Duckweed are free floating plants whose population can attain nuisance levels in Ohio ponds. Their explosive reproductive capacity can quickly cause a pond to be completely covered in the green plants in just a few weeks.
There are many ways how to figure out what is affecting the duckweeds, causing it to be harmful towards it. When the temperature is cool, massive duckweeds will die off. Removing the weed naturally, administering weed control chemicals and also installing a bubble aerator will eliminate duckweed.
Duckweed and Watermeal: Prevention and Control, Ohio State University Extension Fact Sheet Retrieved by July 9, 2013 from http://ohioline.osu.edu/a-fact/0014.html