What Causes Duckweed to Die?

Duckweed floats on the surface of still ponds.

Duckweed, from the Lemnaceae plant family, is the smallest flowering plant in the world. It is aquatic, growing on the surface of still waters in warm weather. Duckweed reproduces quickly, sometimes in as little as one day, requiring some type of control to limit its growth.

Biological Control

Biological controls work best against duckweed as a preventative measure. Effectiveness depends on the extent to which the duckweed has already spread. Examples of biological controls include fish such as koi and goldfish, which eat the duckweed as food.

Mechanical Control

Mechanical options include skimming or draining the pond. These plants reproduce best in water with high nitrogen or phosphate levels, so limit these by stopping any fertilizer or feedlot runoff, along with other decomposing materials such as leaves.

Chemical Control

Fluridone works well to control duckweed, especially watermeal duckweed that cannot be controlled through other means. Treatment must be applied to the entire pond, not just problem areas. Results occur only after a period of at least 30 days. Choose 2,4-D as a liquid ester formulation to control duckweed more quickly. This herbicide kills fish as well as duckweed, however, so use it only in ornamental ponds. Reapplication may be necessary after about one week.

References

• Iowa State University Extension: Duckweed
• Purdue University: Control of Duckweed and Watermeal

Resources

• Ohio State University Extension: Duckweed and Watermeal: Prevention and Control