Duckweed Fact Sheet    

Is Duckweed a curse or a useful plant?  Of course it can be either, or both.   I would 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 reproduced 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.

Duckweed is a good food for many animals.  Of course 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.