State aiming to get grip on invasive plant

The invasive water chestnut has roots that anchor it loosely to the bottom of waterways, and then it spreads quickly. (Jay Connor/Globe Correspondent)

Earlier this summer, people were wondering what was happening at Fisk Pond. The popular fishing and boating spot in Natick seemed to be disappearing under a green carpet of plants that clogged propellers and stymied even the most avid anglers.

"It was wall to wall, shore to shore, so to speak, with water chestnuts," said Robert Bois, environmental compliance officer for Natick's Conservation Commission. "When you have the water chestnuts, you can't do anything. There are some great bass in Fisk Pond, and once you get those water chestnuts, you can't get to them. Good for the bass, bad for the fishermen."

Water chestnut is an invasive species that resembles a lily pad, but it grows thick enough to fill acres of open water with dense leaves and underwater tendrils that choke out native plants and fish. State environmental officials said the plant is especially a nuisance in Boston's western suburbs, where it has been found in Newton, Waltham, and Watertown along the Charles River, and in ponds in Acton, Berlin, Framingham, and Wayland.

Now officials say they have begun repulsing the invader. The state recently removed 320 tons of water chestnut from Fisk Pond, said Anne Monnelly, acting director of water resources at the state Department of Conservation and Recreation. Around $300,000 has been budgeted to hire floating harvesters to scoop up the plants, which blanketed 40 acres of the 67-acre pond. The project is expected to continue for two more years.

About 135 tons of water chestnut were harvested from 30 acres of the Charles River last month, the second annual cleanup in a three-year project costing around $390,000, said Monnelly. The figure for the harvest reflects the plants' weight after they have been allowed to dry. The estimate for the plants removed from Fisk Pond was significantly heavier because the plants were disposed of before drying, Monnelly said.

Natick is sending the plants pulled from Fisk Pond to the Wheelabrator plant in Millbury, where they are added to the trash that the facility burns to generate electricity, Bois said.

It is believed a Harvard University botanist inadvertently released water chestnut into the region's ecosystem more than a century ago, said Monnelly. Since then, the plant has invaded approximately 20 bodies of water, mostly in the Boston area, she said. The plant is native to Africa, Asia, and Europe, but differs from the edible water chestnut used in Asian cooking.

When the floating mechanical harvesters can't reach water chestnut growing in shallow water, officials organize hand pulls, where volunteers venture out in kayaks and yank the plants out of the water. Last month, after mechanical harvesters finished their cleanup on the Charles, a hand pull yielded enough plants to fill three dumpsters, Monnelly said.

It's not hard to uproot a water chestnut, which is only loosely attached to the bottom silt in a waterway. In less than an hour, a puller can fill a few laundry baskets to the brim.

"It's a floating weed," said Phil Rogers, a Pepperell resident who was on the Nashua River recently in his canoe for a hand pull. "You just pull it up. That's the easy part."

Pullers have to wear gloves, however. The water chestnut's seed is a rock-hard, spiny black nut when it is fully developed. "It will go right through your shoes," said Conner.

The spiny seeds are why the water chestnut spreads so rapidly. Clinging by their spines, they hitch rides on boats, ropes, and even wildlife. "They attach to birds and animals and then move to another pond," said Bois.

A sprouting water chestnut seed can send a stem as long as 16 feet to the surface before blooming into 10 to 15 rosettes, or leafy clusters, said Monnelly. Each rosette can produce as many as 20 seeds.

With so many seeds, said Bois, water chestnuts can quickly proliferate in slow-moving waters with soil rich enough to support them. They block the light for underwater plants, suck up oxygen, and displace other plants and fish. And when they die, they alter the chemical composition of the water, harming native species that depend on the previous conditions.

"It's a very efficient plant," said Bois. "You have the broad leaves on the surface that take in light for growth and then you have leaves under the water that use the residual light to grow the plant."

On the plus side, water chestnuts are relatively easy to eliminate. They aren't perennial plants, so if they are removed at the right time, they won't reproduce, Bois said. The trick is to catch them when they've germinated, but before their seeds have spread, and to come back again to grab the plants that remain.

"Harvesting is a timing issue," he said. "You have to start the harvesting before the seeds on the water chestnut mature and drop. In the late summer, early fall, the seeds mature and drop and now you have a whole new crop for the season. I'm sure we won't get it all so we'll have to come back next year."

The cost of hiring mechanical harvesters and spending hours physically grabbing water chestnuts might seem excessive, but it's worth the money, Bois said, because the plant threatens to ruin important recreational spaces.

Every year, more than 250,000 people visit Lake Cochituate State Park, which includes Fisk Pond, he said. "You're filling in your lake, you're losing your pond, you're losing it as a resource" if the water chestnut is allowed to take root, Bois said.