Duckweed, scientific name Lemna minor, is a small floating aquatic weed with kidney shaped leaves and a small, downward hanging hairy root. "Duckweed is usually most abundant in ponds or areas of ponds with little surface agitation by wind or waves," according to Cornell University. Duckweed grows and spreads easily. It can quickly cover the entire surface of a pond, which cuts off sunlight to underwater plants and reduces oxygen in the pond that fish need. You can control duckweed in ponds by physical removal, biological control, or herbicides or a combination of those methods, according to Clemson University Cooperative Extension Service.
Rake off duckweed from the surface by using the type of rake found at swimming pool supply stores, or use a net to scoop up duckweed. Physical control works on small ponds or small areas of ponds where the duckweed is near the shoreline, but is often too time consuming to control the problem.
Introduce duckweed-eating goldfish or carp to the pond to help control it. Ducks and other waterfowl also consume duckweed. In addition, duckweed is an important food source for "muskrats, beaver, birds (e.g., rails, herons) and small aquatic animals such as frogs," according to the Missouri Botanical Garden.
Use an aeration device. Because duckweed grows readily on ponds that have little surface agitation from winds or waves, an aeration device that supplies surface agitation helps to control duckweed, according to the Cornell University Department of Natural Resources website.
Treat duckweed with herbicides if physical methods of control fail or are not an option. Virginia Tech Cooperative Extension rates herbicides Fluridone and Diquat as "excellent" for controlling duckweed. While Clemson University Extension also ranks Fluridone as "excellent," it only rates Diquat as "good" for controlling duckweed and also recommends 2,4-D herbicides as "good."