You can rip them out, vacuum them up, poison them, or smother them with sheets of plastic, but your chances of eradicating the exotic plants that are invading the region’s ponds and lakes are slim, according to environmental officials.
“Once they are in your lake, you are very unlikely to get rid of them,” said Anne Carroll, director of the Office of Water Resources in the Massachusetts Department of Conservation and Recreation. “You just have to deal with them — like mowing your lawn.”
About a third of the approximately 3,000 freshwater lakes and ponds in the state are affected, according to Tom Flannery, who runs the department’s Weed Watchers program that trains people to spot and report invasive freshwater plants.
Whitman’s Pond in Weymouth, for example, is being choked by fanwort and milfoil, and Hobomock Pond in Pembroke is battling hydrilla. In Scituate, Musquashicut Pond is overgrown with phragmites.
Divers pulled out milfoil from Aaron Reservoir in Wompatuck State Park in Hingham and laid a plastic barrier on the pond bottom to block new growth. And every year, volunteers in canoes and kayaks yank out water chestnut plants from Clarks Pond in Walpole.
“These invasives [weeds] are called invasives for a reason,” Flannery said. “You can take a crystal-clear water body and introduce [an invasive] plant, and literally the entire lake is covered top to bottom with weeds. You can’t fish, you can’t boat, you can’t swim, the wildlife suffers. [The time] varies depending on things like lake chemistry and depth. But I’ve seen a lake go from zero to 40 acres [of weeds] in one growing season.”
That doesn’t mean it’s time to give up the fight, though, both Carroll and Flannery said.
“It still makes sense to find things early and have a chance to save the lake,” Flannery said. “Even if you have a lake chock full of invasives, you haven’t lost the battle. The key is management. You have to work at it year after year — and spend a lot of money.”
He said the state spends about $500,000 annually on the battle, and municipalities and private associations spend about another $1.5 million more.
While he said the figure isn’t close to what’s needed, both he and Carroll are encouraged by a new state law that requires the Department of Conservation and Recreation to write rules to combat the spread of invasives and impose penalties for those who fail to comply. The regulations should be finished by the end of the summer, and will focus on requiring people to clean their boats, Carroll said.
“It’s pretty basic. When you come out of a lake with invasives, look at your boat, at the motor, props, the trailer. A quick visual check should take care of it. Or run it through a car wash, or let it dry. We also ask people to empty out anything they filled with pond water and dump it on the ground far enough away that it won’t get back in.
“It’s not onerous, and it is now the law,” she said. “There certainly are people out there who feel fatalistic about this problem, but we’re not. . . . I think we’re keeping up; we actually feel a little bit hopeful.”
Carroll said the state has been largely successful containing the spread of water chestnut, which forms thick fluffy green mats on the tops of ponds and chokes out all other vegetation. In a single season, an acre of water chestnut can produce enough seeds to cover 100 acres of water the following year.
“It’s difficult to put any boat through, and you certainly can’t swim in it,” Carroll said. “But you can harvest it” and control it.
The state also has managed to slow the spread of hydrilla, she said. Scientists believe the fast-growing, feathery plant entered the country as an aquarium plant in Florida in the 1950s; it has spread throughout the South, and Florida spends upward of $100 million a year trying to manage it.
Hydrilla made its first appearance in Massachusetts in 2000 in a pond in Barnstable and has only been found in a handful of other places here — including tiny Magoun Pond in Marshfield, which is completely covered, Flannery said.
Bill Glover, a middle school science teacher in Braintree, discovered hydrilla all along the edges of 17-acre Hobomock Pond in Pembroke, where he lives, about four years ago. He reported his finding to the state, which provided money to treat the pond with herbicides.
“They were very surprised and very concerned,” Glover said. “It can literally choke a pond in a very fast time; it’s like a blanket across it. Hydrillas have the ability to lie dormant and three ways to reproduce. So it’s aggressive. We have to continually [treat] and monitor.”
To help stay on top of the problem, Glover gives “weed watcher” classes through the Pembroke Watershed Association, teaching volunteers to identify invasives.
Weymouth officials are trying to decide how to deal with a variety of invasive plants that have been growing in the 200-acre Whitman’s Pond since “at least the 1980s” but are increasingly a problem, according to the town’s conservation administrator, Mary Ellen Schloss. The weeds interfere with recreation and the water quality of the pond, which is part of the town’s drinking supply, she said.
“The pond is in tough shape in terms of aesthetics,” she said. “People want to see clear open water, and what they’re seeing is vegetation covering the shallow areas near the shoreline. Of course, that’s where folks live.”
The town’s Conservation Commission rejected a proposal to use herbicides on Whitman’s Pond in 2010, saying the chemicals could harm the herring that spawn there and are a critical part of a larger fisheries food chain. In response, the mayor formed the Whitman’s Pond Working Group to recommend alternatives.
Among the suggestions being considered are spot treatment with herbicides, hand harvesting, raking, covering portions of the pond bottom, and releasing insects that eat the weeds. More novel approaches include putting “floating curtains” on the pond’s surface and dying the water to cut down on sunlight getting to the plants.
A consultant’s report also lists ways to get rid of Canada geese, whose excrement helps create water conditions that encourage weed growth. The report says the most expensive, and most long-term promising, approach would be to dredge shallow parts of the pond, since the majority of the weeds won’t grow in deep water.
Schloss said Weymouth probably would end up taking several approaches and assessing how well they work and what the town can afford. Residents have offered to get out and pull weeds, if that will help, she said.
“Once you get these plants under control, you can go after them in a more benign way. But it’s so bad now, that we have to do something drastic,” she said.