Skip to main content
Most King County offices will be closed on Monday, for Presidents Day.  
King County logo

Class B Noxious Weed

Fanwort drawing

History

Native to the southeastern United States, fanwort is a noxious weed in the Northwest. Fanwort has traditionally been used in aquariums for its beautiful fan-shaped underwater leaves.

Method of spread

Like many problem aquatic plants, fanwort can reproduce from small fragments. Fanwort stems become brittle in late summer, allowing the plant to break apart easily - facilitating its spread to new waterbodies. Once introduced, dense strands quickly form. Although fanwort is a noxious weed, it is still (as of 1997) legally sold as an aquarium plant in Washington.

The fanwort infestation in Washington is in a pioneering stage. It has been found in southwestern Washington and some coastal lakes in Oregon. Close monitoring of our lakes and prompt action may prevent further spread and minimize future management costs.

Methods of control

There has been little research on fanwort biology or management, although grass carp are known to eat it. Unlike most other rooted aquatic plants, fanwort may get most of its important nutrients from the water rather than the sediment, potentially making it sensitive to the reduction of waterborne nutrients.

Identification

  • sometimes confused with aquatic buttercup and aquatic marigold
  • plant sometimes looks reddish, sometimes green
  • fan-shaped leaves on short stalks are submerged and arranged in opposite pairs
  • small (less than one inch long), oval floating leaves with the stem attached in the center are sometimes present
  • underwater stems have a "tubular" appearance
  • flowers have white to light yellow petals and float on the surface

Back to aquatic weed identification

For questions about lakes in King County, please contact Rachael Gravon, Water Quality Planner or Chris Knutson, Project Manager, Lake Stewardship Program.