Duckweed is an aquatic perennial plant that floats on water and can quickly cover an entire pond if left unattended. Duckweeds have leaf-like fronds that typically grow no wider than one-fourth of an inch. Under each frond is a root from where the plant absorbs its nutrients. One duckweed plant has the capability to reproduce into 17,500 plants in just one week. For this reason, attacking your duckweed early is essential to avoid an infestation, which is harder to get rid of completely.
Remove the duckweed manually. As it grows, pull it out. If you start when you first notice the duckweed, this method of getting rid of it will be more successful.
Add fish that eat duckweed. Koi and grass carp are two fish that eat duckweed; however, grass carp will eat your other plants first. Therefore, koi is the more sensible fish to add to control duckweed. Stock about 50 fish per acre of water. For heavy duckweed infestations, combine this step with manual control since the fish will not be able to eat the duckweed as fast as the plant is reproducing.
Apply the herbicide fluridone to get rid of your duckweed. Fluridone is added directly to the pond at works at the roots to kill the plant. Follow dosing instructions on the label, but in general, apply at a rate of about 1½ quarts per surface acre (with a depth of 5 feet or more). Dosing can be separated into two equal applications 10 to 14 days apart.
Apply the herbicide diquat to kill your duckweed from overhead. It works well for spot treatment. Diquat is diluted with water before application, and the recommended amounts are written on the label. In general, use 50 parts water to one part of diquat in a backpack or handheld sprayer. You can also use chelated copper in conjunction with diquat for greater success. It is mixed in with the diquat before application at a ratio of one part chelated copper to two parts diquat.