Aquatic Plant Management

The Mason Lake Project - An Innovative Approach to Using Aquatic Herbicides

The residents on Mason Lake in Mason County, Washington have developed an innovative idea to lower herbicide use and more effectively target small populations of Eurasian watermilfoil (Myriophyllum spicatum) along the lake shoreline. Homeowners on Mason Lake have struggled with finding a good chemical control tool for an early invasion of Eurasian watermilfoil. Mason Lake (over 1,000-acres) has a number of springs and currents and coupled with small patches of milfoil growing along the shorelines, liquid herbicides have not been effective because there isn’t enough contact time between the plants and herbicides. Although residents and their contractors are also hand pulling milfoil, residents found that in spite of thorough attempts to remove all plants and fragments, milfoil keeps returning to the same locations year after year.

The residents came up with the idea of creating plastic “tents”, which are deployed into the water and placed over small clumps of milfoil. They then inject minute (22 ml/100 ft2) amounts of fast-acting selective herbicide (triclopyr TEA - Renovate) into the tents, and leave the tents in place for 24 hours. The tents hold the herbicide in contact with the plants for sufficient time to ensure good kill of the milfoil. Lake residents will be also using other herbicides such as 2,4-D and fluridone to compare effectiveness between herbicides.

Lake residents designed and tested several prototypes before settling on the tent-like design pictured at the beginning of this article. They used commonly purchased items such as tent poles, children's' swimming noodles, and gorilla tape to manufacture the tents. The tents are floated out to the milfoil infested area and when placed in position by a diver, they are deployed by removing the floats (pink floats in the photograph). Within a minute or so the tent sinks over the plants and the diver anchors it in place using rocks (if needed).

The diver injects the herbicide through mesh at the top of the tent and then covers the mesh with a plastic flap. The tent is marked with a buoy. After 24 hours the lake residents pull up the tent using the buoy. The tents are designed to be collapsible so they can be easily pulled up by lake residents, stacked on a boat, and redeployed to other sites. Several tents may be needed to cover all the plants at any one site.

A Department of Ecology Aquatic Weeds Management Fund grant is partially funding the work along with in-kind contributions of Mason Lake residents who have devoted thousands of hours to this and other milfoil removal projects.


The chemical company representative initially recommended using a concentration of 2.0 ppm of Renovate (triclopyr) in the tents. He thought that the tents would allow more water exchange leading to relatively rapid dilution of the herbicide within the tents. However, the tents held the concentration very well and this concentration proved too high (and ineffective in killing the plants). It is likely the plants shut down rapidly (high concentration and long exposure) and did not translocate the herbicide. Concentrations of 0.75 ppm and 1.0 ppm triclopyr worked much better. The lake residents reported 100 percent kill of Eurasian watermilfoil at these lower concentrations.

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