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Aquatic weeds

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last updated Mar 26, 2014
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New Zealand pygmy weed choking a pond. Credit: RHS Advisory.

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? Back to top

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:

Azolla filiculoides
Crassula helmsii
Hydrocotyle ranunuculoides
Ludwigia grandiflora
Myriophyllum aquaticum

Appearance Back to top

Submerged plants (aka ‘oxygenators’)

  • These grow mostly underwater with usually only the flowering shoots appearing above the surface. They are not vital in ponds to provide oxygen, although they do have value as cover for aquatic animals
  • Among the most troublesome are Crassula helmsii (New Zealand pygmyweed), Elodea canadensis (Canadian pondweed), E. nuttallii (Nuttall’s pondweed), Lagarosiphon major (curly waterweed), Potamogeton crispus (curled pondweed) and species of Myriophyllum (water milfoil)

Marginal or emergent weeds

  • Rushes, reeds and sedges grow in shallow water in the margins. Some of the most invasive species in larger areas of water include Glyceria maxima (reed sweet-grass), Phragmites communis (common reed) and Typha latifolia (reedmace)
  • 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
  • Large garden pond species such as Iris pseudacorus (yellow flag iris) may also be a problem in small ponds

Floating plants

  • These form dense, unsightly mats across the whole water surface. They can be dangerous to children and livestock who mistake them for solid ground
  • The most troublesome of the free-floating species are Lemna (duckweeds) and Azolla (water fern)
  • Floating-leaved plants include Hydrocotyle ranunculoides (floating pennywort), as well as those capable of rooting in deeper water such as Nuphar lutea (yellow waterlily), Nymphaea alba (white waterlily), Potamogeton natans (broad-leaved pondweed) and Polygonatum amphibium (amphibious bistort)

The problem Back to top

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

Control Back to top

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.

Non-chemical controls

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

Chemical controls

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.

Quick facts

Common name Various
Botanical name Various
Areas affected Ponds, lakes and water features
Main causes Submerged, floating or marginal weed
Timing Some seen year round, other species seen late spring to autumn; treat when seen.
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