Duckweed Center

Controlling Duckweed with Sonar®

Sonar A.S. provides excellent control of duckweed. Sonar A.S. is considered the product of choice for controlling duckweed and provides seasonal control when the following procedures are followed.


Sonar A.S. should be applied starting in early spring through midsummer, at the first sign of weeds floating to the surface. The best and most cost-effective time to control duckweed is prior to May 1st, before the plants have had a chance to mature and become established. If you’ve had trouble with this stubborn vegetation in the past and tried to control it later in the summer, you know that it can be tough to eradicate once it has become established. As the tiny nuisance aquatic plants emerge in the spring, they are extremely vulnerable to a quick and easy treatment of Sonar.

Pond Level

Best control is obtained when discharge can be stopped for thirty days or longer. Ponds should be at or near normal water level when treated. Excessive out-flow dilutes Sonar A.S. and reduces the chance of control. Outflow can be reduced by lowering pond levels prior to treating with a siphon made from a rubber or PVC pipe.


The dose for the first Sonar A.S. treatment is normally one quart per surface acre. Refer to the label or consult a SePRO pond advisor for assistance. After the initial application, duckweed can be prevented with an annual treatment of one pint per surface acre applied in the spring before duckweed becomes heavily established.

Application and Coverage

The labeled rate for Sonar A.S. should be split and applied in two applications about 10 - 14 days apart. For small ponds, application can be made with a hand sprayer from the bank or the middle, if possible. Dilute the product in a large container with water and empty the solution directly into the water. Apply fairly evenly over pond surface and weeds. Total pond coverage is not necessary for control and it is not necessary for Sonar A.S. to come into direct contact with the plants for effective control as the active ingredient will spread throughout the pond for uptake into the target plants. Avoid stirring the bottom sediments if possible. If infestation is heavy, disperse thick mats with a moving boat or aerator in order to get the product directly into the water. Be sure to triple-rinse the container when application is complete. The easiest way is to fill the container three times by submerging it into the pond and rinsing it out directly back into the water.

Control Time

Plants should start turning white as control begins within 2 to 4 weeks. It may take 5 to 10 weeks for final results. A small amount of duckweed may continue to germinate and float to the surface all summer.

No Restrictions

Sonar carries no restrictions for swimming, fishing, boating, or livestock use, even immediately after application. Some precautions may apply for irrigation. Refer to the label or consult a SePRO Aquatic Advisor for more information.