PITTSFIELD — Summer’s here, which means lazy days drifting on the lake — and the vengeful return of Eurasian watermilfoil, an invasive plant that spreads aggressively throughout the city’s lakes.

The watermilfoil plants, which resemble vines covered in leafy fronds, thrive in shallow waters that give them access to sunlight. Warm weather makes the plants abundant along the shoreline — and a nuisance for anyone looking to swim, fish or take out a boat.

Next week, Pittsfield will begin treating its two main lakes with herbicides to kill off the milfoil. Pontoosuc Lake will be treated on Wednesday, and Onota Lake will be treated on Tuesday, June 23.

Jim McGrath, the city’s park, open space and natural resource program manager, said the treatments will keep the overgrown watermilfoil from wrecking havoc on native species.

Fronds of Eurasian watermilfoil grow up to the surface in the shallow water at the Onota Boat Livery on Onota Lake in Pittsfield.

“As these aquatic plants grow, they eventually will die,” he said. “As they die and decay in the water, they suck up available oxygen.” Enough oxygen loss could kill off some of the lake’s fish. Additionally, the thick mats of watermilfoil can block out sunlight for native plant species.

“If we don't do anything, there’s a likelihood that vast portions of both lakes would be overtaken by Eurasian milfoil,” McGrath said. “And that wouldn't be good for anyone.”

Boating, fishing and swimming will not be permitted the same day as treatment, but can resume the following day. Irrigation, however, will take longer to resume. After treatment, Pontoosuc cannot have its water used for watering plants or lawns until June 23; Onota Lake will have the same uses prohibited until July 8.

Comprehensive Environmental Inc., an environmental consulting firm serving clients in the Northeast, conducted a pretreatment survey on June 1 to make recommendations to SOLitude Lake Management, the company contracted to administer the chemicals. About 53 acres of Pontoosuc’s nearly 500 acres will be treated, while Onota Lake will have approximately 218 acres out of 617 treated.

According to McGrath, different chemicals will be used to treat each lake. Onota will be treated with ProcellaCOR, a systemic herbicide that kills off the entire plant. It’s the more expensive option, he said, but more effective.

Meanwhile, Pontoosuc Lake will be treated with diquat dibromide, under the brand name Tribune. As a contact herbicide, diquat dibromide kills plants but does not target their roots.

Treatment for Onota Lake will cost the city $256,150 overall, McGrath said, while treatment for Pontoosuc will come to approximately $9,100, with costs split between Pittsfield and Lanesborough.

The City Council voted on April 28 to allocate $210,000 in free cash for Onota Lake’s herbicide treatments. The Lake Onota Preservation Association also chipped in $46,150 to support treatment.

Brittany Eberling, executive director of the Berkshire Environmental Action Team, said the advocacy organization is currently directing its focus on the environmental impact of runoff from chemicals used in agriculture rather than the use of herbicides in waterways.

But in regards to herbicide treatment in general, Eberling said that notifying people far in advance and being transparent about chemical usage are important steps to take for anyone administering them.

“Our organizational stance is that we should very thoughtfully consider how prioritizing recreation and human-centered desires can sometimes be at odds with what is best for the ecology of our waterways,” she said. “We encourage thinking very differently about the use of herbicides and pesticides across the board.”