How to Kill Duckweed and Filamentous Algae

Controlling Duckweed

• 1

  Uproot as much duckweed as possible from your pond by hand. This step is best performed on a windy day, when the breeze can push the uprooted duckweed to one side of the water.

• 2

  Scoop out the uprooted duckweed with the long-handled swimming pool net.

• 3

  Mix your diquat solution. Combine diquat with water at a 50:1 ratio in your storage bottle and screw on the cap tightly. Shake to blend the diquat and the water.

• 4

  Mix the diquat solution with the chelated copper. Pour the diquat solution into the bottom of the herbicide spray pump. Add the chelated copper in a ratio of 1 part chelated copper to 2 parts diquat solution. Screw on the top of the spray pump.

• 5

  Spray the herbicide mixture on the remaining, rooted duckweed, thoroughly drenching the floating weed's exposed leaves and flowers. You'll need 1 gallon of the herbicide solution for 1 surface acre. Treated plant tissues should turn brown and die within several days. You may need to repeat the herbicide spraying multiple times, about 2 weeks apart, to gain full control of duckweed infestations.

• 6

  Stock your pond with koi, a carp that enjoys feeding on duckweed. Koi can help control fresh duckweed outbreaks once you've manually or chemically removed most of the plants, but the fish cannot eat the weed quickly enough to eliminate an existing duckweed infestation. Duckweed prevention has been achieved at the rate of 50 koi per acre of pond.

Controlling Filamentous Algae

• 7

  Pour copper-sulfate granules into a hand scoop. Read the instructions on the copper-sulfate package to determine the manufacturer's established safe-dose rates.

• 8

  Sprinkle the copper-sulfate granules over the dense surface mats of algae in your pond. Smaller grains will dissolve more quickly and effectively in the water than larger crystals. If your pond has hard water, copper sulfate will bind with calcium and fail to work as an algaecide. Hard water requires the use of alkylamine salt of endothall, which also comes in fine-grain form and can be sprinkled over the algae with the hand scoop.

• 9

  Repeat several times over a warm-weather season to maintain control of the algae if it begins to reemerge.