What Kills Duckweed?

Written by carly reynolds
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What Kills Duckweed?
Ponds with duckweed infestations can reduce the health of fish and pond wildlife. (Jupiterimages/Photos.com/Getty Images)

Duckweed is an aquatic plant that reproduces rapidly, causing problems for ponds. As this plant reproduces, it covers the surface of the water. As it spreads, the weed absorbs the oxygen in the water and deprives the pond's wildlife of oxygen and sunlight. The two most prevalent types of duckweed are common duckweed and watermeal. Both varieties can be treated with biological, mechanical and chemical control methods.

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Biological Control

There are many water organisms that act as a biological control method for duckweed. If you wish not to introduce chemicals to your pond, this control method is a great alternative. Goldfish, koi fish and grass carp are all animals that feed on the duckweed, helping to control the amount of the plant that is present in the pond. Small amounts of duckweed are not harmful to a pond and the presence of these fish helps to prevent the rapid spreading of the plant. Biological control is only effective if small amounts of duckweed are present when the fish are introduced. These fish can not eliminate a pond that is already covered with duckweed or watermeal.

What Kills Duckweed?
Koi fish feed on watermeal and duckweed. (Jupiterimages/Photos.com/Getty Images)

Mechanical Control

A second nonchemical control method for the infestation of duckweed involves skimming the surface of the plant to mechanically remove the duckweed. This is most effective if done in the early spring. To remove the duckweed, use a seine net or a dip-net to collect the plant from the surface of the pond. This method will not completely eliminate the duckweed but will help keep it at less harmful levels. This method can be completed in combination with the biological control method to improve effectiveness.

Chemical Control

There are a number of chemical control methods that can be used to control the presence of both duckweed and watermeal. Diquat, fluridone and 2,4-D herbicide have all proven effective. These products are available at local garden supply centres and should be used per manufacturer's instructions. Possible drawbacks of these control methods are that livestock may no longer be able to drink the water from the pond due to the chemical presence, and the cost associated with these treatments.


Save money on costly control methods by taking steps to prevent the infestation of duckweed. Nutrient reduction and bubble aeration are two preventive methods that have proven effective. Nutrient reduction is a method that prevents too many nutrients from entering the pond as a result of fertiliser, field run-off, drainage and septic systems. Take care not to allow fertiliser to be spread, animals to roam or geese to swim in ponds. Over a single year or several the reduction in nutrients will prevent duckweed from growing.

Bubble aeration involves using a compressor to blow air through the bottom of the pond, were sediments tend to settle. This added presence of air helps to maintain pond health by reducing organic matter in the pond, by reducing the quantity of organic matter and reducing the amount of nutrients needed for duckweed growth.

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