The best way to control duckweed is to prevent its growth through aeration and nutrient control. The next best ways are to remove the weed and chemically treat the area, which is necessary after overgrowth has occurred. Older and undisturbed ponds are prone to buildup of sediment, which is food for duckweed.
Ponds without aeration and water movement develop a buildup of leaves and sediment, creating stratification and the perfect nutrient-rich breeding ground for duckweed. Netting over the pond can catch leaves and prevent addition to sediment. Bubble aeration, especially toward the pond bottom, creates movement, stirring up sediment and preventing the growth of more nutrients. After overgrowth occurs, physical removal of duckweed is necessary. This is done by skimming the duckweed to the pond side and pulling it out. Removal, though helpful, is not permanent and must be repeated.
Biological treatments, such as introducing Koi, goldfish, grass carp and waterfowl, all of which eat duckweed, can also help reduce the overgrowth. Finally, as a last resort, chemical duckweed killers may be added to pond water to control growth of the plants. Before adding chemicals to a pond, try a combination of aeration, sediment reduction, removal and biological additions to reduce and prevent the growth of duckweed.