HADLEY — A five-year plan for addressing the continued growth of water chestnuts at Lake Warner in North Hadley will include use of the weed killer Diquat on nearly 11 acres of the 70-acre pond.

The Conservation Commission last week approved the plan submitted by the Friends of Lake Warner and the Mill River for spraying herbicide to better control the invasive species, which can reduce oxygen and light levels in the body of water and harm biodiversity.

“The plan is to use an herbicide treatment two to three times a year during the first one to two years, and then reducing it as we see the plant reduction over the course of the five-year plan,” said Friends member Brian Pearson.

The herbicide being put on 10.75 acres of the lake will be supplemented with hand pulling of the water chestnuts that has been done for 20 or more years without success, Pearson said. Working with the Connecticut River Conservancy, the hope is increase hand pulling, which last year included 10 hand pulls, each with up to 20 volunteers, but “we just are not able to stay on top of it,” Pearson said.

Water chestnuts were first spotted on lakes and ponds in the Connecticut River watershed in 1997. A 2002 article in the Gazette reported that a small amount of water chestnuts were in Lake Warner.

Now, Pearson said there are concerns about the water chestnut growth spreading and traveling beyond the lake, including going over the dam and heading toward the Connecticut River.

While setting a series of conditions for how the herbicide will be deployed, the commission took under advisement the future use of Phoslock to remove high levels of phosphorous from the lake. That chemical is not yet approved by the state of Massachusetts.

The conditions set by the commission include that Diquat only be applied by a certified specialist licensed by the state’s Department of Agricultural Resources, that another notice of intent be filed if there are changes to the management plan, that 48-hour notice be given before the spraying happens and that a year-end report be provided.

The spraying will take place in late June or early July.

Pearson said the success of this effort will be determined by before and after photographs. The likelihood is that water chestnut, which has an 11-year lifespan, will not be immediately eradicated by the herbicide, but will be better managed in combination with the pulls.

“We’re just trying to get ahead of the invasive species in the lake at this point, that we can’t get ahead of,” Pearson said.

Jonathan Carr, who runs Carr’s Ciderhouse on River Drive, said he is concerned about the use of chemicals and whether it would compromise 40 acres of agricultural land adjacent to the pond.

“Diquat’s pretty strong stuff,” Carr said, adding that it’s highly toxic to aquatic invertebrates, the basis of the food chain, and can be ingested by fish and birds.

Kayla Loubriel, the town’s conservation agent, though, explained that the commission isn’t empowered to deny a project that uses Diquat, but sets the conditions that might reduce some of the concerns.

The Friends group, a nonprofit that oversaw repair of the nearby dam in 2017 and which continues to work on water quality and watershed issues, is working on several water chestnut pulls. These pulls are regularly scheduled on Saturdays from 9 a.m. to noon, with the next of six this spring and summer set for June 7. Those interested in volunteering should go to:

https://ctriver.vomo.org/opportunity/lake-warner

Scott Merzbach can be reached at smerzbach@gazettenet.com.