Duckweeds are small, free-floating aquatic

Perennials are any plant living for at least three years. The term is also commonly used for herbaceous perennials which grow for many years (To compare: annual = one year, biennial = two years).

Common duckweed (Lemna minor) has yellow-green flattened fronds with three veins. They sometimes have reddish colouring, which is more intense on the upper side. Fronds have a single root and are widest in the middle.  

Fat duckweed (Lemna gibba) has larger, rounder fronds that are widest at the tip. They are noticeably swollen on the lower side, with four or five veins. The fronds often have reddish margins and become red in autumn.  

Least duckweed (Lemna minuta) has smaller fronds than most other species. They are translucent, grey-green, flattened on both sides and have just one vein. It is a non-native species and, unlike the others, remains green and floating over winter. 

Ivy-leaved duckweed (Lemna trisulca) has flattened, pointed fronds with three veins and a single root. Its fronds tend to float just below the water’s surface and form branched chains. 

Red duckweed (Lemna turionifera) is a non-native species with flattened fronds. The fronds have three veins, and the central vein has a row of raised bumps (pappules) along its length. They are often reddish, with the colouring most intense on the underside at the point of root attachment.  

Great duckweed (Spirodela polyrhiza) has large, flattened fronds with multiple veins and roots. The fronds are red underneath, and the whole frond becomes red in autumn. 

Rootless duckweed (Wolffia arrhiza) has tiny, bright green fronds that are swollen on both sides, making them look like beads. The fronds have no veins and no roots.

It isn’t necessary to identify which type of duckweed you have, as they are all controlled in the same way, and it is common to find multiple species growing in the same pond. However, if you would like to identify your duckweed species, RHS members can use the Gardening Advice service for help.

Native duckweeds are a natural part of water habitats in this country and a valuable plant for wildlife. They provide food for ducks and fish as well as the larvae of small China-mark moths, who build floating cases from fragments of the fronds. A carpet of duckweed also provides cover for aquatic invertebrates and spawning amphibians. 

Aside from the wildlife benefits, duckweed also helps to reduce the amount of algae in ponds and has been found to improve water quality by absorbing pollutants.

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Duckweed is a protein-rich food for ducks like mallards

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A carpet of fronds provides cover for amphibians

However, duckweed multiplies rapidly and can quickly cover large areas of still or slowly-moving water. It can shade out submerged pond plants, outcompete plants for nutrients and reduce the amount of oxygen in the water. It is therefore generally treated as a weed and controlled to keep numbers in check.

Here are our answers to your most common questions about dealing with duckweed: 

How invasive is duckweed? 

Duckweed is often unknowingly introduced to ponds hidden among shared or newly bought pond plants. It may also be brought in on the feet of visiting water birds or arrive in watercourses. Duckweed spreads by vegetative propagation (cloning), where two daughter plants bud off from an adult one. This takes place every two to three days in warm summer conditions, meaning a single duckweed plant can very quickly become a large mat that completely covers the surface of a pond.  

The good news is that there are several easy ways to control duckweed if it becomes a nuisance.

Do I need to get rid of duckweed? 

It is a good idea to regularly control duckweed to stop it smothering and outcompeting ornamental pond plants. However, as it is a valuable wildlife plant, providing food and shelter, you should aim to reduce its numbers down to an acceptable level rather than try to eradicate it completely.

A small carpet of duckweed is a great addition to wildlife ponds

What is the easiest way to control duckweed? 

Complete control of duckweed is nearly impossible and not necessary for a healthy pond. To remove excess and stop it reaching nuisance levels, try the following options: 

• Quarantine new pond plants – keep new (or shared) aquatic plants in a bucket of water for a few weeks before introducing them to your pond. If duckweed appears in the bucket, keep removing it by hand until the water stays clear.  
• Remove duckweed by hand – use a net or rake to regularly scoop fronds from the water. Do this weekly during the summer months when duckweed numbers can double every few days. Removed duckweed can be added to your home compost or green waste recycling bin.  
• Use a floating boom on larger ponds – sweep a boom from end to end to collect duckweed and make removal with a net easier. Booms can be as simple as a wooden batten or old pool noodle, or there are specialist booms available from pond suppliers.  
• Install a fountain – this disturbs the water surface, making it more difficult for duckweed to grow and spread. 
• Stock your pond with floating-leaved pond plants – duckweed doesn’t compete well with other floating aquatic plants, like waterlilies, so keeping ponds well stocked helps to reduce their numbers. Having some of the water surface covered with vegetation is also beneficial for pond wildlife.  
• Remove debris – leaves and other rotting vegetation increase the nutrient content of pond water, encouraging duckweed growth. Place mesh over your pond to collect leaves as they fall in autumn or use a hand net to remove any that land in the water.  
• Create some shade over your pond – duckweed grows and spreads fastest in full sun. Create shade by planting tall marginals on the south bank of the pond.  
• Fit stop-boards at upstream inlets – this prevents duckweed entering ponds via waterways. 
• Introduce duckweed grazers – in large ponds you could consider introducing fish, such as goldfish, koi and grass carp, or water birds, such as domestic ducks, to eat duckweed.  
• Use a non-chemical duckweed treatment – Ecopond Duckweed Control contains a bacterial culture that removes nutrients from the water to discourage duckweed. According to the manufacturers, it is best applied in spring as soon as duckweed growth is seen.

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Removing duckweed with a net is quick and easy

Stop leaves landing in your pond with mesh nets

Should I use weedkiller?  

No – there are no chemical treatments or weedkillers approved for use by home gardeners for the control of duckweed. Although time-consuming, duckweed numbers can easily be kept in check using the methods described above.