Duckweed thrives in ponds and other bodies of water with stagnant water and excessively high nutrient content. Once duckweed establishes itself, it reproduces rapidly. It can completely cover the surface of a pond in a very short period of time. And once it does that, it will block sunlight from reaching the water and remove the nutrients that other, more desirable plants depend on. Duckweed is fairly easy to control. However, because it is frequently reintroduced by aquatic birds or flood waters, it may spread again if not closely monitored.
Physically remove duckweed from the surface of your pond. In small bodies of water, this can be done with a pool skimmer. In larger ponds, tie a rope to a piece of timber and drag the timber from shore to shore. Keep a bucket nearby to deposit the skimmed duckweed. It can be composted or used to feed livestock--chickens are particularly fond of duckweed.
Introduce fish that feed on duckweed into your pond. The type of fish appropriate for your pond will largely depend on the surrounding climate. Tilapia (7 to 15 fish per surface acre) and grass carp (15 to 20 fish per surface acre) are commonly used varieties, but before you purchase them, make sure that there are no restrictions on introducing these fish in your area.
Treat your pond with a chemical herbicide prescribed for killing duckweed, such as Reward (diquat), Avast (fluridone) or Gallen (pnoxulam). But choose carefully. Certain broad-spectrum contact herbicides such as Reward will kill any plant life that it comes into contact with. Others, like Habitat, will only kill floating vegetation and are safe to use around animals and livestock. Carefully read all warning labels and follow the manufacturer's instructions when introducing these chemicals to your pond.
Dredge the pond annually. Duckweed thrives in ponds with high nutrient levels as a result of excessive decaying plant or animal tissue. If you reduce the nutrient levels in your pond by removing decayed matter from its bottom, duckweed will not be able to spread as rapidly.