Duckweed

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Chemical Treatments

Read and follow label instructions before using any chemical in water. Dilute with enough water to permit even coverage over area to be treated. There may be restrictions or waiting periods on treated water.

• Sonar AS (Fluridone) - Use 0.5 to 1.0 quarts per surface acre. Dilute with water and spray evenly over entire surface area of pond.
• Reward (Diquat) - 3/4 gallon plus 8 ounces of surfactant or non-ionic spreader/sticker plus 50 gallons of water. Mix and spray evenly over plants.

The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended toward other products that may also be suitable.

We have taken particular attention to make certain that all herbicide treatments have been registered with the Environmental Protection Agency for use in the manner described. Registrations for specific practices are often changed or deleted; therefore, treatments suggested in this information sheet may not remain current indefinitely. Read and observe the manufacturer's label to prevent misuse of a herbicide. It is not intended or proposed that usage of any given practice suggested in this information sheet be in violation with existing registration or manufacturer's label.

If you fail to carefully read and follow suggested treatments, you could cause extensive damage to the environment, crops, livestock or humans.

One of the most common, and most frustrating, aquatic weed problems we encounter in small impoundments (0.1 to 5 acres) in Mississippi is the duckweed/watermeal group (Lemna, Spirodella, and Wolffia). Small, privately owned ponds in the state often become infested with one or more of these floating plants, historically creating a very frustrating and perplexing control challenge. Although the duckweeds are susceptible to several herbicides, and are a “preferred” food of grass carp, long-term control has eluded most pond owners.

A number of approaches have been utilized in treating duckweed infested ponds, but none have proven consistently satisfactory. Recommendations over the years have included biological and herbicide controls. At one time it was believed that waterfowl, especially domestic ducks, might control duckweeds. Although these birds do eat duckweed, they do not consume enough duckweed to eradicate the plant from the pond environment. Grass carp have been touted as an effective biological control, but our experience in Mississippi is that grass carp do not consistently provide control. Although grass carp relish duckweed, they usually are not able to stay ahead of the rapid regeneration time that duckweed exhibits, and duckweed effectively survives their foraging.

Herbicides, both contact and systemic, have been used with limited success. The problem with contact herbicides such as REWARD and AQUATHOL is that it is very difficult to uniformly apply the herbicide to all areas of the pond, thus contacting all the duckweed. Since contact herbicides only affect the duckweed they touch, invariably some duckweed is left as a regeneration source. Thus, the problem returns within a very short time, since duckweed reproduces prolifically and rapidly. Systemic 2,4-D formulations have not proven consistently effective. Other systemics, although effective, no longer maintain an aquatic label.

We began a demonstration program in 1995 with SePro Corporation to evaluate the efficacy of their product SONAR (AI=Fluridone) on duckweed. The results have been very encouraging. To date we have applied SONAR AS in over 20 counties across the state, in ponds with up to 100% surface coverage of duckweed (some pure Wolffia stands, some pure Lemna/Spirodella, some mixed Lemna/Wolffia, some mixed Lemna/Spirodella/Wolffia). In all cases but one, where the application rate was severely miscalculated, we have attained 100% control of the duckweed.

Rapidity of control has varied, depending upon which species mix was causing the problem. Lemna and Spirodella both showed chlorosis within 20 days, and control is usually attained within 30-40 days post-application. Wolffia, however, is slower to respond, with chlorosis beginning at about 30 days and complete control usually attained at around 45 days. In all cases, SONAR AS at the rate of 75 ppb provided 100% control. Additionally, we have thus far maintained almost 24 months control in ponds that we treated in early 1995, with no additional application of SONAR. We began in 1996 to examine lower application rates, and have attained initial results similar to those attained with the 75 ppb rate. We continue to monitor those ponds treated with a lower rate to determine how long control will be maintained. We suspect that, although lower rates have provided 100% control, we may see more rapid re-occurrence of duckweed.

We now consider SONAR AS to be the best herbicide recommendation for controlling duckweed and water meal in small impoundments. In addition to its demonstrated efficacy, SONAR has several desirable characteristics that make it an ideal herbicide to use in these applications. First, it can be handled by homeowners and others without undue concern over health and safety. It is also environmentally friendly, with an extremely low toxicity to fish and wildlife. Second, it is easy to apply. Unlike some other materials, SONAR moves rapidly throughout the water body, eliminating the need for sophisticated spray equipment and techniques. In small ponds, we have been successful by simply mixing the appropriate amount of SONAR AS with water and pouring the mixture into the pond at several locations. Third, because it acts slowly, there is no concern about killing too much vegetation in a short period, which often leads to oxygen depletion, especially in hot weather. We have successfully treated ponds with 100% duckweed coverage during July and August without incident. Finally, SONAR AS provides extended control. At label rates, we have consistently achieved at least 18 months control, and we’re still counting!

Our extension county agents have been happy to be able to finally provide homeowners and private land owners with an effective and impressive solution to what was previously almost a “no-win” situation. In all cases where we have recommended SONAR AS for duckweed the clients have been highly pleased with the results. The control they achieved has been excellent, and they are very happy with that. They have, however, asked us the question “Why didn’t you tell us about SONAR before now?”

Biological Treatments

Although grass carp have a high preference for eating duckweed, they are ineffective at controlling it. See more information on grass carp.