Canadian waterweed – Elodea canadensis

Invasive alien species included in the national list (National list of alien species)

Canadian waterweed is included in the national list of invasive alien species. Invasive alien species may not be released into the wild, imported into Finland from outside the EU or from other EU countries, transported, transferred, cultivated, disposed of, passed on, sold, marketed, or otherwise handed over.

Canadian waterweed causes competition of nutrients and habitat. They grow in periods of rapid and low growth in cycles of about 5—6 years causing mass occurrences during the rapid growth. The mass occurences restrict the growth of native species or displace them. Susceptible species include e.g. watermilfoils (Myriophyllum spp.).

Canadian waterweed alters the nutrient cycle in water bodies and also affects water quality. Photosynthesis of vegetation increases the pH of water, and the decomposition of the biomass of dead plants, in turn, consumes oxygen, thus decreasing the pH of water. The drastic pH fluctuations caused by this can affect crabs, fish, and zooplankton. The decrease of oxygen in water can also cause oxygen depletion in winter. In addition, a decrease in the pH of water can release phosphorus from the bottom, causing eutrophication.

The large vegetation and the overgrowth caused by it impedes all water use, such as boating, fishing and swimming. Also, the large occurences of waterweeds may reduce the value of a body of water or beach.

The harmful effects of Canadian waterweed should be restricted and spread prevented as cost-effectively as possible. So far, no effective method have been found for eradicating Canadian waterweed. Therefore, it would be vital to prevent the species from spreading to new areas. For instance, special attention should be paid to the cleanliness of fishing equipment, boats and tools.

Canadian waterweed does not have actual roots and it propagates from even the tiniest fragments of the plant. Therefore, the eradication is difficult by methods like mowing; if done carelessly, mowing can even make the situation worse as every single fragment may form a new plant. Collecting harvesters and sweep nets are used for the removal of submerged plants. The best results are obtained by repeating the removal of vegetation for several years in a row and by combining various control measures.

Among the control methods, the clearing-seining is the most preferable. Thus, usually shoots do not need to be cut, and less pieces of plants come off and spread in the waterbody. The method is used to remove abundant vegetation and it is relatively inexpensive. However, the clearing-seining requires that plants are not firmly attached to the bottom. It is best suited for removing growths that have surfaced in late summer, but removal of overwintered shoots in early summer can also be effective.

Waterweed can also be removed with a collecting mower. However, it is likely to shed more abundant plant fragments, so there is a high risk of waterweed to spread instead. The spreading can be prevented by isolating the area of removal from the rest of the water body with a filter cloth. The mowed plant mass can release plenty of high-nutrient water before it reaches the shore. Also, the method is more expensive than the clearing-seining.

Small scale waterweed removal can be done, for example, on your own beach. If waterweeds will be removed or mowed by machine, a notification must be given to the ELY Centre and to the owner of the water area 30 days prior to the work. Extensive mowing of aquatic plants may require a permit from the Regional State Administrative Agency (AVI). Best results are obtained by repeating the vegetation removal for several years in a row and combining different control efforts.

Also essential in controlling waterweeds is to reduce the external nutrient load entering the water body or, for example, remove nutrients through management fishing. The offending odours from the decomposing waterweed stands have been reduced by aerating the water, i.e. mixing air with the water to increase the latter’s oxygen level.

You can prevent the spread of Canadian waterweed by cleaning all fishing gear of any plant fragments sticking to it and by keeping bilge water clean. Remember that Canadian waterweed must not be released into natural waters in aquarium water.

You can remove Canadian waterweed from restricted areas (less than 500 m3) on an annual basis.  Mechanical removal requires the notification of the owner of the water area and the local ELY Centre.

Observations of the species can be reported to the Invasive Alien Species Portal or through the järviwiki portal. You can also raise awareness among coastline residents and people who move in the waters about the harmfulness of Canadian waterweed and measures for preventing it from spreading.