Control - Littoral Zone Endothall Treatment

Endothall (aquatic herbicide)

Endothall (active ingredient) is a fast-acting contact herbicide (an herbicide that burns back the above-sediment vegetation, but doesn’t kill the roots) that is believed to disrupt the plant biochemical processes at the cellular level. The dipotassium salt of endothall is used for aquatic plant control and is formulated as Aquathol® K (liquid) and Aquathol® Super K Granular. The Washington State Department of Ecology recently completed a risk assessment and an environmental impact statement for endothall. The risk assessment and the impact statement can be viewed by clicking here.

Endothall has been used for years in Washington lakes to spot treat milfoil along shorelines because it is rapidly-acting, and when used at higher concentrations (2-3 parts per million (ppm) needs only a short contact time to remove milfoil vegetation. Recently, lower concentrations (1-1.5 ppm) of endothall have been used to treat milfoil in whole lake or littoral zone treatments. Milfoil can be controlled (vegetative growth removed) at 1 mg/l active ingredient endothall with an exposure time of 48 to 72 hours. At this concentration, endothall impacts some native plant species to a lesser degree (Skogerboe and Getsinger, 2001).

The benefit of using low levels of endothall is to remove exotic weeds like milfoil, while allowing native species to recover. While this is not an eradication technique, it may be useful for maintaining more acceptable levels of milfoil in a lake by periodically treating the littoral zone with low concentrations of endothall. It is possible that treatments can occur as infrequently as every three years. Ecology, along with the Department of Fish and Wildlife, and the endothall manufacturer, Cerexagri, is conducting a study on a small western Washington lake (Kress Lake) to determine the efficacy of using low levels of endothall to control milfoil.

Waterbodies suitable for endothall treatment:

Whole littoral zone treatment with endothall cannot be considered as an eradication method. Endothall will suppress the growth of milfoil and may allow native plants to recover and therefore increase species diversity within a lake. Lakes and ponds considered suitable for littoral zone treatment are heavily infested with Eurasian watermilfoil. This method may be used where it is considered too expensive, or the waterbody is too large to use milfoil eradication strategies.

Special considerations:

The endothall label has a three-day fish consumption restriction in the area of treatment and an irrigation and stock watering restriction for 14-days after treatment. Ecology advises waiting 24 hours after any herbicide treatment before swimming, although there is no official label restriction for swimming. Care must be taken with the application so that low oxygen conditions do not develop as plants decompose.

Any whole lake or widespread herbicide treatment, such as littoral zone endothall treatment should be conducted under an integrated aquatic vegetation management plan. A permit called a National Pollutant Discharge Elimination System Permit (NPDES) permit is needed to treat waterbodies with aquatic herbicides. You can obtain NPDES coverage under the Washington Department of Agriculture Permit for noxious weed control. Click here for more details of the permitting process.

Description of the Kress Lake project, using endothall:

A detailed report about the treatment and sampling methodology and the results of the Kress Lake project can be seen in Ecology’s Aquatic Plants Technical Assistance Program: 2001 Activity Report. The information/data below were taken from that report. The project is still ongoing and additional data will be collected in August 2002 and June 2003.

Kress Lake, a 30-acre manmade lake in Cowlitz County, is a popular fishing lake with a nuisance population of milfoil.

Kress Lake is owned and managed by Washington Department of Fish and Wildlife as a warm water fishery (bass, channel catfish, and sunfish) and has no inlet or outlet. Trout and surplus steelhead are also stocked into this landlocked lake. Prior to treatment, aquatic plants were found growing throughout the lake with milfoil as the dominant species. Both fishing and the fishery of the lake were being negatively impacted by the milfoil plants (Stacey Kelsey of Fish and Wildlife, personal communication). She reported that excessive vegetation was contributing to a stunted fish population, and milfoil mats, especially along the shoreline, were interfering with fishing. The endothall study was undertaken to see if a low concentration of endothall could selectively remove milfoil, increase species diversity, and improve fishing and the fishery.

On June 21, 2000, a state-licensed applicator applied Aquathol® K at rate of 1.5 ppm to ten acres around the edge of the lake. A second treatment took place a month later, with an additional 10 acres treated from the shorelines toward the center of the lake using the same application rates.

Assessment of the treatment project is ongoing. Three months after treatment the endothall treatment reduced the frequency with which the vascular plants (flowering plants like milfoil) were found, while not affecting the macroalgae muskgrass (Chara sp.). During this period, vascular plants were reduced to the point of eliminating plant cover completely in locations throughout the lake. By one year after treatment and throughout that summer (June 2001 and September 2001) the frequency of muskgrass appeared to level-off while some of the vascular plants increased (e.g. waterweed [Elodea candensis], milfoil [M. spicatum], and bladderwort [Utricularia sp]). This recovery appeared to fill in areas left bare of plants the previous summer. The pondweeds (Potamogeton sp.) did not appear to be rebounding.

Two species showed a significant change in their biomass before and after treatment. The biomass of waterweed (native plant species) increased significantly one year after treatment. About one third less milfoil biomass was collected after treatment (76 g/m2 - before treatment versus 23 g/m2 - one year after treatment).

The species list from each sample date shows that the species diversity was greatest in June 2001; one year after treatment. A total of 12 different plant types were present at that time. This is almost double the number found before the herbicide treatment. The number of plant types observed decreased to 9 by the September 2001 sampling event. This may have been due to sampling variability, increased dominance by a few species making locating less common species more difficult, or the seasonal die off of selected species.

Endothall (Aquathol KÒ) significantly reduced both the biomass and frequency of observation of milfoil, over the study period. However, by 1.3 years after treatment milfoil was showing a significant increase in frequency, so the duration of the control may be ending. The results also show an increase in overall submersed aquatic plant species diversity one year after treatment.

Although the June 2002 data have not been statistically analyzed, surprisingly milfoil did not appear to have increased in frequency or biomass when compared to the previous year (Kathy Hamel, personal observation).

General impacts of endothall treatment:

Generally endothall is used to spot treat areas and therefore impacts are not widespread. Using low levels over the lake littoral zone does cause adverse impacts in the short term, since many vascular plants are affected by the treatment. Within a few weeks of treatment, most plants in the treated area are brown and dropping from the water column. In Kress Lake, an algal bloom was observed a few weeks after the herbicide treatment. This may have been caused by the nutrients released from the decaying plants. (Note: an algal bloom was also observed in August 2002, although no herbicide treatment had taken place for two years. Many lakes are naturally nutrient-enriched.) Sampling ten weeks after treatment showed mostly dead and decaying plants lying along the bottom and bright green healthy muskgrass populations. A year after treatment, the native plant community was recovering, but milfoil, though present, did not dominate the plant population.

Fish and Wildlife staff have been pleased with the results, indicating that anglers are now able to fish without tangling their gear in milfoil.


This is potentially a new method available for the control of milfoil in heavily infested lakes. The results from Kress Lake have been excellent. The lake was treated in 2000 and no further treatment was needed in 2001 or 2002. At this stage of assessment, we do not know how often the lake will need to be treated to continue the suppression of milfoil.


Parsons, J., B. Dickes, and A. Fullerton, 2001. Aquatic Plants Technical Assistance Program: 2001 Activity Report. Washington Department of Ecology

Skogerboe, J.G. and K.D. Getsinger. 2001. Endothall species selectivity evaluation: southern latitude aquatic plant community. J. Aquat. Plant Manage. 39:129-135.

Questions about this page? Contact us.