The city of Maple Valley is treating Lake Wilderness with an herbicide in an effort to control invasive, non-native aquatic weeds.

Officials planned to apply the herbicide July 8. Swimming was restricted for 24 hours during and following the treatment.

Last month, divers found Eurasian Watermilfoil (milfoil) in the southwest part of the lake and north of the boat launch. They removed 19 pounds of it by hand, but taking all of it that way may have taken up to three weeks, officials said.

The plant can muscle out native aquatic vegetation and form a dense monoculture in the water, officials said. The lake was heavily infested with milfoil in 1998 until being eradicated by herbicide.

Milfoil may have returned to the lake on boat trailers, boats or fishing gear.