The Cities of Maple Valley and Covington, in cooperation with the King County's Lake Stewardship Program, continue with a multi-year effort to eradicate Hydrilla verticillata (hydrilla) from lakes Lucerne and Pipe. The project was initiated in 1995 and is expected to continue for several more years.
Hydrilla is a Class A noxious weed that spreads by tubers and plant fragments. Once established, the plant grows quite quickly, out-growing native species. In affected areas, the plant
fills the water column, restricting swimming, boating, and fishing activities.
Both California and Florida spend millions of dollars each year to stop new hydrilla infestations and maintain open waters for agriculture and recreational uses in affected areas. Over the last six years, Washington has spent $390,000 to eradicate hydrilla from lakes Lucerne and Pipe.
Recent research results from Florida verified that hydrilla tubers could remain viable in a lake for over ten years. Observations at Lucerne and Pipe suggest that new plants germinate each year from tubers which have remained in the sediment since 1995 when management actions were first initiated. Generally, the density of hydrilla plants has markedly declined since the beginning of the project. This decline is consistent with Florida findings where management actions have also been ongoing to prevent plant reproduction and eradicate an established tuber bank in an affected lake.
Thus far, annual management actions at Lucerne and Pipe have focused on preventing further plant reproduction. Herbicide has been the primary tool used to control reproduction. As the population of plants continues to diminish, management actions will shift to hand pulling and ongoing survey. Once hydrilla is eradicated, the native plant community will be restored.
In 2001, only hand pulling was be used to control hydrilla in the two lakes. In Lucerne, hydrilla appears nearly controlled while in Pipe, hydrilla still remains in low densities largely at a depth of 10 to 15 feet. The specific findings and management actions for each project year(s) are highlighted.
In 1994, hydrilla was first documented in Washington at Lucerne and Pipe. This documentation marks the only known
infestation of hydrilla in the Pacific Northwest. The plant was possibly introduced into the lakes from an aquarium
dumping or as a plant contaminant associated with another introduced species such as the non-native water lilies found
in the lakes.
In 1995, County and state agencies developed a response program and began hydrilla eradication efforts on Lucerne and
Pipe lakes. This response program included: (1) notifying and educating the public; (2) documenting hydrilla levels; and
(3) applying herbicide (fluridone) to prevent further plant growth and reproduction.
From 1995 to 1997, the County used whole lake herbicide treatments to control hydrilla reproduction in the two lakes.
Each summer, herbicide treatments were conducted over an eight to ten week period maintaining a concentration of 10-20
ppb. With each year's treatment, hydrilla plant numbers were significantly reduced from levels observed in the prior
year. The most significant reduction in plant coverage occurred after the initial treatment in 1995.
In 1996, the County also completed work to evaluate the germination patterns of hydrilla tubers at Lucerne and Pipe.
Through this special study, the critical temperature for tuber germination was determined (about 15°C), which allowed
for better timing of spring diver surveys and initiation of subsequent management actions.
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.
In June and July, divers again 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.
On June 22, a diver survey was completed.
In Lake Lucerne, hydrilla appears nearly eliminated except for the canal area.
Ongoing survey and hand-pulling will be used to manage remaining plants.
In Pipe Lake, plant densities remained similar to levels observed in 1998 and 1999. Most plants are concentrated along
a narrow band at 10-15 feet with a few plants scattered above or below that depth. Additionally, several areas were
identified where no, or only scattered, plants were identified. In 2000, herbicide and select hand pulling will be used
to continue controlling plant reproduction.
On June 22, a diver survey was completed.
In Lake Lucerne, a few hydrilla plants were found scattered mostly at depths
of 10-15 feet. No plants were observed in the canal area. Ongoing survey and hand pulling were scheduled to be used to
manage remaining plants in both Lake Lucerne and 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. Given the relatively low density of hydrilla in Pipe Lake, hand pulling
was used to remove remaining plants.
To accomplish this, divers began regular hand pulling of hydrilla in Lucerne, Pipe, and in the adjoining canal area
in July of 2001. A motorized boat was used on a limited basis to move divers around the lakes. Handpulling activities
continued periodically through August.
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