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Hydrilla verticillata, a Class A noxious aquatic weed.

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The Cities of Covington* and Maple Valley,* in cooperation with the King County's Lake Stewardship Program, are currently partnering on a multi-year effort to eradicate Hydrilla verticillata (hydrilla) from Lake Lucerne and Pipe Lake. The project began in 1995 and is expected to continue until it can be verified that the plant is gone from the lakes entirely.

Hydrilla is legally designated as a Class A noxious weed. The plant spreads by tubers, turions (vegetative buds), and plant fragments. Once established, the plant grows rapidly, overcoming native species and making monotypic stands (only one plant). In affected areas, the plant fills the water from bottom to surface, restricting both agricultural and recreational uses.

Photo of hydrilla tubers.

Both California and Florida spend millions of dollars each year to stop new hydrilla infestations and maintain open waters in affected areas. Over the last nine years, Washington state and local agencies have spent $575,000 to eradicate hydrilla from Lake Lucerne and Pipe Lake.

Research studies in Florida verified that hydrilla tubers can remain viable in a lake for as many as ten years. Observations at Lake Lucerne and Pipe Lake suggest that new plants germinate each year from tubers in the sediment, although the numbers of plants have substantially declined since the beginning of the eradication project. This decline is consistent with Florida findings where and eradication management actions are also ongoing in an affected lake.

Herbicides have been the primary tool used to eradicate hydrilla from Lake Lucerne and Pipe Lake, although a method where SCUBA divers pull hydrilla by hand was used exclusively in 2001 and 2002. This was the result of a lawsuit filed in Oregon under the Clearn Water Act (CWA) that called into question whether aquatic herbicides should be considered pollution.

There was a hiatus in granting aquatic herbicide application permits until the U.S. Ninth Circuit Court of Appeals decided that aquatic herbicides were to be held to the standards of the National Pollution Discharge Elimination System (NPDES) permitting requirements under the CWA. In 2003, applications of the slow-release form of Sonar to the lakes were resumed after hydrilla assessments showed that hand pulling was not decreasing the hydrilla population.

Currently, hydrilla appears to be nearly eradicated in Lake Lucerne, while in Pipe Lake it still remains in discrete areas, largely at depths of 10 to 15 feet. The specific findings and management actions for each project year(s) are summarized below.

*Indicates a link leading outside the King County Web site.

Project history
The project began in 1995 and is expected to continue until it can be verified that the plant is gone from the lakes entirely.

Project reports & plans
The project's annual report, Integrated Aquatic Vegetation Management Plan and hydrilla survey results are available online.

Hydrilla information & resources
Hydrilla identification, statistics on Pipe and Lucerne lakes, links to the Noxious Weed Control Board, the cities of Covington and Maple Valley and more.

*Indicates a link leading outside the King County Web site.

News and announcements

Oct. 21, 2009
Hydrilla considered eradicated in two King County lakes after lengthy battle

For questions about lakes in King County, please contact Rachael Gravon, Water Quality Planner or Chris Knutson, Project Manager, Lake Stewardship Program.