Invasive Alien Species in Northern Ireland

Elodea canadensis, Canadian pondweed

Elodea canadensis Michaux
A submerged aquatic plant of lakes, ponds, canals and slow rivers. When first introduced into British and Irish waterways in the mid–nineteenth century it spread rapidly and became a great nuisance by blocking many waterways, but has since declined in abundance to become a more ‘well–behaved’ member of the flora.
The plant is a uniform dark green in colour, with thin narrow translucent leaves 5–13mm long, produced mostly in threes at each node and often curled backwards to some degree. Male and female flowers on separate plants, but only female recorded in Ireland. Female flowers tiny, solitary, floating at the water surface on long stalks. It can grow rooted or unrooted.
Country of origin
North America.
Current distribution
Spread throughout the British Isles and also now established on the European continent and in parts of Asia, Africa, New Zealand and Australia.
Location in Ireland
Virtually ubiquitous in suitable habitats throughout Ireland.
Life cycle
Most of the plants grown in Great Britain and Ireland are female and consequently incapable of producing seed. Spread is mostly by fragmentation of plants, either as a result of mechanical damage (for example, by boat propellers or cleaning of watercourses) or the natural production of leafy fragments which become detached from the parent plant in autumn, and which can float away, root, and start new plants. Plants can remain alive and green in Ireland’s milder winters but survive colder winters by production of overwintering buds called turions that sink to the bottom of the water in the autumn and then begin growing again in the spring. Even in its native North America, seed production plays only a minor role in dispersal of the plant.
Wildlife and habitat impacts
Canadian pondweed is now an established part of Ireland’s aquatic ecosystems. It provides good habitat for many aquatic invertebrates and cover for young fish and amphibians and food for waterfowl. However, in spite of a general impression that this species is no longer a serious problem in Ireland, there is at least one report of a locally serious impact from Scotland where this species formed a bloom in one lake which may be the result of either nutrient enrichment or the recent introduction of the plant into an enriched lake. Such blooms have a direct adverse impact on other, native, submerged aquatics by competing for light, space and nutrients. On the more positive side blooms provide increased food for wildfowl such as mute swans, but in the longer term any bloom is inevitably followed by a crash in the population which could cause turbidity or deoxygenation of the water. E. nuttallii or Lagarosiphon major may replace E. canadensis following eutrophication.
Introduction of this species into another Scottish loch has also threatened the existing population of a rare native aquatic, Naias flexilis.
Human impacts
The plant caused problems to canal boat users when it first colonised Irish and British waterways in the nineteenth century because of the way it choked the canals. This has now declined as a problem as the plant has formed a balanced relationship with other vegetation.
Key vectors
Dispersal is by fragments of plants being carried to new sites. This can be by wildfowl, or by being attached to boat propellers, fishing gear, dredging machinery etc.
What you can do as an individual
This is a commonly recommended ‘oxygenator’ of garden ponds. Do not dispose of excess material by dumping it into or beside a natural water body or stream.
Advice to specific groups relevant to this species
Managers and users of water bodies should be aware of the ease with which this plant can be transferred to new sites by accidental transport of fragments of the plant.
Management measures
Although perceived as a nuisance up until the mid–twentieth century, it is doubtful if this view can be sustained today. Since the plant is now an established part of Ireland’s aquatic ecosystem, it would be impossible and inadvisable to attempt to eradicate it. It may cause local problems (for example, in artificial ponds) because of excessive growth in which case physical removal is probably the best option, taking care to dispose of the excess material responsibly (by composting or burning). It can also be controlled by suitable herbicides and there is a biological method of control using grass carp (Ctenopharyngodon idellav) which graze the plant. Seek expert advice before using any of these methods.
Potentially, Canadian pondweed also seems able to produce blooms following nutrient enrichment of waterbodies as in the Scottish example quoted above, a potential which reinforces the importance of the avoidance of nutrient enrichment. There is evidence that E. nuttallii or Lagarosiphon major may increase and supplant E. canadensis following eutrophication.
Management groups
In Northern Ireland a number of bodies are responsible for various aspects of watercourse and water body management, water quality, recreational use and aquatic habitat management. These include the Rivers Agency, the Water Service, Environment and Heritage Service and Waterways Ireland.
Further information
Washington State Dept of Ecology web site gives an account of the plant in one part of its native range
Elodea spp. on Flora of North America on line
Methods of control – see: Centre for Aquatic Plant Management – Information Sheet 7
Simpson, D.A. (1986). Taxonomy of Elodea Michx. in the British Isles. Watsonia 16: 1-14.
(available on-line at
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum