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Unlisted Noxious Weed (1998)

Water hyacinth drawing


Water hyacinth is native to South America, and was introduced to the United States in the 1880s. Its beautiful, large purple and violet flowers have made it a popular ornamental, and the plant is now naturalized in most of the southern United States.

Method of spread

As of 1998 water hyacinth hadn't found in the wild in Washington State, but was sold as an ornamental in plant nurseries. Its use as an ornamental means that it could be introduced to our lakes and rivers, and this is expected to be its primary method of spread.

Water hyacinth has been called the worst aquatic plant in the world! Its growth rate is among the highest of any plant known: hyacinth populations can double in as little as 12 days. Incredibly dense mats of free-floating vegetation block boat traffic and prevent swimming and fishing, and keep sunlight from reaching the water column and submerged plants.

Methods of control

Water hyacinth can be controlled by harvesting, aquatic herbicides, and biological control agents. Locally, the best way to manage water hyacinth is to prevent it from becoming established. Plants purchased at local nurseries should be disposed of away from waterbodies. Identification

  • free-floating, robust plant grows up to three feet off the water's surface shiny green leaves are round to oval, four to eight inches in diameter, with gently incurved sides leaf veins are dense and numerous so leaves stand erect stalks are bulbous and spongy, and help keep the plant buoyant flowers have six petals, purplish blue or lavender with yellow several flowers grow at the top of a single stalk
  • a mass of fine purplish black and feathery roots hangs in the water underneath the plant

Back to 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.