Photo credit: Alison Fox, University of Florida, Bugwood.org
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Species at a Glance
Eurasian watermilfoil is a feathery, submerged, aquatic plant that was once commonly sold as an aquarium plant. Generally found in water less than 6.1 m (20 ft) deep, it quickly forms thick damaging mats that are causing harm in shallow areas of rivers and lakes throughout North America.
Leaves: Feathery whorls of 3-6 leaves (four leaves per whorl is common) are openly spaced along the stem with 1-3 cm (0.3-1.2 in) between nodes. Leaves are threadlike, uniform in diameter, and have 12-24 leaflet pairs. They are aggregated into a submersed terminal spike and the tips often have a blunt, snipped-off appearance. Note that the occasional Eurasian watermilfoil leaf may have as few as five leaflet pairs. For this reason, it is always advised to count leaflet pairs on several leaves taken from various points along the stem.
Flowers: Tiny whorls of flowers are located on floral bracts atop slender spikes that rise above the water’s surface. Flowers either have four petals or are without petals.
Fruits/Seeds: Hard segmented capsules contain four seeds.
Stems/Roots: Slender stems, which often curve to lie on top of the water’s surface, begin to thicken before blooming and double their width further down.
Without fruits or flowers, it can be difficult to distinguish Eurasian watermilfoil from the native northern milfoil (Myriophyllum sibiricum) and coontail (Ceratophyllum demersum). It may also be confused with other leafy milfoil species including bladderworts, hornworts, mermaid weeds, and water crowfoots, which generally have fewer than 14 leaflet pairs. Counting leaflets can provide helpful identification clues.
This extremely adaptable plant can thrive in a variety of conditions. It grows in a wide temperature range in still to flowing waters and even survives under ice. While Eurasian watermilfoil grows best in fertile, fine-textured sediments and nutrient-rich lakes with lots of sunlight, it will readily inhabit disturbed lake beds.
Eurasian watermilfoil does not rely on seeds for reproduction but instead reproduces by fragmentation. Plant fragments break off and float via water currents, allowing it to disperse long distances and hitchhike on boats, boat trailers, motors, and fishing equipment.
Native to Europe, Asia, and northern Africa, Eurasian watermilfoil was first discovered in the eastern United States in the 1940s, but may have arrived as early as the late 1800s. It is now established in nearly every U.S. state and at least three Canadian provinces. It is common in lakes, ponds, and rivers all throughout the Mid-Atlantic region.
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.
Eurasian watermilfoil forms thick mats that can interfere with swimming, fishing, waterfowl hunting, boating, and other recreational activities. Plant fragments can also become wrapped around boat propellers and other equipment. Fish and wildlife are impacted when Eurasian watermilfoil shades out nutrient-rich native plants that provide food, shelter, and spawning areas for other organisms. Heavy infestations can also clog industrial and power plant intakes and even reduce local property values.
Information for this species profile comes from the Mid-Atlantic Field Guide to Aquatic Invasive Species (2016).