MERRIMACK — The public boat launches have been closed at Naticook Lake because of an infestation of milfoil.

Last week, the Veteran’s Park boat launch and the boat launch at Camp Sargent Road were closed to all motorized boats because of the milfoil, according to Sherry Kalish, director of the Parks and Recreation Department.

“A major factor in the spread of invasive variable occurs when propellers and other activities chop up the existing plants and the fragments drift and re-root in new areas,” Kalish said in a release.

The presence of milfoil near the Veteran’s Park boat launch was confirmed this past summer, at which time the Department of Environmental Services became involved, according to Kalish.

She said a significant spreading of milfoil in the lake was documented throughout the summer, as divers performed hand removal and diver-assisted suction harvesting work on the areas of milfoil growth in August and September.

“DES is seeking bids for both herbicide treatment and diver-assisted suction harvesting activities to be done in 2013,” said Kalish, explaining individuals who are interested will be trained in the spring about weed watcher programs.

Currently, only non-motorized boats, such as canoes and kayaks that can be carried in, are permitted entry into Naticook Lake, she said, adding use of the boat launches will be reevaluated after milfoil management operations are done in the early summer of 2013.

In July, when the milfoil was first found in the lake, DES explained that the infestation had gone unnoticed for an extended period of time.

“When infestations reach this level, they are more difficult to manage, and eradication is not often feasible,” DES said in a statement at the time, adding there is not currently a program for prevention and early detection in place for Naticook Lake.

The local infestation was well-established when found, and appears to have been present for at least two or three years before being reported, according to DES officials, who explained that the early ice-out this year, coupled with warmer than average temperatures, have contributed to more pronounced and rapid growth of exotic species in New Hampshire waters.

According to a release from DES, exotic aquatic plants like milfoil may lead to water quality impairments, can reduce shorefront property values, and can be problematic to the aesthetic and recreational values of waterbodies.

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Kimberly Houghton may be reached at khoughton@newstote.com.