Phone calls from landowners having problems with aquatic weeds are common for Extension Agents every spring. This is especially true following mild winters when temperatures may not have gotten low enough to kill or suppress the previous year’s growth.
Some aquatic vegetation is beneficial to a healthy pond ecosystem. Plants attract small invertebrates that feed upon them. Predatory insect larvae and other invertebrates feed upon those insects. Little fish eat the insects and invertebrates, big fish eat the little fish, and so on. Vegetation can also provide shaded, cooler water during hot months, and some of the tangled stems provide escape cover for fish avoiding predators. However, vegetation should not cover more than about 15% of a pond’s area, including below the surface. Most folks don’t mind a few pond lilies around the edges of their pond, but when their fishing lures and line repeatedly get fouled by duckweed or filamentous algae, that’s usually when our phones start ringing. Some calls are also from people who just don’t like the looks of a pond full of plants.
While there are multiple strategies to eliminate aquatic weeds, or at least keep them manageable, one of the most popular and effective methods is the use of aquatic herbicides (herbicides labeled for use in systems where water is present). Often, herbicides are an important part of an integrated pest management system, used in combination with cultural and biological control methods. One of the most important pieces of information we pass along to the public is that when using an herbicide, it is essential that the user read the entire product label and adhere to its instructions. Along with safety warnings, appropriate personal protection equipment, instructions for application, and much more information, you will find extensive descriptions of where the chemical can be used and for what purposes. If a particular use that you are interested in is not listed on the label, then it is not legal to use that chemical for that purpose. Labels carry the authority of Federal law, and penalties for misuse can be substantial. THE LABEL IS THE LAW! This applies to not only aquatic herbicides but to all commercial herbicides and other pesticides.
Extension Agents frequently receive questions from landowners about whether a product called Karmex® DF would control the weeds in their pond. Either they have used it before or know somebody who has used it, and “it did a real good job on their pond”. The active ingredient in Karmex® DF is diuron, an herbicide commonly used in agricultural operations for control of weeds in a variety of terrestrial crops (those grown on dry land). Nowhere on the label is there information on using diuron for the control of weeds in aquatic systems. In fact, language on the label specifically states, “Do not apply directly to water, to areas where surface water is present, or to intertidal areas below the mean high-water mark.”
Although some products containing diuron were once labeled for aquatic use, that label has since been revoked. Why is this? According to the PubChem Database of the National Center for Biotechnology Information, diuron is very toxic to aquatic life, both acutely and in the long-term (1). By choosing to apply an herbicide containing diuron, a person would not only be breaking the law, but they may also kill the fish and aquatic invertebrates they are trying to promote. Furthermore, diuron may severely damage or kill shrubs and trees along the shoreline by uptake of chemical through the roots. Appropriately, there are no diuron products labeled for use in aquatic systems.
Fortunately, there are many other safe and effective herbicides available that are labeled for aquatic use. If you are unsure of the weed you are dealing with and or the appropriate herbicide to use, contact your local Clemson Cooperative Extension office and ask to speak with one of our Agents. We will be glad to help! You can also find information in the Clemson Home and Garden Information Center publication, HGIC 1720 Chemical Control of Aquatic Weeds (2).
- National Center for Biotechnology Information. PubChem Database. Diuron, CID=3120, https://pubchem.ncbi.nlm.nih.gov/compound/Diuron (accessed on Mar. 23, 2020).
- HGIC 1720 Chemical Control of Aquatic Weeds