Advice
RHS Help & Advice
Aquatic weeds
Aquatic plants can create problems when they make excessive growth. In garden ponds control is relatively easy, but in larger ponds and lakes it is more difficult.
Aquatic plants are usually a problem only during the warmer months of the year when water temperatures rise above 6C (43F). Many then grow rapidly and can quickly take over garden ponds. Larger, shallow ponds and lakes are also likely to become choked especially where there is nutrient rich run-off from surrounding agricultural land.
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 can provide details of suitably qualified contractors to carry out spraying of aquatic weeds. To visit the website (www.naac.co.uk) click here
It is very important that water weeds that are removed from garden ponds are either 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.
Types of water weed
Submerged plants
These weeds are rooted in the mud with usually only the flowering shoots appearing above the surface. They often increase rapidly and can quickly fill even large lakes, smothering more desirable water plants. It is a myth that submerged plants are 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).
In garden ponds growth can be checked by frequent thinning using a rake. In larger, shallow ponds and lakes it may be possible 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. Because Crassula helmsii will regrow from tiny stem fragments it should not be controlled by cutting.
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.
Marginal or emergent weeds
Emergent plants, such as rushes, reeds and sedges grow in shallow water in the margins and increase by means of spreading rhizomatous roots. 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).
In garden ponds species such as Iris pseudacorus (yellow flag iris) may need lifting every three to four years and dividing before replanting. For small natural ponds hand-pulling is highly effective but try to ensure the roots are pulled out. Alternatively, plants can be dug out by hand. 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
In areas of still water floating-leaved plants may completely cover the water surface cutting out sunlight to submerged plants. These unsightly mats can also be dangerous to children and livestock who mistake them for solid ground.
Free-floating plants
Among the most troublesome are the free floating species of Lemna (duckweeds) and Azolla (water fern). 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 water lilies. Proprietary products are sometimes offered and can be obtained from suppliers of aquatic sundries. Whatever methods are used, complete control is usually impossible. Regular inspection is therefore necessary to prevent re-establishment.
Floating-leaved plants
Hydrocotyle ranunculoides (floating pennywort) roots in the margins and forms floating mats. Plants capable of rooting in deeper water include Nuphar lutea (yellow waterlily), Nymphaea alba (white waterlily), Potamogeton natans (broad-leaved pondweed) and Polygonatum amphibium (amphibious bistort). They 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.
Cultural control
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. Typically no more than 30-50% of the surface area is covered to conserve fauna that can later recolonise the treated area and to reduce the risk of deoxygenation.
Useful links
Applied Vegetation Dynamics Laboratory
Centre for Aquatic Plant Management
Pond Conservation:The Water Habitats Trust (previously The Ponds Conservation Trust)
Tony Dickerson