This article has been checked for horticultural accuracy by Oliver Parsons.
Are weeds good for a pond?
Ponds are just like any other part of the garden in that weeds do occur and the plants growing in the water need some control from time to time. Pond weeds are known as aquatic weeds as they live in, on, and at the edges of water. Some can be good in small amounts, providing food and shelter for aquatic invertebrates and larvae. However, pond weeds are prone to vigorous growth and, if left unchecked, they can smother other plants, or completely cover the water surface so the water is no longer visible. Restricting the growth of pond weed ultimately means a healthier pond. It also prevents the pond from gradually silting up, because every year the dead growth builds up in the pond and rots down. The most problematic times for pond weed are mid-spring to mid-summer, when water temperature rises and day length is increasing, which stimulates growth.
Avoiding pond weed problems
When introducing new plants to your pond, avoid well-meaning gifts of plants from friends because even small fragments of pond weed can easily hitch a ride in and then rapidly spread to become a nuisance.
Also, a number of pond plants that were once cultivated and sold as ornamental pond plants are now classed as invasive weeds. Although many are now forbidden from sale, these are still growing in countless garden ponds and may be unwittingly passed on, albeit with the best of intentions. Always buy from a reputable source to be sure of having clean, legal, weed-free plants.
Common pond weed types
Floating pond weeds
The most common floating pond weeds are duckweed (Lemna species) which has bright green leaves in pairs, and fairy moss (Azolla) which forms crinkled clusters of reddish leaves. Individually, these plants are tiny but grow at great speed and soon form dense mats that, if not tackled, may cover the entire surface of the pond. Water that isn’t visible is a real danger as visitors and young children may not realise the pond is there, step on an apparently solid surface, and go straight in.
Floating invasive plants that are banned from sale are floating pennywort (Hydrocotyle ranunculoides) and New Zealand pygmyweed (Crassula helmsii).
Submerged pond plants
Also known as oxygenators because they take up nutrients and release oxygen into the water, these plants live in the deeper parts of the pond and form clumps below the surface, usually rooting in the silt at the base of the pond.
Invasive submerged plants that are banned from sale include curly pond weed or curly water weed (Lagerosiphon major), Nuttall’s waterweed (Elodea nuttallii), parrot’s feather (Myriophyllum aquaticum), and water milfoil (Myriophyllum heterophyllum).
Blanket weed is a common type of algae which forms masses of very fine, hair-like, bright green strands. These grow on pond sides, plants, or form cotton-wool-like clumps in the water. Blanket weed thrives in water that is high in nutrients, which may come from one or more sources such as tap water, garden fertilizer or manures, fish waste or food, and organic debris such as leaves. While small amounts of blanket weed are not a cause for concern, particularly early in the year, an abundance of weed may cause problems by inhibiting plant growth and affecting fish health by reducing oxygen levels in the water.
How to control pond weeds
Weeds in a pond are almost impossible to totally eradicate, so regular control two or three times a year is the most achievable aim. Because of the sensitive nature of the aquatic environment and the number of creatures that live in or use the water, physical control is the only way to tackle pond plants that have become weeds. Using weedkillers is not possible for the gardener without killing wildlife and fish. There are more control options for blanket weed and other forms of algae, which are very simple plants.
Remove unwanted weed growth by hand-pulling, using a rake, and using a net to clear floating weeds and any other weed fragments left over from pulling. Take care not to damage or puncture the pond liner when using any tools in the pond.
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Pile collected weed by the side of the pond for several days so any small creatures can make their way back to the water (bear in mind that tadpoles aren't able to crawl back in so avoid removing material when tadpoles are in the pond).
When to tackle pond weeds
Autumn is the ideal time to tackle pond weeds, because there are fewer species using it. In spring frogs, toads and newts lay their eggs in the weed, while in summer the tiny tadpoles and other larvae may become entangled in piles of removed weed and are unable to escape.
Floating water weed is best tackled little and often to stop it taking over.
Getting rid of pond weed
Care must be taken when disposing of weed taken out of a pond because some of the more invasive types continue to grow and may spread into the wild, with the potential to cause serious problems. Put leafy growth and stems only in the garden compost bin. Compost any roots by bagging up in old compost sacks, turn over the top to exclude light, for at least a year, which kills growth. On no account dump pond weeds in the wild.
Chemical weedkillers and ponds
Using chemical weedkillers either in or around a pond is only permitted for professional contractors and is strictly enforced by the Department of the Environment, Food, and Rural Affairs (DEFRA) under regulations that require Environment Agency approval. There are no pond weed killers licensed for use by home gardeners.
How to kill weeds growing near a pond
The safest way to tackle garden weeds growing close to a pond, or on ground above where rainwater runoff will go into the pond, is by physical methods of control such as hoeing, hand-digging, using a weed burner, or pulling, rather than using a chemical weed killer.
Invasive pond plants
Plants that were once cultivated and sold as pond plants but have been discovered to have ultra-vigorous, weed-like tendencies are known as invasive pond plants. A number of these are now banned from sale and it is an offence to plant or dispose of them in the wild because of the threat they pose to natural ponds, lakes, and watercourses. As the effects of climate change increase, plants that are causing major problems in other countries, such as water hyacinth (which chokes huge areas of waterways in warmer countries) could become an issue in the UK.