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Hydrilla is a submersed perennial herb that 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 mosquitoes. 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.

After being discovered in two small lakes in King County, Washington, an aggressive eradication plan was put 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 and after over ten years of repeat treatment, all the tubers were depleted and no new populations of hydrilla emerged.

Legal status in King County, Washington

Hydrilla is a Class A noxious weed only ever found in 2 connected lakes in King County and since eradicated. Hydrilla was first identified in June 1995, although it is suspected that it was present for a few years before it was identified. If it were discovered again in King County, it would be the highest priority for immediate removal. For more information on the legal status of noxious weeds in King County, see Noxious weed lists and laws.


Additional information on hydrilla

Hydrilla photo

hydrilla plant

Report hydrilla in King County, Washington

Locate hydrilla in King County, Washington

Related information

Related agencies

Program offices are located at 201 S. Jackson St., Suite 600, Seattle, WA 98104. To contact staff, see the Noxious Weed Control Program Directory, send an email, or call 206-477-WEED (206-477-9333).