Eurasian water-milfoil is an invasive aquatic plant native to Europe, Asia, and northern Africa. Introduced to North America in the 19th century, it is now one of the most widely distributed invasive aquatic plants on the continent. It is suspected to have been introduced via ballast water, but was then moved around within the province through recreational watercrafts and natural expansion.
Eurasian water-milfoil prefers shallow water 1-3 m deep, but can root in up to 10 m of water. Being a fast-growing perennial, it forms dense underwater mats that shade out other aquatic plants. When large stands begin to die off in the fall, the decaying plants can reduce oxygen levels in the water, potentially affecting the fish communities.
The plant can hybridize with our native milfoil, northern water milfoil (Myriophyllum sibericum), creating a more aggressive form of the invasive species. Because tiny plant pieces can develop into new plants, Eurasian water-milfoil is easily spread when water currents, boat propellers, trailers, or fishing gear carry plant fragments to new areas.
Eurasian water-milfoil was first discovered in Canada in Lake Erie in 1961. Since then, it has spread to each of the Great Lakes, the St. Lawrence River, many inland lakes throughout southern and central Ontario, and much of the United States. Outside its native range, the plant has spread across every continent, except Antarctica.
For an up to date distribution map of European water-milfoil in Ontario, visit EDDMapS.org/Ontario/distribution.
Impacts of Eurasian Water-Milfoil
- The plant reduces biodiversity by competing aggressively with native plants.
- Reduces oxygen levels in the water, caused by decomposing plants, which can lead to fish die-offs.
- Thick mats of Eurasian water-milfoil can hinder recreational activities such as swimming, boating, and fishing.
- Dense stands can create stagnant water, which is an ideal habitat for mosquitoes.
How to Identify Eurasian Water-Milfoil
- The plant is a perennial that grows under the water surface.
- Feather-like green leaves circle the stem in groups of four or five.
- Leaves have 12 or more thread-like segments.
- Tiny, reddish flowers grow on spikes 5 to 20 cm long that rise above the water towards late summer (August-September).
- Eurasian water-milfoil looks similar to two other aquatic plants: the native northern water-milfoil (Myriophyllum sibiricum) has leaves with 11 or fewer leaf segments. Whereas, the invasive parrot feather (Myriophyllum aquaticum) has not been found in the wild in Ontario.
What You Can Do
- Learn how to identify Eurasian water-milfoil and how to prevent accidentally spreading this plant with your watercraft or fishing equipment.
- Avoid infested areas or reduce your speed when travelling near Eurasian water-milfoil infestations. Your propeller can dislodge fragments and spread the pieces to new areas. New plants can grow from small pieces of the plant.
- Clean, Drain, Dry your boat, trailer, and equipment after each use. Remove all plants, animals, and mud before moving to a new waterbody.
- Avoid planting Eurasian water milfoil 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.
- Never release unwanted aquarium plants or pets. Return or donate unwanted plants to a garden centre, pet store, or put them in the garbage. Unlike terrestrial plants, aquatic plants can be composted so long as the compost is at least 30m from the water’s edge.
- If you find Eurasian water-milfoil or another invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, visit EDDMapS Ontario, or search for the ‘Invasive Species in Ontario’ project on iNaturalist.org to report a sighting.
OFAH/OMNRF Invading Species Awareness Program. (2012). Eurasian Water-milfoil. Retrieved from: www.invadingspecies.com.
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Header photo by Alison Fox, University of Florida, bugwood.org