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Aquatic weeds (or pond weeds) can normally be tolerated in small numbers, but it is when they make excessive growth that they become a nuisance, particularly in summer. In garden ponds control is relatively easy, but in larger ponds and lakes it is more difficult.
New Zealand pygmy weed choking a pond. Credit: RHS Advisory.
Aquatic weeds are usually a problem only during the warmer months of the year when water temperatures rise above 6°C (43°F). Many plants grow rapidly in the warmer temperatures and can quickly take over garden ponds.
All ponds, from small shallow ponds, to larger lakes can become choked with weeds especially where there is nutrient rich run-off from surrounding agricultural land.
In recent years a growing problem has been posed by a number of introduced aquatic plants. These can be very invasive and can have seriously detrimental effects on gardens and the wider landscape. In early 2013 Defra announced a ban on sale of five of the worst invasive water plants in the UK which will come into force in April 2014. The five species that will be banned from sale are:
Invasive water weeds are troublesome in various ways:
It is very important that weeds removed from ponds or lakes are composted, buried or burnt. On no account should they be transferred to rivers, other ponds or lakes. Several introduced pond weeds, widely available from garden centres, cause enormous problems where they escape or are introduced into the wild.
Different approaches will be needed depending on the type of aquatic weed:
Submerged plants (aka ‘oxygenators’)
There are no weedkillers approved for the control of aquatic weeds in gardens, but there are a small number approved for use by professionals. Because of the danger of water pollution their application is very carefully controlled and prior approval for their use must be obtained from the Environment Agency or equivalent authority.
The National Association of Agricultural Contractors (NAAC) can provide details of suitably qualified contractors to carry out spraying of aquatic weeds.
Freshwater Habitats Trust
Invasive non-native species
Pond algae and blanket weed
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