There are actually seven species of duckweed in the UK, including five native species – common duckweed (Lemna minor), ivy-leaved duckweed (Lemna trisulca), great duckweed (Spirodela polyrhiza), fat duckweed (Lemna gibba) and rootless duckweed (Wollfia arrhiza), which is the world's smallest flowering plant. Both introduced species are from North America – the more common of the two is least duckweed (Lemna minuta) and the other is red duckweed (Lemna turionifer), which was only recently introduced.
The duckweeds most commonly found in garden ponds are least duckweed, common duckweed and fat duckweed. To the untrained eye, they are tricky to tell apart.
As its name suggests, duckweed does provide food for ducks, but it can also provide shelter for frog and toad tadpoles. It's high in protein and can provide wildlife with an importance source of food. Even humans eat duckweed in some parts of the world.
Why is duckweed a problem?
Duckweeds are not a problem in small doses. In fact, they can contribute to the ecosystem of your pond – the small China-mark moth feeds on duckweeds, making floating cases from small pieces of the leaves. Look closely and you may also spot the duckweed weevil (Tanysphyrus lemnae). This tiny weevil measures just over 2mm in length, and can be found on or around duckweed, where it burrows into the leaves.
Duckweed can also help control growth of algae in ponds. Like algae, duckweed needs nitrogen to thrive, so the more duckweed you have in your pond, the less likely you are to have algae.
However, duckweeds can multiply rapidly (doubling in size every two to three days) and, if not cleared regularly, can quickly cover the entire surface of a pond. Not only does this look unsightly, but huge carpets of duckweed can form and block light to plants growing beneath the surface. Large amounts of duckweed can also out-compete other pond plants for nutrients.
How to get rid of duckweed
Duckweed is usually introduced to garden ponds accidentally, typically on the roots or leaves of pond plants. Birds may also bring duckweed to ponds on their feet.
It's not in the best interests of you or wildlife to completely eliminate duckweed, so care should be taken to manage it, rather than remove it completely.
Use a fishing or pond net to gently sweep duckweed from the surface of the water, leaving it in a pile at the side of the pond to ensure any creatures that ended up in the net can make their way back to the pond. Avoid removing duckweed while tadpoles are in the pond as you are likely to scoop them into the net too, and they will be unable to return to the water. Take the time to remove as much duckweed as possible while frogspawn is still intact, and then do so again immediately after all the froglets and toadlets have left your pond in summer.
In larger ponds, some species of water birds, including ducks, moorhens and coots, along with fish, including grass and koi carp, will eat duckweed. But before introducing these species to your pond, bear in mind that both water birds and ornamental fish eat large numbers of pond invertebrates, including tadpoles.