|Duckweed in Spain|
The plant is generally considered a pest species. If not kept in check it can actually become a serious issue. If the pond is completely covered, photosynthesis cannot occur and this oxygen levels will drop in the water, leading the death of any fish or other aquatic creatures present. To help prevent this from occurring, there are a wide variety of products and methods used to control the plant. One of the basic methods is simply removing the plant by hand, but this will not eliminate it. Ducks and carp (including koi) are effective biological controls, as both will eat duckweed. Chemical methods can also be used, with some being more effective than others. Preventive measures can also be taken to prevent duckweed from becoming a problem in the first place.
Despite this, duckweed has the potential for doing a lot of good. It is being promoted by the UN for use as a food for poultry and other animals, particularly ducks. It is cheap to produce, and thus can be a quite quite useful component of animal feed.
Sources are Ohio State University, Texas A&M University, and the Food and Agricultural Organization of the United Nations. Image is from Wikimedia Commons and is copyright free.