Another unwanted weed has invaded Maine’s waters.

The Department of Environmental Protection verified Thursday that curly-leaved pondweed has been found in Round Pond, located in Parsonsfield, near the New Hampshire border.

Pondweed is the third invasive aquatic plant to be found in Maine that appears on a list of 11 “most unwanted species” identified as harmful by state biologists.

Variable-leaf milfoil has spread to at least 10 Maine lakes and ponds, while hydrilla was found last fall in Pickerel Pond in Limerick.

Field biologists from the Department of Conservation’s Maine Natural Areas Program found the new plant in mid-July, while conducting a routine assessment of the pond.

They found about a dozen stems of curly-leaved pondweed, Potamogeton crispus, floating on the water’s surface, but a two-hour search of the pond’s bottom did not reveal any rooted plants.

That’s to be expected with pondweed, said John McPhedran, head of the invasive species program for the DEP.

Curly-leaved pondweed blooms early and dies down by July, leaving behind only winter buds called turions. A single plant can produce hundreds of turions, which winter over in the lake sediment then grow into new plants in the spring, McPhedran said.

Pondweed also can spread by leaf fragments and by seed from its small blossoms.

The plant forms a dense mat of plants with long, wavy ribbonlike leaves in June and July. It can interfere with the early growth of beneficial native plants, as well as hamper boaters and swimmers.

And when a great number of curly-leaved pondweed plants die down in midsummer, the water can become oxygen-deprived, potentially harming fish.

State biologists plan to study Round Pond next spring to determine the extent of the infestation.

Round Pond is 10 feet deep on average and covers 175 acres.

Until the Round Pond discovery, Maine, South Carolina, Alaska and Hawaii were the only states without curly-leaved pondweed infestations.