An aquatic weed that makes swimming dangerous, boating difficult and diminishes other animal and plant life in the water has taken over half of Namaske Lake in Pinardville, according to a state official.

Amy Smagula, a limnologist and exotic species program coordinator for the Department of Environmental Services, said the lake has become the worst case of milfoil infestation in New Hampshire, because of the density of the milfoil, also known more formally by its scientific genus name of Myriophyllum.

About half of the 194-acre Namaske Lake is coated with the aggressive weed, which is native to the United States, but not New England, according to Smagula.

Smagula said the milfoil, which generally is found in shallower areas, has grown to the maximum extent possible in the lake, which exists where the Piscataquog River backs up to the Kelley Falls Dam. The large body of water was named “Namaske Lake” just this year by an act of the state Legislature.

Milfoil grows 10 to 15 feet tall, rising to the surface in Namaske Lake. It can clog the engines of motor boats and makes it dangerous to swim because it is so thick. In other states, people have drowned in milfoil-infested waters.

“Picture like a corn crop under water, more or less,” Smagula said. “It’s literally an aquatic jungle.”

Milfoil also can harm aquatic life by blocking out the sunlight and depriving the water of oxygen, causing fish to suffocate.

Michael Allard, who has lived by Namaske Lake for more than 40 years, said he first noticed the milfoil in July 2006 and contacted the state immediately.

Allard noted that the appearance of the weed came after the Mother’s Day flood earlier that year, when Scobie Pond upstream in Francestown had overflowed. The pond had a significant case of milfoil.

Smagula said the state had to focus on eradicating the infestation in Scobie Pond. Otherwise, efforts downstream would have been futile, she said. The state also treated a small patch of milfoil from Glen Lake earlier this year.

Last month, Allard formed the Namaske Lake Association to tackle the milfoil problem. The association has been working on an application for a state grant for a herbicide treatment of the milfoil early next summer. Smagula estimated it would cost between $50,000 and $90,000. The state, she said, could afford to pay for half.

Some towns, she said, have used conservation funds for the purpose. Others have annual warrant articles that replenish a standing milfoil fund. It is possible the town could get a Moose Plate grant, which is for conservation projects, from the state, according to Smagula.

After the initial treatment next year, the town would have to follow up with smaller-scale antimilfoil activities every three to five years.

“You are not going to ever be milfoil-free in that lake at this point because there’s always going to be a little bit of persistent milfoil within the Piscataquog River system,” Smagula told selectmen. “I think that if we get ahead of it we really need to work hard to keep on top of it,” she added.

She said the herbicide the state uses targets milfoil and will not kill other plants. The state does not allow swimming for seven days after the herbicide has been applied, but that rule is under review and the prohibition could be reduced to two or three days.

Smagula has been talking with officials in Manchester about sharing some of the cost, since some properties on the lake are within the city limits. There are 150 homes on Namaske, according to Allard. Selectman Vivian Blondeau questioned how she could justify spending taxpayer money on a private lake. Smagula responded that it is a public body of water owned by the state.

“We can assure you, it is used very much by the public and by the very people in the town of Goffstown,” Allard said.

Right now, the only public access ramp to the lake is on Electric Street near the West Side Arena in Manchester. Blondeau said she knew people who had tried to use it had been harassed and had their tires slashed. Selectman Scott Gross said the town could build its own public access ramp o n land it owns off Bay or Cove streets.