Fanwort is an invasive aquatic plant native to the southeastern United States and parts of South America. Today it has invaded parts of the Crowe River watershed in central Ontario, as well as waters in the northern United States, Asia and Australia. Fanwort is a popular aquarium plant. It may have been introduced to Ontario by someone emptying the contents of an aquarium into a waterway, or boats may have carried plant fragments from an infested area outside the province. Fanwort grows and spreads aggressively, forming dense mats under or at the surface of the water. It is most often found in slow moving or stagnant water less than three metres deep. The plant can tolerate cold temperatures and stay green throughout the year. It prefers acidic water with a pH of about 4.8 to 7.8.


In Ontario, fanwort was first found in Kasshabog Lake, part of the Crowe River watershed northeast of the City of Peterborough. Since its discovery in 1991 it has spread within the watershed to North River, South Lake and Big Mountain Lake. Fanwort has also been introduced in Australia, India, Japan, Malaysia and the northeast and northwest United States.

Impacts of Fanwort

The fanwort population growing in the Crowe River watershed is the only known wild population in Ontario. If fanwort spreads outside this area it could disrupt the plant and animal life in other waterways and interfere with recreation.

  • The fast-growing plants form thick mats that crowd out native plants, block sunlight to submerged plants, disrupt fish communities and clog drainage canals and streams.
  • Dense stands of fanwort can hinder swimmers and boaters and prevent other recreational uses of waterways.
  • Because fanwort thrives in acidic water it could spread to lakes on the Canadian Shield, which tend to be acidic.

How to Identify Fanwort

  • Fanwort is a submerged plant that roots on the bottom of lakes and rivers.
  • Under the water, pairs of finely divided fan-shaped leaves grow on opposite sides of the main stem, creating a feathery effect.
  • The plants also have small, floating oblong leaves up to three centimetres long.
  • Fanwort flowers from late spring to early fall. The flowers usually rise above the surface of the water. They are 0.6 to 1.5 centimetres wide and white to pale yellow, sometimes with a purple or pink tinge.

Fanwort looks similar to other aquatic plants, including bladderwort (Utricularia vulgaris), white-water crowfoot (Ranunculus aquatilis), northern water-milfoil (Myriophyllum sibiricum), water marigold (Megalodonta beckii) and coontail (Ceratophyllum demersum). Only fanwort has opposite, finely divided, fan-shaped leaves on distinct stems.

What You Can Do

  • Learn how to identify fanwort and how to prevent accidentally spreading this plant with your watercraft. This is especially important in Kasshabog Lake, Crowe River and connecting waterways.
  • Avoid infested areas or reduce your speed when travelling near fanwort infestations. Your propeller can break off fragments and spread the pieces to new areas.
  • Inspect your boat, trailer and equipment after each use. Remove all plants, animals and mud before moving to a new waterbody.
  • Avoid planting fanwort in your aquarium or water garden. Aquarium hobbyists and water gardeners should only use native or non-invasive plants, and are encouraged to ask retailers for plants that are not invasive.
  • If you find fanwort or other invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, or visit EDDMapS Ontario to report a sighting.

OFAH/OMNRF Invading Species Awareness Program. (2012). Fanwort. Retrieved from:
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Header photo by Ontario Federation of Anglers and Hunters