How to Kill Lake Weeds

Written by elizabeth knoll
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How to Kill Lake Weeds
Enjoy a crystal-clear lake by killing lake weeds. (Sunset lake landscape. Chany lake, Novosibirsk area, June 2007 image by Igor Zhorov from

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.

Skill level:
Moderately Easy

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Things you need

  • Gloves
  • Lawn mower
  • String trimmer
  • Dragline
  • Inert pond dye
  • Vegetation-eating fish
  • Copper sulphate
  • Herbicide containing 2,4-D or diquat dibromide
  • Pump-up sprayer

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  1. 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. 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. 3

    Introduce an inert dye into your lake to filter sunlight. Less sunlight means the plants will not be able to photosynthesise 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. 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. 5

    Apply copper sulphate at the rate of 1.22kg per acre-foot of water to control floating weeds like filamentous algae. Dissolve the copper sulphate in water and spray using a pump-up sprayer over the algal mat.

  6. 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.

Tips and warnings

  • Some herbicides may have restrictions on when and where they can be used. Some may kill young fish and you may not be able to swim in the water for a period of time after applying the herbicide. Read the label of your herbicide before applying to understand its restrictions.
  • Killing large amounts of floating weeds may result in oxygen depletion in the lake. This could result in a large fish kill. Overcast and warm days pose the highest risk. If your lake has almost total coverage of floating weeds, kill half one day and then wait a few days to kill the rest.

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