Submerged Plant Control

Aquatic plants are a beneficial and necessary part of Missouri ponds and lakes. Without them, most other organisms cannot survive. Plants keep the water oxygenated, provide food, cover and nesting sites, and stabilize the shoreline and pond bottom.

Submerged plants grow under water, are rooted in the bottom and have stems and leaves that grow towards the surface. Several types of submerged plants are commonly found in Missouri waters. Coontail, elodea, naiad, pondweed and milfoil are examples of submerged plants in a healthy pond. Refer to our publication “Nuisance Aquatic Plants in Missouri Ponds and Lakes” for more information on the identification of aquatic vegetation and the benefits and drawbacks of having aquatic plants in your pond.

Ideally, 10 to 20 percent of a pond’s bottom and surface should have aquatic plants. If more than 20 percent of the pond has aquatic plants, or if aquatic vegetation is not allowing some pond uses, mechanical, biological or chemical control methods should be considered.

MECHANICAL CONTROL

SHADING

Shading areas with large sheets of black plastic (8 millimeter thickness) will kill virtually all aquatic plants under the sheet within 30 days. Float the plastic on the surface and anchor it by fastening the corners to concrete blocks, or sink the sheet over the weed bed with weights. Be sure to puncture the sheet in a number of places so gasses can escape.

BLANKETING

Gravel, sand or clay may be used to “blanket” submerged plants in swimming or fishing areas. Plants have a difficult time trying to grow through physical barriers such as gravel or sand. However, submerged plants will return to the treated area sooner or later.

WEEDING

Submerged plants can be removed by pulling or raking them out. Although weeding is not a particularly enjoyable task, it may be effective in swimming or fishing areas

DEEPENING POND EDGES

Aquatic plants, like all other plants, cannot live without sunlight. The depth to which aquatic plants can grow in a pond or lake is totally dependent on how deep sunlight penetrates. The clearer the water, the deeper plants will grow. Deepening many of the pond’s shallow areas to a depth below where light penetrates (3 to 4 feet) may reduce the severity of plant problems.

Usually this technique requires that the water level be drawn down and the pond bottom be allowed to dry enough to allow access for a bulldozer or backhoe. If you can see the bottom of your pond or lake past a depth of five feet (a common characteristic of water bodies in the Ozarks), deepening the edges may be impractical as a means of plant control.

WATER LEVEL DRAWDOWN

Exposing sediments to prolonged freezing and drying during the months of December, January and February can be very effective in controlling certain aquatic plants, if exposure lasts 2 to 4 weeks. However, both freezing and drying may be difficult to achieve in the typically unpredictable weather of a Missouri winter. If pond sediments remain wet, especially if insulated by a layer of snow, plant roots may not be sufficiently damaged to achieve the desired level of control.

Drain no more water than necessary to expose the unwanted plants and always leave at least eight feet of water in the deepest part of the pond to reduce the chance of a winter fish kill. Keep in mind that if the pond does not reach its normal level by spring and water clarity allows light to penetrate many feet below the surface, aquatic plants may grow even deeper into the pond basin. 3.

REMOVING NUTRIENTS

If the pond is old and has become shallow, due to accumulation of black muck on the bottom, it may be necessary to drain, dry and deepen the pond. The black muck is a storehouse of nutrients that fuel the excessive growth of aquatic plants. All excavated material should be removed from the pond’s watershed.

MINIMIZING NUTRIENT INPUTS

Excess nutrients (nitrogen and phosphorus) should not be allowed to wash into lakes and ponds. Submerged plants in ponds can grow to nuisance levels in a short time if given the extra nutrients. Sources of nutrients may include runoff from feedlots, fertilized fields or yards, and septic tank seepage. Nutrients will also accumulate naturally as the pond gets older.

Establishing and maintaining a 100 foot or wider buffer strip of grass and trees around the pond’s edge will help filter excess nutrients from runoff water. The construction of small silt retention ponds in the watershed will help settle out nutrients before they enter the pond.

Localized nutrient inputs from feedlots or other sources may be avoided by tilling, or constructing a water diversion terrace below the nutrient source to direct its runoff away from the pond. Fencing livestock from the pond’s edge and watering them from a tank below the dam is also a helpful protective measure. The Natural Resources Conservation Service (NRCS) office for your area can provide information on these and other practices.

BIOLOGICAL CONTROL

GRASS CARP (Ctenopharyngodon idella)

The grass carp (or white amur) is a plant eating Asian member of the minnow family. Stocked correctly, they can be effective in controlling many submerged plants. See Grass Carp For Weed Control.

For additional information on Chemical Control and Determination of Acre-Feet to Calculate Total Amount of Herbicide needed, download the full Aquaguide.

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