Emily Lucas inspects a boat for invasive plants before it gets put in the water at a boat launch on Cobbossee Lake in Monmouth in June 2022. Officials recently found several areas of the lake infested with milfoil in Winthrop and Manchester. Joe Phelan/Kennebec Journal file

WINTHROP — Following the discovery of invasive water milfoil in Cobbossee Lake earlier this month, officials have treated the area with an herbicide but found scattered plants in another part of the lake, near Horseshoe Island.

A newly trained volunteer of the Cobbosseecontee Lake Association spotted two areas on the west side of Horseshoe Island infested with the invasive plant that continues to threaten aquatic habitats across the state. More plants were also found along the stretch of Horseshoe Cove.

Horseshoe Island in Winthrop is one of the biggest islands in Cobbossee Lake and has about a mile of trails maintained by the Kennebec Land Trust.

The recent discovery comes about two months after a new law went into effect requiring people to clean, drain and dry boats before and after entering inland waters to curb the spread of invasive species.

These measures were previously recommendations, but boaters can now be fined up to $2,500 for violating the rules and transporting an aquatic plant.

Similar laws exist in 20 other states, including Vermont, New York and New Hampshire.


Cobbosseecontee Lake Association President John Weaver said the scattered plants near Horseshoe Island have been removed.

“Friends of the Cobbossee Watershed confirmed the sighting of the plants. You can imagine, if you find one plant, you look around to see if there is more,” said Weaver. “Since then, because they were scattered, they have been pulled out manually, and herbicide treatment was not required. But the surveying of the area continues because they were scattered along a wide area. So, that work continues.”

Eurasian water milfoil has long been a concern in Maine. The plant, if introduced to a water channel, can grow at a rapid rate of several inches in a day. The spread, if not curbed, can thwart water access for humans and harm aquatic life by sucking out the oxygen in the infected area.

A survey team found large areas of milfoil in a portion of the lake in Manchester almost three weeks ago. The invasive plant had infested parts of Cobbossee Lake between the Outlet Bridge on Pond Road and a nearby dam. Having discovered such large infestations, lake associations have cordoned off the area to limit the spread.

Because of the large area and moderate depth of the infestation, the Maine Department of Environmental Protection stepped in.

The DEP placed a floating warning buoy on the lake side of the bridge to warn boaters and has treated the area with the herbicide ProcellaCOR.


The herbicide mimics a growth hormone in plants, accelerating growth until the plant can’t support it and breaks down.

After the treatment last week, officials are still surveying the areas to ensure it worked. The herbicide usually takes over a week to cull the invasive plants.

Local associations are also doing their part in furthering the clean, drain and dry mission. Cobbosseecontee Lake Association has directed funds to Friends of the Cobbossee Watershed to allow the group to hire two boat inspectors who monitor for plants at two public boat launches, one in Monmouth and the other in East Winthrop.

“We are also supporting businesses like the Lakeside Marina and the Birches that are requiring people to have their boats inspected before use,” said Weaver. “We don’t have enforcement capabilities, but what we can do is support those who do have these capabilities as best as we can.”

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