Watermeal (Wolffia) is the world’s smallest flowering plant, but it quickly becomes a nuisance for pond owners. This tiny, free-floating aquatic plant is often mistaken for duckweed, but its size and lack of roots make it distinct and harder to control. When conditions are favorable, watermeal reproduces rapidly, forming a dense, bright-green blanket across the water’s surface. This mat blocks sunlight from reaching submerged plants and hinders the natural exchange of oxygen, potentially leading to fish kills as dissolved oxygen levels drop. Eliminating watermeal requires a comprehensive strategy addressing both the existing population and the underlying environmental factors that allow it to flourish.
Identifying Watermeal and its Growth Factors
Accurate identification is the first step in effective treatment. Watermeal plants are incredibly small, typically measuring only 0.2 to 1.5 millimeters, or about the size of a pinhead, giving them the appearance of floating cornmeal or green grits. Unlike true duckweed, which has a small root dangling into the water, watermeal is completely rootless and consists of a single, oval-to-spherical frond. It is commonly found in calm, slow-moving, or stagnant waters where it can accumulate without being dispersed by wind or current.
The primary driver of watermeal’s explosive growth is an excessive amount of nutrients, specifically phosphorus and nitrogen, in the pond water. These nutrients often originate from external sources, such as agricultural runoff, lawn fertilizers applied too close to the shoreline, or seepage from faulty septic systems. Decaying organic matter, like leaves and bottom sludge, also acts as an internal source, constantly feeding the plants. Watermeal is highly efficient at absorbing these nutrients, allowing its population to double in size in as little as a few days under warm, sunny conditions.
Immediate Mechanical and Cultural Removal Strategies
Before resorting to chemical treatments, physical removal is a necessary step to immediately reduce the mass of the watermeal population. Due to the plant’s minuscule size, standard rakes or nets are ineffective, requiring the use of fine-mesh skimming devices or specialized aquatic nets. This process, often called “booming” or “seining,” helps pull the floating layer of plants to the pond’s edge for collection. It is important to remove all skimmed material completely from the pond area, as any remaining fragments can easily re-establish the infestation.
Physical removal is labor-intensive and temporary, but it reduces the biomass that would otherwise decay and deplete oxygen after chemical treatment. Another immediate, non-chemical approach involves cultural strategies focused on disrupting the water surface. Installing or upgrading an aeration system, such as a bottom-diffused aerator or a surface fountain, can create water circulation and movement. This constant motion breaks up the still surface environment that watermeal prefers, pushing the floating plants toward the edges for easier collection. Aeration also improves overall pond health by boosting dissolved oxygen levels, which helps the pond ecosystem process excess nutrients.
Targeted Chemical and Biological Control Options
Chemical control is the most effective method for eradicating watermeal, requiring specific aquatic herbicides. Flumioxazin, a fast-acting contact herbicide, provides excellent control when applied to actively growing plants. Another option is the systemic herbicide Fluridone, which is absorbed slowly by the plant and can take 30 to 90 days to achieve full control; its slow action helps prevent rapid oxygen depletion. Diquat dibromide, a contact herbicide, is also labeled for watermeal but is generally less effective than Flumioxazin or Fluridone.
When using any herbicide, follow the product label instructions precisely, including application rates and water use restrictions. Treating a severe infestation all at once risks a fish kill because the massive amount of decaying plant matter consumes dissolved oxygen. To mitigate this danger, it is best practice to treat only one-quarter to one-third of the pond’s surface at a time, waiting 7 to 14 days between treatments to allow oxygen levels to stabilize. For contact herbicides, an aquatic-approved non-ionic surfactant is often recommended to help the chemical penetrate the plant’s waxy surface and increase effectiveness.
Biological control offers a long-term, self-regulating option, though watermeal is not the preferred food of all herbivorous fish. Mozambique Tilapia are known to consume watermeal, but they are a warm-water species that cannot survive in temperatures below 55 degrees Fahrenheit, making them a seasonal solution in many regions. Sterile triploid grass carp, which are widely used for submerged weed control, will also feed on watermeal, although it is not their first choice. Pond owners must obtain a permit from their state’s fisheries department before stocking triploid grass carp, as these sterile fish are regulated to prevent their escape into public waterways.
Long-Term Pond Management and Prevention
Long-term prevention requires addressing the source of excess nutrients. Maintaining a healthy buffer zone around the pond is a highly effective strategy; this involves planting a strip of dense vegetation, like native grasses or shrubs, along the shoreline. This buffer intercepts and filters nutrient-rich runoff from surrounding lawns or fields before it enters the water. Furthermore, pond owners should avoid using phosphorus-containing fertilizers on turfgrass near the water’s edge.
Another strategy is to remove nutrients from the water column and the bottom sediments, where they accumulate as sludge. Routine pond maintenance should include removing leaves, grass clippings, and other organic debris that decay and release phosphorus. Specialized products like PhosLock or aluminum sulfate (alum) can be used to chemically bind phosphorus into an insoluble form, locking it in the bottom sediment and making it unavailable for watermeal uptake. Combining these nutrient-reduction methods with a consistent aeration program and routine skimming will create an environment less hospitable to watermeal, ensuring the pond remains clear and healthy.