Absence of hydrilla in two King County lakes could signal victory in battle to eradicate invasive weed

Though not yet ready to declare victory in their dozen-year-long battle against a fast-growing exotic water plant that chokes out native vegetation, King County and state environmental managers are relieved that extensive surveys of Pipe and Lucerne lakes this year have turned up no sign of hydrilla.

It wasn't long ago when King County was still battling the threat of a significant hydrilla infestation, even after many years of treatment. In 2003, surveyors found 470 of the fast-growing plants in the two lakes, which are connected and are the only two water bodies in the Pacific Northwest known to have hosted the invasive plant.

Stepped-up management and survey programs in recent years have shown remarkable results: No hydrilla have been found in Lucerne Lake during the past three years, while Pipe Lake had just two hydrilla plants in 2006 and none in 2007.

"Some states spend millions of dollars every year to address hydrilla infestations in their waterways, so it is very important that we completely eliminate hydrilla from these two lakes and prevent its spread to other water bodies," said Sally Abella, manager of the King County Lake Stewardship Program.

Native to the warmer areas of Asia and first discovered in waters of the United States in 1960, hydrilla has caused numerous environmental problems in lakes and rivers – particularly in the Southeastern U.S. In the past, it was frequently sold to the public as an aquarium plant, but has been banned in Washington.

It's thought that most hydrilla infestations begin when aquarium owners release fish into lakes or rivers, or when water lilies imported from states with hydrilla infestations are planted. However, it can also be carried by boats between lakes because only small fragments of the fast-growing vegetation are needed to start a new infestation.

Abella said the success in dramatically reducing hydrilla is due in large part to the partnership forged between King County, the Washington Department of Ecology (DOE) and the cities of Maple Valley and Covington.

The cities and state agency have provided funding for the program, while King County has performed both herbicide treatments and monitoring.

Abella said that after conferring with DOE experts, King County is following the protocol that the State of California used to eradicate hydrilla from several locations.

"California treats water bodies for three years after the first survey that finds no plants," she said. "Using that scenario as our guide, we will continue to treat Pipe Lake for at least two more years to ensure that we have no re-emergence of this invasive plant."

Abella said both lakes would be monitored for at least another five years beyond treatment.

More information on the hydrilla eradication program is available at http://dnr.metrokc.gov/wlr/waterres/smlakes/stophydrilla.htm on the Internet.