Photo credit: Robert Vidéki, Doronicum Kft., Bugwood.org
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Species at a Glance
Fanwort is a submersed freshwater perennial that can be found rooted in the substrate or floating on top of the water. When rooted, stems may reach lengths of up to 6m (20 ft). It is persistent, aggressive, and competitive, bringing with it the potential to take over aquatic ecosystems.
Leaves: Two types of leaves include submersed and floating. Submersed leaves are delicate, fan-shaped, and usually green in color, averaging 5 cm (2 in) in diameter. They are finely divided and arranged in opposite pairs along the stem. Floating leaves, which are not always present, are small and narrow (less than 1.3 cm [0.5 in]), oval to diamond in shape, and arranged in an alternating pattern.
Flowers: Small white, pink, or purple flowers with a diameter less than 1.3 cm (0.5 in) grow from the tips of the stems and float on the water’s surface.
Stems/Roots: Range from grass to olive-green and sometimes reddish-brown. Shoots are upturned extensions of the horizontal rhizomes and may reach lengths of up to 6m (20 ft).
Fanwort is often confused with other leafy milfoils, Beck’s water-marigold (Megalodonta beckii), some bladderworts, hornworts, mermaid weeds, and water crowfoots. The leaves of watermilfoils are whorled and it has small flowers growing from where the leaves meet the stem. Beck’s water-marigold has yellow composite flowers and sessile leaves, while fanwort has white flowers and slender leaves. Water marigold also has opposite leaves that attach directly to the stem with no petiole between the leaf and stem.
This very hardy plant is usually found rooted in muddy areas of slow moving waters such as streams, small rivers, lakes, and ponds. It can establish in a wide variety of environments and tolerate a wide range of temperatures, allowing it to overwinter in frozen lakes.
Fanwort is thought to have spread from intentional and unintentional release in the aquarium trade. Its fragile stems break off easily and most pieces can re-sprout and grow into new plants.
Native to the sub-tropic areas of South America and Gulf of Mexico region of the United States, fanwort has been introduced to the Northeast and the Pacific Northwest. It can be found in scattered populations all throughout the Mid-Atlantic states except for West Virginia.
Note: Distribution data for this species may have changed since the publication of the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016), the source of information for this description.
Fanwort is highly competitive and persistent, forming dense mats at the water’s surface that block sunlight into the water column, negatively impacting native plant species, biodiversity, and water quality. It can also clog waterways, impacting recreational activities such as boating, fishing, and swimming.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).