Eurasian watermilfoil

Identification & Biology
Alias' : milfoil
Latin Name : Myriophyllum spicatum
Category : Aquatic Plants
Description :
  • Rooted, submersed plant inhabiting the shallow waters of lakes in British Columbia. 
  • Long, underwater stem that branches profusely when it reaches the surface of the water. 
  • Leaves are whorled on the stem at each node, and there are generally four leaves per whorl. 
  • Leaves are finely divided and feather-like in appearance. There are usually 12 to 21 pairs of leaflets. Each leaflet is thin, fine and about 1- 1.5 cm long.  
  • Produces small reddish flowers that emerge several inches above the water on a spike grown from the tip of the stem.  


Eurasian watermilfoil plants form thick underwater stands and dense mats on water surfaces. Floating plant fragments produced by waves and boaters are spread by water currents, making the plant difficult to contain. New plants develop when the fragments sink, rooting best in protected locations. Boats and boat trailers carrying plant fragments are thought to be the most common form of spread from one water body to another. 

Eurasian milfoil is sometimes confused with Northern watermilfoil (Myriophyllum sibiricum), which has fewer leaflet pairs (5-10). 

Look Alikes

In the Okanagan, milfoil was first identified in the Vernon Arm of Okanagan Lake in 1970. By 1974, the plant was well established in all of the mainstem lakes of the Okanagan. The maximum depth where rooted plants are found will vary with the depth of light penetration; for milfoil in the Okanagan this is typically about 5 to 6 metres deep, with some plants found up to 8 metres deep. Milfoil is well adapted to rooting in a variety of substrates, from sandy bottom to very silty substrate. Gravel substrates are not preferred.

Impact & Risks

Milfoil is very aggressive and once introduced to a waterbody will displace native aquatic vegetation in a couple of years. In British Columbia, problems caused by milfoil include: 

  • Growing and spreading rapidly. 
  • Invading and replacing native plant communities. 
  • Obstructing swimming, boating, waterskiing and fishing. 
  • Reducing the appeal of beach areas due to the accumulation of plant debris.  
  • Impeding flood control, water conservation, drainage and irrigation works. 
  • Reducing the economic benefits of tourism where dense growth limits recreation. 

Many uninfested water bodies remain susceptible to the introduction of this plant.

Prevention & Mitigation

The most effective way to prevent the spread of milfoil is by prevention. Here are some recommendations for how to prevent milfoil invasion:

  • Clear all plant material from boats, motors, trailers, wet wells and anchors. 
  • Dispose of plants far away from water bodies. 
  • Learn how to identify Eurasian watermilfoil. 
  • Report suspected new infestations to the Ministry of Environment. 
  • Contact local authorities and seek expert advice when concerned about aquatic plant problems in your community. 
Treatment & Disposal

 Management strategies currently in use in British Columbia include: 

  • Preventive efforts (e.g. surveillance of non-infested areas and public information to discourage spread, particularly by boaters).  
  • Placing bottom coverings on new populations to prevent lake-wide infestations.  
  • Root removal (maintenance of priority areas by rototilling or shallow water cultivation). 
  • Harvesting (cosmetic control by cutting the plant below the water surface). 
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.