Canadian pondweed
Canadian Pondweed or Waterweed - Elodea canadensis - is native to North and South America where it occurs in lakes ponds, canals and slow flowing water. It was introduced to
Ireland in 1836 as a fragment on an imported log from Canada, and rapidly spread to Europe soon after, occurring in similar habitats. In many regions of the world it is
considered a pest. It grows from stolons (creeping stems) and has vertical, narrow, sparsely branched stems with leaves in whorls of three. The leaves are flat and it can form dense mono-specific strands. It does not reproduce by seed in the UK and relies entirely on vegetative reproduction for its spread. Although it is now regarded as a naturalised aquatic plant, it causes problems by competing for nutrients and outgrowing many native species. However, it is now considered preferable to xertain related weeds like Lagarosiphon major and E. nuttallii and where there is a danger of invasion from these species after control, care should be exercised not to eradicate all of the plant.
Control
Mechanical control - This plant is easily cut and controlled for short periods (1-2 months in summer) by mechanical control methods. The cut weed should be removed from the water to avoid deoxygenation. The cut weed can be left to decompose in small heaps away from the
side of the water, taking care to avoid seepage of the liquor back into the water.
Cutting early in spring may delay the onset of the peak biomass period. If the weed can be kept at a low level by regular cutting, then doing this then peak biomass should not be reached.
Continued cutting will weaken the plant and may lead to its disappearance from the system. Our amphibious weed cutter is an excellent way to remove or control this plant.
Chemical control - There are no approved herbicides to control Canadian Pondweed as it grows below the surface. Cutting and removal is the only option.
Biological control - The use of herbivorous Grass Carp is appropriate as a control method for this plant. Common Carp, and other bottom feeding fish, which create turbid water, can also be effective in preventing regrowth of the plant after mechanical removal or control by a herbicide. There have been reports of sudden population crashes of this species and it may be that some form of self-regulation occurs in some situations. It is not known if this is due to a pathogen.
Environmental control - Shade will control most submerged aquatic plants. This can be achieved by
planting trees on the south side of waterbodies or by using a floating sheet of
opaque material. Care must be taken when using the latter to prevent
sudden deoxygenation.