Duckweed and Lagoons: Your Questions Answered

Contact: Bob Schultheis, natural resource engineering specialist
Headquartered in Webster County
Tel: (417) 859-2044
E-mail: schultheisr@missouri.edu

Q: In May, our family moved into a rural home with a lagoon septic system.  In the short time the house was empty, the duckweed completely covered the lagoon.  Can you tell me what steps I can take to bring the duckweed back under control?

A: First, confirm if the plants are filamentous algae (http://mdc.mo.gov/discover-nature/field-guide/filamentous-green-algae-moss-pond-scum) or duckweed and watermeal (http://mdc.mo.gov/discover-nature/field-guide/duckweeds-watermeal). If it’s the former, the lagoon is operating normally. If it’s the latter, then it needs to be removed for the lagoon to operate properly.

Duckweed and watermeal are difficult to control. The Missouri Department of Conservation aquaguide at http://mdc.mo.gov/sites/default/files/resources/2010/05/4889_2842.pdf gives instructions for chemical control of the plants. Reward (active ingredient is diquat) is most often suggested because it is least expensive. But we are seeing limited success with it on watermeal. The products below are more expensive, but provide better control. Usually more than one application is needed. Unlike ponds, it’s best to treat the whole water surface to kill the plants. Adding a septic tank ahead of the lagoon (if it doesn’t already have one) will lessen the nutrient load on the lagoon and lengthen the time before the duckweed might reappear.

Herbicide product labels and example prices

Sonar RTU: http://www.sepro.com/documents/SonarRTU_Label.pdf and https://www.lakelawnandpond.com/SonarRTU.aspx?ItemGroupID=51 and http://www.amazon.com/Sonar-RTU-Sepro-Herbicide-quart/dp/B00QXNS55K/ref=sr_1_5?ie=UTF8&qid=1432761369&sr=8-5&keywords=sonar+herbicide

Sonar A.S.: http://www.sepro.com/documents/sonaras_label.pdf and http://www.amazon.com/Sonar-AS-Aquatic-Herbicide-Pint/dp/B00HND1QXC/ref=sr_1_2?ie=UTF8&qid=1432761369&sr=8-2&keywords=sonar+herbicide

You can also skim the duckweed from the surface and compost it or feed it to livestock, but you have to be sure to remove it all (don’t leave any on the lagoon bank where it can wash back in) or it will usually repopulate the lagoon within a month.

Our lagoon guide at http://extension.missouri.edu/p/WQ402 gives more management tips.

None of these are restricted-use herbicides, so no license is required to purchase them.

For a typical 1,400 sq.ft. lagoon with a 3 ft. water depth and using Sonar RTU, you would need [1,400 sq.ft./(43,560 sq.ft. per acre x 0.5) x (3 ft. x 0.33 avg. depth) x 64 oz. per  half acre] = 4 oz. of chemical for the first application, and 2 oz. each for application #2 and #3.

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