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.
What are aquatic weeds?
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.
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’)
- 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
- 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
- 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
- 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
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.