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Class A Noxious Weed

Hydrilla drawing


This plant is native to Africa, Australia, and parts of Asia but was introduced to Florida in 1960 via the aquarium trade.

Method of spread

Hydrilla (Hydrilla verticillata) is considered the most problematic aquatic plant in the United States. It can grow an inch a day, forming dense mats that interfere with recreational uses and destroy fish and wildlife habitat. It will grow with less light and is more efficient using nutrients than other plants. Hydrilla reproduces easily - it can sprout new plants from root fragments or extremely small stem fragments, as well as from seeds, tubers and turions. The only known occurrence of hydrilla was discovered in King County in 1995. Eradication efforts are underway. The infestation most likely came from contaminated water lily rhizomes or through a aquarium.

Methods of control

Different methods or combined methods can be used to control hydrilla depending on the management goal. In recreational waters the challenge is to control hydrilla selectively amid native vegetation. Management methods include herbicide, grass carp, and mechanical removal.


  • commonly confused with Brazilian elodea (Egeria densa) and native American waterweed (Elodea canadensis)
  • long, sinewy, underwater plant
  • leaves are small and pointed, oppositely arranged, and generally grow in whorls of five
  • leaves are sometimes serrated along the edges; midrib of leaf is often reddish and has one or more sharp spines
  • flowers are tiny, white, and grow on long stalks
  • distinct tubers are 1/4 to 1/2 inches long, off-white to yellowish, potato-like structures that attach to the roots

Other information:

Hydrilla Eradication Project

Aquatic weed identification

For questions about lakes in King County, please contact or call the Water and Land Resources Division front desk at 206-477-4800.