×15 feet which were colonized completely by this species with 100% cover. Another large area 40×50 feet had about 90% cover. Many plants were emergent from the water.”

Little is known about how susceptible the mermaid weed is to the herbicide tricolpyr.  It is a perennial dicot, the class of plants affected by this auxin.  Besides milfoil, the only aquatic dicot widely found in the lake are the water lilies.  They suffered some effect from the triclopyr in 2010, but they quickly recovered.

One bit of irony: marsh mermaidweed is widely advertised as a desirable aquarium plant.  Many believe that Eurasian milfoil found its way into this country’s waterways when it was discarded by aquarium enthusiasts who had purchased it for the same purpose!