Eurasian water-milfoil prefers shallow water one to three metres deep, but can root in up to 10 metres of water. A fast-growing perennial, it forms dense underwater mats that shade other aquatic plants. When large stands begin to die off in the fall, the decaying plants can reduce oxygen levels in the water.
The plant can interbreed with native milfoils, 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 all 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.
Impacts of Eurasian Water-Milfoil
- The plant reduces biodiversity by competing aggressively with native plants.
- Reduced oxygen levels in the water caused by decomposing plants can kill fish.
- Thick mats of Eurasian water-milfoil can hinder recreational activities such as swimming, boating and fishing.
- Dense stands can create stagnant water, which is 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 five to 20 centimetres long that rise above the water.
- The plant blooms in late July and early August.
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. The invasive parrotfeather (Myriophyllum aquaticum) has not been found in the wild in Ontario, but it is common in the aquarium trade. It has leaves that rise above the surface of the water, while only the terminal flower spike of Eurasian water-milfoil emerges from the water.
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 break off fragments and spread the pieces to new areas. New plants can grow from small pieces of the plant.
- Inspect 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 or pet store, or put them in the garbage. Do not put them in the compost or discard them in natural areas. Discarded plants may produce seeds than can sprout.
- If you find Eurasian water-milfoil or other invasive species in the wild, please contact the toll-free Invading Species Hotline at 1-800-563-7711, or report a sighting online.
- Invasive Species: A Threat to Ontario's Biodiversity
- Ontario Invasive Plant Council
- Invasive Species Centre
OFAH/OMNR Invading Species Awareness Program. (2012). Eurasian Water-milfoil. Retrieved from: http://www.invadingspecies.com. This factsheet may be reproduced for non-commercial purposes.
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