Over the next six-to-nine weeks, lake-watchers will see the Curlyleaf Pondweed in that part of Hayden Lake struggle to overcome the effects of the herbicide. It will even appear to grow, although the new growth will be sparse, stringy, and unhealthy-looking.  Five to six weeks from now, the Curlyleaf will finally look like it’s dying. In nine weeks, we expect it to be gone. Any Eurasian Watermilfoil, which is early in its growth cycle, will die back such that ISDA anticipates few issues with it later in the season. The herbicide will affect the native species as well.

SonarOne, a Fluridone-containing herbicide is in the form of time-release pellets. If you were in the North Arm today, you might have seen the applicator spraying these across the top of the water. The pellets will sink to the lake bottom where they will slowly release the herbicide to maintain an effective concentration near the root-level of the plant.

Concentration is Key.

ISDA carefully selected the SonarOne application rate to arrive at a Fluridone concentration that will kill the Curlyleaf but will have the least impact on residents’ and visitors’ use of the water.   The desirable range is 5 to 20 parts per billion (ppb). If below 5 ppb, the herbicide might stress but not kill the plants; if above 20 ppb, drinking water restrictions would need to be imposed. The target is 12 ppb.

The application rate is somewhat higher along the shoreline and lower in the center of the North Arm.  This gradient will accommodate the way things move out there. In that dynamic system, the forces of wind, friction, and gravity move the water and all it contains toward the middle and northward along the arm. The variable application rate, working with the natural water exchange, should yield a uniform Fluridone concentration across the treatment area.

Exposure Time is Important Too.

Time is the other factor affecting the success of this treatment.  The plant absorbs Fluridone through its shoots and stems and transports the chemical throughout the plant system. In the plant cells, the Fluridone disrupts the plant’s ability to build new resources.  The plant continues to grow, using up the resources that it has in store. Provided it can’t out-grow the damage done by the herbicide, it dies. Thus, the lengthy exposure is the time that it takes the plant to use up its resources while it’s unable to produce new. A second and third application in the North Arm will maintain the herbicide concentration throughout the needed exposure time.

Minimal Water-Use Restrictions are in Place.

Even though ISDA designed the treatment plan to minimize water-use restrictions, there are a few that are worthy of attention.  According to the label, homeowners cannot use water from within the treatment area to irrigate established crops for the first seven days post-application.  After that, feel free to use lake water on your trees and yard only.  NEW plantings should not be irrigated with treated lake water until testing confirms that Fluridone levels are below 5 ppb. The label does not call for any swimming or potable/domestic water-use restrictions.

The first water-testing, called a FasTEST, will take place 2-and-a-half weeks from now. The results will reveal whether affected property owners can irrigate their seedlings with lake water for the half-week until the next herbicide application. It will also tell ISDA whether the system met their expectations for water movement and chemical concentration. ISDA and HLWID will post results within a day of testing.

Please provide feedback.

As always, ISDA welcomes your feedback. Please participate in our poll and share more feedback, questions, and thoughts in the comments below.

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