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How to Kill Lake Weeds

Elizabeth Knoll

Lake weeds may severely reduce water clarity and interfere with fish and other aquatic life. They also can make swimming uncomfortable or impossible. Microscopic plants are sufficient for oxygen production and fish food, so large plants are not needed in lakes. When you have undesirable plant life in your lake, you can control it a few different ways: mechanically, chemically or biologically.

Enjoy a crystal-clear lake by killing lake weeds.
  1. Remove weeds along the lake's edge by pulling them out, cutting them, or mowing them. Repeatedly removing the leaves of plants will deplete the energy supply in the plants' roots and they will die.

  2. Eliminate the shallow areas around the lake. Steepen the sides of the lake to a 3:1 slope with a dragline. Shallow-water plants will not grow if the water is at least 3 feet deep around the shore.

  3. Introduce an inert dye into your lake to filter sunlight. Less sunlight means the plants will not be able to photosynthesize and they will die. Dye is effective in water that is at least 2 feet deep. Follow the directions on the dye for application rates.

  4. Fill your pond with vegetation-eating fish like triploid white amur, also known as grass carp. Stock your pond with 10 to 15 of these fish per surface acre of pond.

  5. Apply copper sulfate at the rate of 2.7 lbs per acre-foot of water to control floating weeds like filamentous algae. Dissolve the copper sulfate in water and spray using a pump-up sprayer over the algal mat.

  6. Use an herbicide containing 2,4-D or diquat dibromide for emergent weed and submerged weed control. Emergent weeds should be sprayed with a pump-up sprayer of properly diluted herbicide and water. Inject the herbicide below the water surface to kill submerged weeds. Read the label to see how much concentrate should be mixed with water.