Because the Main Body of the lake and the North Arm differ greatly, ISDA’s efforts have led to different targeted actions around the lake. This week, they treated parts of the Main Body of Hayden Lake with the systemic herbicide ProcellaCOR. Diver-assisted mechanical EWM removal will also take place near Gem Shores. And Jeremey Varley, ISDA’s Noxious Weed Specialist, announced that they would not be treating the North Arm.

Treatment in the Main Body of the Lake

Clean Lakes, contracted by ISDA, has already applied the systemic, targeted herbicide, ProcellaCOR to treatment areas totaling 45.8 acres. These areas lie primarily in the shallow bays along the eastern shore of Hayden Lake. The northern tip of Bervin Bay also required treatment. There are no drinking water, recreation, fishing, or turf irrigation restrictions associated with this action. Irrigation restrictions of ornamental and broadleaved plants and crops will be in place until sample testing shows residual herbicide levels below two (2) parts per billion (ppb).  ISDA will conduct the first set of tests on 7/24/21. Watch the interactive 2021 Treatment map on ISDA’s Invasive Species website for the lifting of this restriction.

In addition, ISDA will contract for diver removal of EWM in 21+ low-density acres along Gem Shores. The diver removal project areas are not under any water restrictions.

Treatment of the North Arm of the Lake

ISDA has completed a thorough plant survey, conducted localized plant investigations in response to residents’ concerns, and consulted herbicide studies and experienced herbicide applicators. The findings emphasize these interacting concerns:

• Native plant growth in the area is a good sign. It indicates that invasive weeds have succumbed to previous herbicide treatments so that the natives can regain dominance in the water. The diversity of native plants will help to keep invasives from overtaking the area. The invasives can therefore be controlled with smaller herbicide treatments or even diver-assisted mechanical treatment.
• 95% of the plant mass currently in the north arm consists of native species. The high density of the native species interacting with the low water level would slow or prevent the dispersal of the herbicide so that only the invasive plants near the point of application would be affected. Parts of the 5% invasive population would not feel the impact.
• Native plants will take up the ProcellaCOR herbicide just like the EWM. But, since ProcellaCOR targets specific receptors in EWM, not in the native species, they would not be extensively damaged by the herbicide. In the current still-water scenario, they would essentially rob the invasive species of the herbicide.
• Fish, certain plants, beneficial algae, and other organisms require oxygen-rich water to survive. Low water levels and high temperatures are stressing the North Arm at this time. An herbicide treatment that causes rapid plant death and decay would reduce the dissolved oxygen in the water. This would potentially push the ecosystem into an unhealthy state, and oxygen-dependent aquatic life would die.

Read Jeremey Varley’s explanation of these and other factors driving ISDA’s decision not to treat the North Arm again in 2021.