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Hydrilla, a Class A noxious weed, is a submerged perennial herb that forms dense stands in still and flowing freshwater, outcompeting even other invasive plants like Eurasian watermilfoil and Brazilian elodea. Toothed-looking leaves grow in whorls of 3-10. This plant reproduces via stem and rhizome fragments, as well as vegetatively through rhizomes and stolons, seeds, tubers, and other methods. It's a major nuisance: it can raise water pH, decrease oxygen, and increase temperature. It also interferes with recreational activities and provides breeding grounds for mosquitoes.

Distribution and Impacts

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).