MANCHESTER, N.H. – New Hampshire is joining Maine in a campaign to prevent the spread of milfoil, the invasive plant that chokes lakes.

Inspectors will staff boat ramps at 42 Granite State lake sites this summer to examine hulls and remove all vegetation on boats coming in and out of lakes. The plant has already appeared in dozens of state lakes.

The effort is part of a $260,000 National Oceanic and Atmospheric Administration grant.

In addition, New Hampshire lawmakers recently passed legislation that increases boat registration fees $3 to help raise money to fight milfoil and other exotic waterweeds. It takes effect in 2003.

Maine, which also is developing a milfoil problem, already requires special anti-milfoil stickers on all motorboats using inland waters. The fees – $10 for Maine boats and $20 for out-of-state registrations – help pay for the fight against invasive plants.

Maine has launched a public information campaign involving toll collectors who warn motorists entering the state with boats. Warning signs are posted at border crossings, and TV ads remind boaters to be vigilant.

Three state departments have been reviewing a request from property owners in Belgrade, Maine, to close a popular boat ramp to Messalonskee Lake because of fears that fragments brought in by boats will cause a spread of milfoil.

Experts say that in New England, the invasive plant problem is widespread and getting worse. Since there is no natural deterrent to their growth, the plants can grow wildly and absorb nutrients, air and other valuable resources that native plants would otherwise use.

Experts have found that milfoil not only affects aquatic life and recreation activities on lakes – it also can hurt property values. A study by the University of New Hampshire last year found that lakefront property value can plummet at least by 15 percent if there is milfoil present in the water outside the home.

The loss in value could be permanent, because once a lake gets milfoil, it can never get rid of it. Right now, you can only hope to control it, said Jody Connor, director of the limnology center for the New Hampshire Department of Environmental Services.

Connor said the summer program is a key opportunity to show boat owners where the milfoil picks up.

“You would be surprised the number of people who don’t care,” he said, noting perhaps as many of a quarter of all boat owners do not inspect their hulls or make any effort to prevent milfoil’s spread.

That, he said, is the source of the problem.

The New Hampshire Department of Environmental Services also is involved in a genetic study of milfoil at Dartmouth College.

Getting its genetic code is the first step toward eradicating the weed, Connor said.

“That is the basis or understanding how to kill it,” he said.