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 or bog plants. These can be very invasive and can have seriously detrimental effects on gardens and the wider landscape. Species currently banned from sale are:

Alternanthera philoxeroides (alligator weed)
Azolla filiculoides (fairy fern)
Cabomba caroliniana (Carolina fanwort)
Crassula helmsii (New Zealand pygmy weed)
Eichhornia crassipes (water hyacinth)
Elodea nuttallii (Nuttall's waterweed)
Gunnera tinctoria (Chilean rhubarb)
Hydrocotyle ranunculoides (floating pennywort)
Lagarosiphon major (curly waterweed)
Ludwigia grandiflora (water primrose)
Ludwigia peploides (creeping water primrose)
Lysichiton americanus (American skunk cabbage)
Myriophyllum aquaticum (parrot's feather)
Myriophyllum heterophyllum (broadleaf watermilfoil)

Submerged plants (also called ‘oxygenators’)

• Among the most troublesome are the non-natives: Elodea canadensis (Canadian pondweed), E. nuttallii (Nuttall’s pondweed), Lagarosiphon major (curly waterweed) and species of Myriophyllum heterophyllum (broadleaf watermilfoil)

Marginal or emergent weeds

• The non-native Ludwigia spp. (water primrose) is a problem weed on water courses in France and has now been found at sites in the UK where it is being eradicated 
• Rushes, reeds and sedges grow in shallow water at the margins of a pond. Some of these plants can be too vigorous even for large garden ponds, covering over the open water entirely in a few years. For this reason, regular removal (usually annually) of UK-natives including Glyceria maxima (reed sweet-grass), Phragmites australis (common reed) and Typha latifolia (reedmace) is needed to limit the spread in garden situations
• Large garden pond species such as the native Iris pseudacorus (yellow flag iris) may also be too vigorous in small ponds without regularly reducing the size of the clumps

Floating plants

• The most troublesome of the free-floating species are the non-native Lemna (duckweeds) and Azolla (water fern), plus the floating-leaved plant Hydrocotyle ranunculoides (floating pennywort) and Crassula helmsii (New Zealand pygmyweed)
• Vigorous native plants capable of rooting in deeper water may need regular removal to prevent them filling a garden pond. Examples include Nuphar lutea (yellow waterlily), Nymphaea alba (white waterlily), Potamogeton natans (broad-leaved pondweed) and Persicaria amphibia (syn. Polygonum amphibium) (amphibious bistort)

Invasive water weeds are troublesome in various ways:

• Submerged aquatic plants root in the mud. They often increase rapidly and can quickly fill even large lakes, smothering more desirable water plants
• Marginal weeds usually increase by means of spreading rhizomatous roots
• Floating-leaved plants root in the margins or sediment at the bottom of the pond and form floating mats
• Floating-leaved and free-floating aquatics may completely cover the water surface, especially on still water, cutting out sunlight to submerged plants

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.

The RHS believes that avoiding pests, diseases and weeds by good practice in cultivation methods, cultivar selection, garden hygiene and encouraging or introducing natural enemies, should be the first line of control. If chemical controls are used, they should be used only in a minimal and highly targeted manner.  For example, where pests, diseases or weeds pose a serious threat to the wider environment, to important heritage specimens, to habitat, or to native wildlife.

Cultural control

Different approaches will be needed depending on the type of aquatic weed:

Submerged plants (aka ‘oxygenators’)

• In garden ponds thin the weed frequently using a rake
• In larger, shallow ponds and lakes try thinning using a long-handled scythe to cut by hand. In deeper water use a chain scythe. For large areas specialist contractors can be employed using weed cutting boats or weed bucket attachments
• It is likely that cutting will be required twice during the growing season
• Do not cut Crassula helmsii as it will regrow from tiny stem fragments
• Most water weeds float to the surface when cut and it is essential that as much as possible is removed from the water; left in place it decays leading to de-oxygenation. Where there are flow outlets, booms should be placed to prevent the weed washing downstream
• With heavily silted ponds and lakes it may be necessary to drain and dredge

Marginals

• Lift and divide Iris pseudacorus (yellow flag iris) every three to four years
• For small natural ponds, hand-pulling is highly effective but try to ensure the roots are pulled out. Alternatively, plants can be dug out
• Cutting in July or August limits the time for regrowth before the end of the growing season but has to be repeated annually
• Livestock can also be used to manage bankside growth of some rushes and reeds

Floating plants

Free-floating plants:

• In garden ponds they can be removed with a rake or net or hosed to the side of the pond for removal
• The use of a fountain to disturb the water surface may also reduce infestations
• In larger ponds and lakes, a floating boom can be used to sweep the surface from end to end
• Stop-boards should also be fitted at upstream inlets to prevent weeds entering
• Duckweeds do not compete well with other floating-leaved plants such as waterlilies
• Whatever methods are used, complete control is usually impossible. Regular inspection is therefore necessary to prevent re-establishment
• A biological control (Stenopelmus rufinasus weevil) for Azolla is available

Floating-leaved plants:

• These can be cut and cleared the same way as submerged water weeds. With waterlilies, however, cutting gives only short-term control as new leaves will regrow from the rhizomes
• In garden ponds, plants can be lifted out every two or three years, thinned and replanted. The use of planting baskets makes the job easier
• Many aquatic weeds are intolerant of shade. This can be created by bankside planting of taller marginals or trees and shrubs on the south side
• In larger, still waters, with heavy infestations, black polythene sheet weighted at the corners can be used to shade out water weeds but it should remain in place for at least four to six months. Don't cover more than 30-50 percent of the surface area so as to conserve fauna and reduce the risk of de-oxygenation

Weedkiller control

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.