There are meany types of WEED. This is but one.

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 (not recurved like Lagarosiphon major and pointed like E. nuttallii). It can form dense mono-specific stands. 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 both major and 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.

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. However, if large amounts are to be disposed of then it should be taken away for composting or alternative disposal.

Cutting early in spring may delay the onset of the peak biomass period. Dragging trailing knives across the bottom in March is the best time for this. If the weed can be kept at a low level by regularly doing this then peak biomass should not be reached.

Continued cutting will weaken the
plant and may lead to its disappearance from the system

There are several appropriate methods of mechanical control, removal by hand, raking, chains, weed bucket weed boat or dredging. All are suitable.

Chemical control

There are no methods for chemical control of
Elodea species in Europe. Outside Europe, some herbicides are approved, and readers should consult their local government Environment Agency or equivalent.

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, or a stem mining fly which is yet to be investigated.

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.

The use of dyes has been successful in static waters. Early application of the dye is critical to the success of this technique, preferably before the plant has started to grow in spring, or when water temperatures are still less than 8 to 10ºC. A further application may be required after 6 – 8 weeks, depending on dilution from rainfall, or degradation by UV light. Blue dyes are generally cheaper than other colours, but all colours will reduce or completely control
E. canadensis.

Best option

In mixed stands: Remove as much of the plant as possible by mechanical means. If mechanical removal is not possible then treat early with a pond dye in static waters only. Repeat application of dye throughout the season as necessary.

If you prefer a biological control option then use Grass Carp