Plans to treat Eurasian milfoil at Vancouver Lake have been delayed because the invasive weed has stopped growing and would no longer effectively absorb an herbicide.
Friends of Vancouver Lake, which has raised private and public dollars to treat the infestation, announced Monday that treatment will be delayed until the late summer or spring of 2020.
AquaTechnex, a Centralia-based company that specializes in invasive species control and toxic algae management, was scheduled to treat the lake’s milfoil with ProcellaCOR this week.
The company’s manager, Terry McNabb, said cyanobacteria blooms, also known as blue-green algae, likely have prevented daylight from penetrating the lake, which has halted the milfoil’s growth.
“Plants need light to photosynthesize and be healthy,” he said. “There is so much algae in the water, the light is absorbed in the first 4 or 5 inches.”
Milfoil leaves and stems that are not growing will not carry the herbicide down to the plant’s root crowns, which would yield marginal results, McNabb said.
The lake’s milfoil is “sick,” McNabb said, but he cautioned it’s only a temporary condition.
“As soon as those root crowns start getting light … they are going to come roaring back and be as healthy as they can,” he said.
Two months ago, drones and hydroacoustic mapping were used to identify 614 acres infested with milfoil, which is about 27 percent of the 2,300-acre lake. At that time, the milfoil was growing and healthy.
Mark Heilman, senior aquatics technology leader with SePRO Corp., ProcellaCOR’s manufacturer, visited Vancouver Lake on Thursday and observed the changing conditions.
“We’re doing a bunch of projects around the Northwest, including a couple for the Army Corps of Engineers and the state of Montana,” McNabb said. “He was out to look at those.”
McNabb said he or someone from his company will visit Vancouver Lake every couple of weeks to examine conditions and determine when the milfoil should be treated.
“Next spring’s got a lot of advantages to it, when the plants are young and taking off,” he said.
Larry Cassidy, chairman of Friends of Vancouver Lake, said his group’s board of directors decided Monday to delay treatment to maximize its effectiveness.
“We are as disappointed as anyone, but it’s the right thing to do,” Cassidy said.