Targeting invader taking root in river

In the past, state officials have battled the plant in this section of the Charles with large machines that yank it out by the roots. But since money has been scarce for mechanical harvesting, volunteers have taken to the river to pull out the water chestnuts by hand.

The task is made daunting by the spiky water chestnut seeds, which are so sharp that they can injure anyone who steps on them, and so durable they can survive in sediment for as long as 12 years.

“At this point, it’s entirely volunteer-driven,’’ said Elise Leduc, a fellow at the Charles River Watershed Association, as she looked out onto the Charles River in Newton’s Auburndale section earlier this month.

“There’s no mechanical harvesting. You can’t eradicate the problem by hand. You’re pretty much in a holding pattern.’’

Water chestnuts have been found in dozens of locations around the state, including the Sudbury River, Bare Hill Pond in Harvard, Ice House Pond in Acton, and Pepperell Pond, where volunteers plan to pull out the plants this afternoon.

Leduc’s organization has held three volunteer sessions this summer, leading a flotilla of canoes into the river. The group also takes out corporate volunteers to hand-pull the plants.

The work needs to be done quickly, because August is when water chestnuts drop the next generation of seeds into the sediment.

“If the plants are completely uncleared, they would get worse and worse each year until the Charles River would be completely overgrown from bank to bank in some places,’’ said Kate Bowditch, director of projects for the watershed association.

The interest in keeping the Charles free for canoes, kayaks, and sailboats is an indication of how people are looking differently at the river, once so polluted that recreation in its waters was rare, she said.

“I think it’s a real indication of how much the Charles River has improved in terms of getting cleaner,’’ Bowditch said. “And how much people value the Charles River now in a way that they’re getting used to.’’