The term ‘weed’ describes a plant that is growing where it isn’t wanted. In most cases, it is up to you to decide what you call a weed and what you choose to retain or remove. However, there are several invasive non-native aquatic plants that can have seriously detrimental effects on gardens and the wider landscape. These plants should be treated as weeds and there are measures in place to control their spread.  

Species currently banned from sale are: 
Alternanthera philoxeroides (alligator weed) 
Azolla filiculoides (water fern) 
Cabomba caroliniana (Carolina watershield) 
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) 
Ludwigia uruguayensis (syn. Ludwigia grandiflora subsp. hexapetala) (water primrose)  
Lysichiton americanus (yellow skunk cabbage) 
Myriophyllum aquaticum (parrot's feather) 
Myriophyllum heterophyllum (broadleaf watermilfoil)

Aquatic plants can be divided into three main categories depending on how and where they grow. Below we give examples of the most problematic plants in each category.    

Submerged plants (oxygenators) 

These grow mostly underwater with usually only the flowering shoots appearing above the surface. They produce oxygen during the day and provide cover for aquatic life. 

Submerged aquatic plants root in mud. They often increase rapidly and can quickly fill even large lakes, smothering more desirable water plants. 

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

Floating plants 

Free-floating aquatic plants and floating-leaved plants (rooted in sediment) can form dense mats across the whole water surface, especially in still water. This blocks sunlight from reaching submerged plants and 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), plus the floating-leaved plants 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).

Marginal or emergent plants 

These are plants that live along the edges of ponds, either directly planted in the muddy bank or planted in pond baskets positioned at the edge. Many marginal plants that spread quickly, do so by rhizomes.

They provide habitat for wildlife, but if they are too large and vigorous, they can easily choke a garden pond and may need to be controlled. 

The non-native Ludwigia (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 can be too vigorous even for large garden ponds, covering open water entirely within 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 (bulrush) is needed to limit their 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, unless sections are regularly removed.

It is a good idea to remove or control the spread of any aquatic plants that are outcompeting and smothering more desirable pond plants. A large amount of a single plant species will affect the health, biodiversity and attractiveness of your pond.  

Aquatic plants listed on Schedule 9 of the Wildlife and Countryside Act and Species of Special Concern list should not be allowed to spread, and a relevant environmental authority can order control measures if they deem it necessary.

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

Submerged plants (oxygenators) 

• Thin out weeds using a rake. Do this regularly in a garden pond to keep growth under control.  
• Cut and thin out weed growth in larger ponds and lakes. Use a long-handled scythe to cut by hand in shallow water. In deeper water use a chain scythe. For large areas, specialist contractors can be employed using weed cutting boats or weed bucket attachments. 
• Cut twice during the growing season to achieve good control.  
• Avoid cutting Crassula helmsii as it will regrow from tiny stem fragments. 
• Remove cut weeds from the water. Most submerged weeds will float to the surface after cutting and you should aim to remove as much as possible. They decay if left in place and this causes de-oxygenation of the water. Where there are flow outlets, booms should be placed to prevent the weed washing downstream. 
• Consider draining and dredging heavily silted ponds and lakes.  

Floating plants 

Free-floating plants: 

• Use a rake or a net to remove unwanted free-floating plants. It may be possible to hose them to the side of your pond to make removal easier.    
• Install a fountain to disturb the water surface, making conditions less favourable for free-floating plants.    
• Use a large floating boom to sweep across the surface of larger ponds and lakes.  
• Fit stop-boards at upstream inlets to reduce the amount of weeds entering your pond or lake.  
• Stock your pond with a variety of ornamental aquatic plants to help outcompete free-floating weeds. For example, duckweeds do not compete well with other floating-leaved plants, such as waterlilies.  
• Check regularly to assess the level of weed cover, bearing in mind that whatever methods are used, complete control is usually impossible.  
• Consider using biological control (Stenopelmus rufinasus weevil) to reduce Azolla numbers.  

Floating-leaved plants: 

• Cut and clear floating-leaved plants the same way as submerged plants. With waterlilies, bear in mind that cutting only gives short-term control because new leaves will regrow from the rhizomes. 
• Lift, thin and divide plants every few years to control their spread. Replant the divisions, ideally into planting baskets to make the job easier.  
• Create some shade over your pond. Many floating-leaved plants won’t thrive in shady conditions. Create shade by planting tall marginal plants or trees and shrubs on the south side of your pond.  
• Consider covering areas with black polythene sheeting. This option should only be done as a last resort if you have a very high weed population in a large pond or lake with still water and other control methods haven’t been successful. The black polythene will need to be weighted down at the edges to secure it and should be left in place for at least 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.  

Marginals: 

• Lift and divide herbaceous marginal plants that you want to keep, such as Iris pseudacorus (yellow flag iris), every three to four years.  
• Hand-pull plants. This is highly effective if you can lift the roots out. Alternatively, plants can be dug out but take care not to damage the liner or base of your pond.  
• Cut back growth in July or August; this limits the time for regrowth before the end of the growing season but must be repeated annually to achieve good control.  
• Use livestock to manage bankside growth of some rushes and reeds, if the size and access to your garden makes this feasible.

Should I use a weedkiller? 

No, there are no weedkillers approved for use by home gardeners for the control of aquatic weeds.

There are a small number of aquatic weedkillers approved for use by professionals, but 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 control of aquatic weeds.