How to Kill Underwater Plants in a Pond

Garden and farm ponds can quickly become overrun with aquatic weeds. They can choke out more desirable plants and make the pond inhospitable for fish. Learn how to kill underwater plants in your pond to restore the balance of the ecosystem.

  1. Put on wading boots or a wading suit. Drag a large scoop net across the surface of the water to push surface plants aside or remove them. Many methods of killing underwater plants are also toxic to surface plants; if you wish to save them for future use, remove them and place them in large plastic tubs or garbage cans filled with water.

  2. Kill microscopic, erect and green filamentous algae. These can form dense clouds of vegetation and heavy mats of grasslike growth along your pond's border. Starve and kill the algae by reducing water runoff into your pond, a common source of nutrients. Adding an algae herbicide containing copper sulfate is one of the best options for heavy algae growth. Blue or black pond dyes such as Aquashade can also be effective algae killers; they act by blocking the sun's rays from entering the pond. Apply the algae herbicide or dye according to the manufacturer's guidelines.

  3. Drag a hard tine rake along the bottom of the pond to manually remove underwater vegetation. Drag and twist the rake slightly to help uproot any sunken plant tubers. You may need to use a rowboat if your pond is large. Pile the removed plants in direct sunlight to kill them.

  4. Administer a dosage of the aquatic herbicide fluridone, the active ingredient in products such as Sonar. The chemical is absorbed by the underwater plants' leaves and prevents the plants from maintaining chlorophyll levels, killing them within 10 days. Fluridone comes in liquid or pellet form. Apply it according to the manufacturer's instructions.


  • Many herbicides are applied in dosages according to your pond's surface area. Failing to calculate the correct pond area can cause you to use the wrong amount of herbicide, making the product either too toxic or not toxic enough. Calculate the surface area of rectangular ponds by multiplying the length and width, then dividing by 43,560 for your final surface acreage. Calculate the area for a round pond by squaring the pond's shoreline footage and dividing by 547,390 for total surface acreage.

About the Author

Joshua Duvauchelle is a certified personal trainer and health journalist, relationships expert and gardening specialist. His articles and advice have appeared in dozens of magazines, including exercise workouts in Shape, relationship guides for Alive and lifestyle tips for Lifehacker. In his spare time, he enjoys yoga and urban patio gardening.