Hydrilla is a Class A noxious weed only found in 2 connected lakes in King County. Hydrilla was first identified in June 1995, although it is suspected that it was present for a few years before it was identified.

Hydrilla outcompetes Eurasian watermilfoil and Brazilian elodea. Hydrilla has infested over 65,000 acres of Florida's lakes, rivers, streams drainage and irrigation canals. In states where hydrilla has become established, millions of dollars are spent each year on management activities.

Dense mats alter water quality by raising pH, decreasing oxygen under the mats, and increasing water temperature. Stagnant water created by hydrilla mats provides good breeding grounds for mosquitos. Hydrilla interferes with recreational activities such as swimming, boating and fishing. In eastern Washington, hydrilla has the potential to impact power generation and irrigation by clogging dam trash pipes and intake pipes.

An aggressive eradication plan has been in effect between the King County Department of Natural Resources (DNRP) and the state Department of Ecology (DOE). Control methods include annual diver surveys, biological assessment of the tuber growth, annual Sonar applications as necessary and annual diver manual removal of plants and tubers. Sonar treatments controlled the hydrilla plant, but it is not effective on the tuber bank in the sediment. The Sonar treatments continue to target the new plants as they emerge and prevent new tuber production. Sonar also prevents any spread by fragmentation.

Addtional hydrilla photos - click thumbnail for larger image

For more information about hydrilla, please follow this link to the WA State Department of Ecology.

If you find hydrilla in King County, please notify us through our online infestation form.

To find out where we have records of this weed in King County, use our interactive noxious weed map and search for hydrilla.