Common Aquatic Plant Herbicides Used in Lakes and Ponds
Aquatic herbicides vary in effectiveness (depending on the weed species), toxicity, and water-use restrictions. Selection of which herbicide to apply depends largely on the identification of the aquatic plant to be treated (Murphy and Shelton 1996). For example, algae (filamentous and single cell) problems are typically treated with herbicides containing copper. Submersed plants (coontail, elodea, and pondweed) are often treated with Fluridone and Diquat.
- Chelated Copper Compounds
- Fluridone (Sonar)
- Glyphosate (Rodeo, Pondmaster)
- Endothall (Aquathol, Hydrothol)
Floating plants (duckweed) can be treated with Fluridone or Diquat. Emergent plants (cattail and bullrushes) are effectively treated with Glyphosate (Rodeo, Pondmaster). The relative effectiveness of aquatic herbicides on different species of water weeds is provided in Table 1, and water-use restrictions are provided in Table 2.
Endothall compounds (Aquathol and Hydrothol) are registered by the EPA as aquatic herbicides, but they are relatively toxic to fish at rates near those needed to kill water weeds. The Hydrothol formulation is the most risky to use in fish ponds. Endothall cannot be used in irrigation water, livestock water, or in food crop or food fish areas without water-use restrictions (Table 2).
Fortunately, there are a number of other less toxic, but effective, herbicides that are registered for use in aquatic systems. The five types of herbicides most commonly used in ponds and lakes include chelated copper, fluridone, glyphosate, 2, 4-D and diquat.
Chelated copper compounds are used to control algae, not rooted aquatic plants. Most algae species are effectively controlled by these herbicides. However, copper is a toxic metal that is long-lived (persistent) in the environment. Copper can be toxic to fish and aquatic animals at concentrations near levels used to control algae, especially in soft water. Copper toxicity increases as water hardness decreases. Copper sulfate is not as safe to use as chelated copper compounds, and it should not be used in soft waters (alkalinity values less than 50 mg/L). No water-use restrictions are associated with copper compounds, although they may be lethal to sheep (please check the label before applying any herbicides)
Fluridone (Sonar) is one of the safest of the registered herbicides to use in fish ponds. It is expensive and will not kill algae, but effectively controls submersed aquatic plants. It is a persistent, slow-acting herbicide. Sonar residue may persist for a period of 2 to 12 months, and results may take 30 to 90 days to be noticeable. Do not use Sonar-treated water for crop irrigation for 30 days after application. There are no restrictions for fishing, swimming, livestock, or human consumption.
Glyphosate (Rodeo) is best used for control of emergent and shoreline weeds such as cattail, reeds, rushes, smartweeds, and some floating-leaf plants like water lily and lotus. It is usually applied to the plant and not directly to the water. It is quickly bound to suspended particles and bottom sediments and is rapidly inactivated. It has no waiting period or withdrawal restrictions for irrigation water, livestock water, fish consumption, or swimming. Use only those glyphosate products labeled and specially formulated for aquatic systems. Some glyphosate products contain additives that are toxic to aquatic organisms.
2,4-D (Aquacide, Aqua-Cleer, Weedar, Weed-Rhap, Weedestroy) is effective for controlling submersed aquatic plants. These compounds rapidly and completely decompose in about 3 weeks. Toxicity of these herbicides increases as pH decreases. They are less effective at pHs greater than 8, and more toxic in acidic waters (pH<6). Depending on the formulation, 2,4-D can be highly toxic to rainbow trout. 2,4-D should not be used in water for irrigation, livestock, or domestic purposes.
Diquat (Reward, Aqua-Clear, Aqua-Quat, Watrol, Weedtrine) is a wide-spectrum herbicide that can be used to control algae and submersed weeds, but it is not especially effective on emergent weeds. A 14-day waiting period is required by law before diquat-treated water can be used for livestock consumption, crop irrigation, or drinking. There are no restrictions for fishing, but a 1-day waiting period is required before swimming. Diquat is rarely found in treated water after 10 days.
Fish kills may occur after herbicide application, even when the herbicide used is not directly toxic to fish. Fish die indirectly from suffocation, rather than herbicide poisoning, because masses of rotting water weeds killed by the herbicide decompose, and reduce oxygen levels.
When using herbicides, treat one-half (or less) of the lake at a time to allow fish freedom to move to untreated, oxygen-rich areas of the pond or lake. Apply herbicides during the spring when water temperatures are cooler and dissolved oxygen levels are higher than in summer. Some herbicides are not as toxic at colder temperatures. Apply in early spring when weeds are small and not well established, and when fewer weeds are present to decompose.