Is Duckweed a curse or a useful plant? It can be either, or both. I caution against risking getting any into waterways that do not have it in already; most rivers and lakes already have some. Duckweed is one of the plants that can grow at enormous rates. Azolla is another one, but some of their characteristics are quite different. For information about Azolla, see “Azolla Fact Sheet”.
In the summer, and if the nutrient content of the water is high, duckweed grows very quickly. Although it is a ‘flowering plant’ its flowers are very rarely seen and in practice it only reproduces vegetatively by growing more plants off the side of the existing ones. In water without plenty of Nitrogen duckweed does not grow quickly. Duckweed contains between 20 and 50 percent crude protein expressed as a dry matter percentage so its Nitrogen requirements are high. Similarly, it is removing excess Nitrogen from the water.
Often the rampant growth of duckweed is a sign that the water has too much Nitrogen and other nutrients. The Duckweed is being blamed, but the real culprits are people who are polluting the water .
Duckweed is a good food for many animals. Ducks eat it. Many types of fish do as well. In our aquarium for large goldfish at the front of our shop, in summer, the fish get through a surprising amount of duckweed. I feed them other things as well (I believe in giving my fish a good variety of food.) The goldfish seem to be very healthy with the amount of duckweed they eat.
I have observed larger tadpoles eating it, and smaller ones grazing the roots. No doubt they are also eating the microorganisms growing on the duckweed.
Duckweed in a pond can supply a very useful amount of food for the fish, as well as shading out some of the algae. You may not want the whole of your pond covered with duckweed. If it succeeds in growing too quickly for your fish to eat, you can take some out and use it in your compost or as mulch round your plants. If you have hens it is an excellent supplementary food for them.
Sources and Picture Credits
The superb picture of the three types of duckweed is by Christian Fischer [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons. A high resolution copy of this picture is below this article.
The top picture of Lemna Minor is by Rasbak (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons.
The second picture of lemna minor on a finger is a public domain picture by Kjetil Lenes.
The picture of Ivy Leaf Duckweed on a hand is By Petr Filippov (Don Pedro28) (Own work) [GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) (https://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons
The picture of Lemna gibba is by Christian Fischer. Desription: ''Lemna gibba''; lateral view (left) and bottom side.
*made in northern Germany in June 2005.
The picture of Lemna perpusilla is By Dalgial (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons.