Project history
Hydrilla Eradication Project

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1994-1997
1994: Hydrilla was first documented in Washington at Lake Lucerne and Pipe Lake. This documentation marks the only known infestation of hydrilla in the Pacific Northwest to date. The plant may have been introduced into the lakes from a personal aquarium that was dumped in the water or through a contaminant associated with another introduced plant species, such as the non-native water lilies found in the lakes.

Early 1995: County and state agencies developed a response program and began hydrilla eradication efforts on Lake Lucerne and Pipe Lake. This response program included: (1) notifying and educating the public; (2) documenting hydrilla levels; and (3) applying a herbicide (fluridone) to prevent further plant growth and reproduction.

1995 - 1997: King County used herbicide (fluridone) treatments to control hydrilla reproduction in Lake Lucerne and Pipe Lake. Each summer, herbicide treatments were conducted over an eight to 10-week period, with the goal of maintaining a concentration of 10-20 parts per billion (ppb) of fluridone in the water column. The number of plants appeared to be successively reduced each year. The largest decrease occurred after the initial treatment in 1995.

In 1996, King County also completed an evaluation of hydrilla tuber germination patterns in the lakes. This special study focused on the critical temperature for tuber germination (determined to be about 15 degrees Celsius), which directed the timing of spring diver surveys and the initiation of subsequent management actions each year.

1998
Divers found hydrilla plants remaining in scattered patches, ranging in density from one to ten plants per square meter. For affected areas, localized application of herbicide was used to control plant reproduction. At the end of the summer, the lakes were surveyed and no actively growing hydrilla plants remained.

1999
In June and July, divers surveyed the lakes for hydrilla. Plant densities remained at levels similar to 1998 with plants concentrated primarily between 10 to 15 feet in depth, covering an estimated 2.2 acres. Because of the number of plants remaining, additional herbicide was used to control remaining plants.

2000
In June, a diver survey was completed. In Lake Lucerne, hydrilla appeared nearly eliminated except near the canal. Divers used a hand-pulling method to manage remaining plants. In Pipe Lake, plant densities remained similar to levels in 1998-99. Most plants were concentrated along a narrow band at 10-15 feet. Herbicide applications and select hand pulling methods were used to treat or remove the plants.

2001
In June a diver survey was completed. No permits were available for the application of herbicides because of a lawsuit. Ongoing survey and hand pulling methods were scheduled to remove all located plants. In Lake Lucerne, a few hydrilla plants were found at depths of 10-15 feet. No plants were observed in the canal area. In Pipe Lake, plant densities were lower than levels observed in 2000. Similar to Lake Lucerne, most remaining plants were found at depths of 10-15 feet with few plants scattered above or below that depth. Additionally, several areas of the lake were identified as having no plants.

2002
In late June, a diver survey was completed and hand pulling was again chosen as the main technique for hydrilla control. Divers worked in both lakes for a total of 26 days through the summer, completing three total sweeps of the littoral zones -- the shallow water around the edges of the lake -- but reporting that other vegetation was appearing in the lake and making it difficult to find small hydrilla sprouts. A quick survey in October found several substantial hydrilla plants in both lakes, making it clear that a return to herbicides would be necessary. Areas around the located plants were treated with the herbicide Aquathol Super K in early November.

2003
An application of slow-release Sonar PR was made in early June, based on location maps of hydrilla found over the previous three years. Applications occurred three more times through the summer at three week intervals. The areas and amounts of each application were adjusted, based on ambient concentrations of the active ingredient fluridone and the findings of frequent SCUBA diver and snorkel surveys through the season. A final map was prepared showing all located hydrilla through the season, to be used at the beginning of 2004.

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For questions about aquatic weeds and lakes in King County, please contact Sally Abella, senior engineer, Lake Stewardship Program.