Duckweed, watermeal, mosquito fern, common salvinia, and giant salvinia are common aquatic pests. Their control is warranted in bodies of water for fish production, irrigation, and aesthetics. Few herbicide formulations provide reliable control, and applications must be focused and precise. This publication outlines the identification of these species and the aquatic herbicides effective for their control.

Duckweed (Lemna spp. and Spirodela spp.), watermeal (Wolffia spp.), mosquito fern (Azolla spp.), common salvinia (Salvinia minima), and giant salvinia (Salvinia molesta) are floating aquatic plants that can become problematic in ponds and slow-moving waters. Proper identification and early and focused herbicide applications are required for their control. In the growing season, these small floating weeds can completely cover the surface of small ponds in just a few weeks. Their growth can interrupt fishing, swimming, and irrigation and reduce aesthetics while also creating the potential for fish kills as they shade out sunlight required for oxygen production in the pond.

Each of these species has distinct characteristics that make them easy to identify. From a distance, they can give the appearance of a heavy plankton bloom or filamentous algae mat, but when standing at the water’s edge, one can see that the floating growth is actually comprised of countless small, individual plants. The wind will often gather or blow growth to the edge of a pond, creating dense mats. When the wind subsides, the plants will be more evenly distributed across the surface of the pond again.

Identification

Duckweed is a light green, free-floating plant with flat, smooth, oblong leaves often 3 to 5 millimeters in length and width, but they can be larger and more oblong, depending on the species. The leaves are often clustered in groups of 2 to 5+. It has 1 to 5+ whitish, hairlike roots that suspend below the surface (figure 1).

Watermeal (the smallest flowering plant on earth) is a light green, free-floating plant with oblong, rootless, grainlike, or gritty texture similar to cornmeal (figure 2).

Mosquito fern often gives a pink or reddish hue at a distance. On the hand, it exhibits pink and green coloration with a textured fernlike leaf surface along with fine hairlike roots that suspend below the surface (figure 3).

Common salvinia is a green-brown–colored plant that often grows clumped together. The leaves are oval shaped and can appear creased, each about the size of a dime. The leaf surface can resemble Velcro, and close inspection reveals tiny hairs that split four ways. Roots are suspended below the surface, clumped, and brown in color (figure 4).

Giant salvinia should not be confused with common salvinia. It grows much larger, with leaf hairs that split four ways and come back together at the tip to resemble tiny mixer beaters (figure 5). Giant salvinia can also form thick, matted layers as it grows or builds upon itself, making it much more difficult to contact with herbicides and to control than common salvinia.

Prevention & Control

Although reducing water clarity with dyes or fertilizer is one of the most effective methods for preventing and reducing submerged aquatic weeds issues, those practices are not effective at preventing infestation of floating species because water clarity does not affect their growth as they float on the surface. Excess nutrients can also substantially increase aquatic weed growth, so reducing nutrient runoff is a good practice. This includes avoiding fertilizing around the pond before heavy rains, slowing runoff through vegetated buffers, and excluding livestock (waterfowl included) from the pond to reduce nutrient input that stimulates infestation and advanced growth. Good sanitation practices when moving a boat from one body of water to the next can prevent these species from hitchhiking from another water body. Spraying off the trailer, bunks, and boat and emptying live wells to allow the boat and trailer to dry out completely can help reduce introductions. Physically removing floating species is possible with constant effort in small water bodies. However, the time and labor involved in dragging, raking, or skimming out these species can be overwhelming and effects short lived, particularly in the heat of the growing season as the plants are rapidly growing and reproducing.

Biological agents are available for these species but are limited or rarely offer satisfactory control. Salvinia weevils have been used to control common and giant salvinia on public water bodies, but their availability is limited for use in private ponds, and their ability to maintain population levels over cool winters is also limited. Stocking grass carp is generally a good option for controlling many aquatic weeds; however, they are not effective for satisfactory control for these plants as some of these species are usually too small for the carp to consume effectively, generally reproduce at rates that exceed their consumption, or are low preference. Tilapia have been shown to control (not eradicate) watermeal, mosquito fern, and duckweed. Although they are generally only cost effective in small ponds, they may be the only option for control in ponds used for irrigation. A mixed sex tilapia stocking at rates of 25 to 50 pounds per acre should take place in spring when water temperatures remain above 70 degrees F. Because tilapia die as water temperatures decline to 55 degrees F, they are only effective for one season and must be restocked annually. Tilapia availability can be limited, so plan to contact producers in the fall to ensure that your order is secured for spring stocking. In ponds with largemouth bass, tilapia should be stocked at higher rates of 25 to 50 pounds per acre and larger than 5 inches to buffer against predation. A list of tilapia producers and suppliers in Alabama may be found on the Alabama Extension website at www.aces.edu.

