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Municipal Services / Urban Planning and Environment / Environment / Eurasian water-milfoil

Eurasian water-milfoil

Eurasian watermilfoil has a pretty name, but in fact it is an extremely aggressive, invasive aquatic plant that, for our lakes, is the equivalent of an alien from outer space. It is a perennial plant from Europe, Asia, and North Africa that easily adapts to a wide variety of environments (lakes, ponds, rivers, and wetlands) and conditions. It can grow at depths of up to ten metres, getting off to an early start in the spring (as soon as the water temperature reaches 15°C), and multiplying rapidly to form dense colonies (300 stems per m2!) below and at the water surface, where it spreads out in a mat.

A formidable enemy

These dense mats compete with native plants for light and nutrients, thus reducing biodiversity. They also encourage mosquitoes and interfere with recreational activities like boating (by getting caught up in motors), fishing (cast your line and catch a bunch of milfoil), waterskiing, and of course swimming. Eurasian water-milfoil also has a secret weapon. It multiplies so quickly because its main mode of reproduction is by cuttings. So if you drive your boat through a colony, your propeller will cut up the stems into a thousand pieces, creating a thousand new plants. Each little stem fragment will grow roots and plant itself elsewhere in the lake bed. This is what makes this invader so hard to control and eradicate. Even when it dies at the end of the season, Eurasian water-milfoil is harmful because it settles to the bottom and decomposes, using up dissolved oxygen in the water and reducing oxygen concentrations in the lake. This can kill certain fish species, further decreasing biodiversity. This suffocation mechanism and the tendency of the plant’s dense colonies to gradually eliminate native aquatic plants important to a number of wildlife species have a significant impact on the environment.

What’s it like to have Eurasian water-milfoil as a neighbour?

Just ask those who live on contaminated lakes. Besides harming the environment, it interferes with the view and recreational activities, is bad for tourism, and brings down property values. As lake associations will tell you, it’s better to avoid getting it in the first place. Once established, it is almost impossible to keep it from spreading. There are ways to combat it, including having divers manually remove it or vacuum it up, or installing jute or geotextile tarpaulins on the lakebed. But these operations are costly and need to be constantly repeated because the plant grows so quickly. There is probably no way of completely eradicating Eurasian water-milfoil, and existing methods only slow its expansion.

An ounce of prevention is worth a pound of cure

First, it’s important to know how it spreads. The main culprit is boat traffic between lakes . There are other sources of contamination, but this is by far the most damaging.

In areas where the plant is already established, it’s important to prevent conditions that help it multiply. For example:

  • avoid driving in shallow water, where your propeller can cut the stems.
  • Don’t spread fertilizer on lawns near the shore, and
  • make sure your septic system complies with environmental standards.

Vectors that help spread eurasian water-milfoil:

  • Pleasure boats (boats, rowboats, canoes, kayaks, personal watercraft, etc.)
  • Seaplanes
  • Anglers and hunters (boats, hunting and fishing equipment, bait pails, water in live wells)
  • Water gardens
  • Aquatic birds (e.g., ducks)

For more information

Ministère du Développement durable, de l’Environnement et des Changements climatiques (MDDECC) where you can find (in French):