Aquatic herbicide applications are generally the most economical and effective control option for each of these species. Those discussed here are not restricteduse and are available for purchase from farm supply stores and online vendors. When purchasing a product for aquatic application to surface waters, be sure that the label permits that use. Products with the active ingredient of glyphosate or diquat are commonly recommended for control of duckweed, watermeal, and mosquito fern, but these active ingredients rarely provide acceptable control because they must make contact with each individual plant, often requiring multiple applications per season or over years in some cases, which can be costly over the long term. Salvinia is an exception; 96 to 128 ounces of glyphosate per surface acre in combination with 12 to 32 ounces of diquat per surface acre can be effective toward control because of the larger leaf surface area of these species, which increases contact and plant kill. Additionally, shoreline vegetation can harbor these plants and make it difficult to effectively contact target plants, reducing effectiveness of these applications. Although there are many herbicide options available for control, contact and control for these species can be best achieved with one of these four active herbicide ingredients: carfentrazone, flumioxazin, fluridone, and penoxsulam (table 1).

Carfentrazone is a liquid for foliar application. It offers good control of these species but requires contact with each plant and must not be washed from the plant surface. This can be difficult to achieve in heavy infestations with overlapping or dense vegetation. Contact is also challenging to maintain in large areas of growth as boat lanes and wake will allow areas to go uncontrolled as herbicide is washed off the target plants. It is important to spray the margin around the pond’s perimeter carefully as plants along the edge and on the moist bank of the shoreline can otherwise escape treatment and allow for reinfestation.

Flumioxazin is available in liquid suspension concentrate (SC) and flowable granule (that is dissolved into solution) and applied as a foliar spray. It requires even distribution throughout the pond and along the shoreline margin to be effective. Flumioxazin is a great choice as it does not have to stay directly on the vegetation to provide control since the chemical applied to surface waters also kills plants through water contact. It is best used in waters with a pH of 8.5 or less, as efficacy is greatly decreased in more basic waters.

Fluridone is a liquid applied as a volume treatment and must remain at the concentration for 45 days to be effective. This application requires an accurate assessment of the pond’s total water volume. Fluridone can be broadcast from shore to mix throughout the pond but is best distributed by boat throughout the water body. It is more costly than the former products but may be a good choice for smaller water bodies, those infested with multiple species of aquatic weeds, or a manager with limited equipment or capability. However, the required residence time of fluridone comes with the risk of failure due to outflow with heavy rains or in systems with consistent outflow, such as ponds fed by springs or those with a large watershed that creates high turnover rates. The pond should be lowered 6 to 8 inches below the outflow (standpipe or dam) to improve water retention and maintain chemical concentration. This practice may also allow weeds, particularly duckweed, watermeal, and mosquito fern, left on the bank to go untreated.

Penoxsulam is a liquid that can be applied as a volume treatment, much like fluridone. It is less costly but requires a 60- to 120-day residence time so weather patterns and turnover rate must be considered. Unlike fluridone, it may also be applied as a foliar spray directly on vegetation and the shoreline for control when a nonionic surfactant is added. Penoxsulam performs best in water with a more neutral pH near 7.

For the described foliar applications, it is highly recommended that an adjuvant, particularly a nonionic surfactant, be included in the tank mix. Surfactants reduce the surface tension of the spray droplets, which helps improve contact with the foliage, increasing coverage and uptake of the chemical by the target plant. Often, surfactants are mixed at a 0.25 percent volume/ volume rate or 0.32 ounces/gallon, but this can vary by the product used, so be sure to read the surfactant label. There are many brand names of nonionic surfactants, and they are widely available for purchase at farm supply stores and online vendors.

Regardless of the weed species, care must be taken when treating water bodies with more than 30 percent weed coverage as the decay after herbicide application can result in decreased oxygen and fish kills. However, this is not the case with volume treatments of fluridone and penoxsulam, as the decomposition process is slow over the treatment period, allowing oxygen production to be maintained. Some products with similar active ingredients are not labeled for aquatic application, so (as with any pesticide application) be sure to follow the label accordingly, only apply when water temperatures are above 60 degrees F, and choose a sunny day with low wind, ideally less than 10 miles per hour. It is important to note that despite a proper application that results in complete control, these species can easily reinfest a pond from the seed bank, from sources within the watershed (i.e., infested wetlands and ponds), by hitchhiking on waterfowl and shorebirds, and by human transfer (i.e., boat or bait bucket). Consistently monitor for growth and understand that follow-up applications may be required for dense infestations or missed areas. Visit the Alabama Extension Fresh from the Field YouTube page and search for “Aquatic Weed Control: Managing Duck Weed, Watermeal, and Mosquito Fern” to learn more about chemical control and see a visual demonstration of the application process.

References and Additional Reading

• SRAC No. 360, Aquatic Weed Management Control Methods
• SRAC No. 361, Aquatic Weed Management Herbicides
• SRAC No. 3601, Aquatic Weed Management Herbicide Safety, Technology & Application Techniques
• Agronomy Facts 37: Adjuvants for Enhancing Herbicide Performance, Penn State Cooperative Extension
• Giant Salvinia, LSU AgCenter website

Norm Haley, Extension Agent, ACES-AFNR-Field, Forestry, Wildlife, and Natural Resources, Auburn University

New May 2025, Aquatic Vegetation Identification & Control: Duckweed, Watermeal, Mosquito Fern, Common Salvinia & Giant Salvinia, FOR-2177