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Controlling Duckweed – Pond & Lake Q & A


Ponds & Lakes Q & A

Q: Duckweed has taken over my pond! What do I do to eradicate it and prevent future outbreaks? – Sue in Michigan

A: Common duckweed, or Lemna minor, can take over a pond in no time. Growing in dense colonies in quiet, undisturbed water, these tiny free-floating plants, if left unchecked, will blanket a pond or lake over the course of several seasons, depleting the water body of oxygen, destroying fish populations, and killing submerged plants by blocking the sunlight.

Most often, these green invaders are transported to your pond on the feet of waterfowl, such as ducks, geese or even herons. The plants stick to their feet or feathers and can be carried for miles. Though water fowl and some fish eat duckweed, it typically reproduces faster than the animals can consume it.

Short-Term Solution

When controlling duckweed, you can use a fast-acting aquatic herbicide, like Ultra PondWeed Defense®, to knock down the plant population. The contact herbicide is designed to work best on mature aquatic weeds in a contained environment, and you will need to apply it to the duckweed multiple times for effective short-term control.

To spot-treat duckweed, mix 1 quart of Ultra PondWeed Defense® with at least 3 gallons of tap water and spray directly on the pond’s surface using the pond sprayer. One quart will treat 5,000 square feet. To ensure safe oxygen levels, treat your pond in thirds, waiting 10 to 12 days between treatments.

Long-Term Control

For longer-term control, use Sonar™ A.S. or Fluridone. It’s the least-expensive method of treating an entire pond, easy to apply, safe for aquatic life and lasts an entire season. The herbicide is absorbed by the leaves, roots and stems directly from the water, and it works by inhibiting the weed’s ability to produce carotene, a pigment that protects the plant’s chlorophyll. Without carotene, the sun quickly degrades the green chlorophyll and the weed dies.

To treat one surface acre of duckweed in a 4 to 6-foot deep pond, we recommend you mix in a tank sprayer 32 ounces or 1 quart of Sonar™ A.S. with enough water to fill the tank. Place the spray nozzle directly under the water and disperse evenly around the pond. It can be applied in early spring before the weed even appears, which means you can get ahead of it before it becomes a problem. Keep in mind that it needs to stay in your pond for up to 90 days, so it’s not recommended to use in ponds with heavy overflow or during times of heavy rain. Also, Sonar™ A.S. will be degraded by the sun so we suggest to add Nature’s Blue™ Pond Dye or Black DyeMond™ Pond Dye right after treatment to ensure the longevity of the application.

POND TALK: What kinds of invasive aquatic plants have taken root in your lake or pond? What did you do to control them?

17 Responses

  1. i have about a half acre lake app 20 to 25 feet deep and duck weed has started appearing. whats the best method for getting rid of it.

    • Hi Dan – Duckweed is a prolific grower; the quicker you get treatment started, the better. To get in control, we recommend using a herbicide like Clipper™ and Ultra PondWeed Defense® are both selective plant killers. They’re a short-term solution—be sure to apply as directed.

      For long-term control of duckweed, you’ll need something a bit stronger. We recommend Sonar™ A.S.. It’s formulated to control stubborn aquatic weeds all season long. Don’t forget to add pond dye, it protects the Sonar™ A.S. from being quickly degraded by the sun.

      • where can i get clipper and ultra pond weed and how expensive is it.

      • Both products can be purchased through our website or calling us at 866-766-3435. If you have any questions about dosage rates or how much product your pond will need, we encourage you to call us to speak with a Pond Tech.
        Clipper™ can be purchased by clicking this link. Ultra PondWeed Defense® can be purchased by clicking this link.

  2. […] if you look very closely, you’ll find that it’s actually duckweed or watermeal. Check out this blog entry to learn more about controlling this invasive […]

  3. by the pic in the duckweed article ,, the sliver of duckweed in the pic looks like my iris pond plants,, is this the same or do i need to view a better pic.??? or is the duckweed the algae //

    • Joe,

      Here is another blog post that shows more of a close up of duckweed as well as compares it to forms of algae and other nuisances.

      Note: Duckweed and algae are two very different things.

  4. I bought an a 9-watt greenfree ultraviolet clarifier. I have a 100gal. pond, the guy told me it would work to destroy the algae. My pond is 3′ deep, and I need to know, what size pump I need to make the water circlate right?

    • Sesa,

      The maximum water flow that a 9 watt GreenFree UV can handle is 900 GPH (gallons per hour). What kind of algae do you have? A UV clarifier will only kill green water algae that flows through the UV. Any string algae or floating algae that does not pass through the filter will be unaffected by the UV.

      Hopefully this helps!

  5. What about for a very large pond? I am concerned that we would inadvertantly kill the life in the pond.

    • Hedy,

      WhiteCap is designed for large ponds. What size is yours? If it is over 1/8 acre (over 5,445 sq. ft.) than WhiteCap is perfect to help control duckweed.

      Usually when fish are killed by any aquatic herbicide treatment its because there is a ton of vegetation and the chemical works extremely fast. When vegetation dies quickly it can take oxygen from the water. Since WhiteCap works very slowly, you don’t have this issue. WhiteCap works wonders when there is a lot of vegetation to get rid of.

      Now if you just want to get rid of duckweed but not any other aquatic plant in the pond, then there would be the issue. When treating for duckweed, whitecap will not only get rid of the duckweed, but most other aquatic plants contained in the pond. The fish however will be fine.

      Hopefully that helps answer your question.

  6. I have duckweed in a small 4′ x6′ pond that came from a plant I bought. Is there are way to purchse smaller quantities of these herbicide or do I keep scouping the duckweed out with my fish net? Looks like 8 oz. would be a lifetime supply. Is it safe for shubunkin and golden orf? M

    • Using WhiteCap is a small decorative pond such as yours isn’t recommended. When using WhiteCap according to the label, there are no issues with the fish that you have.

      Do you have a skimmer in your decorative pond? For floating plants such as Duckweed or Watermeal, a skimmer works great at automatically skimming the surface of your pond instead of having to manually skim with a pond net.

  7. Thanks for your website. I have learned so much from it. Also, thanks for the feature of being able to ask questions and have them answered.

    • Ruth,

      I’m glad you’ve found our site helpful. We are striving continuously to make the site better and more functional. If there is anything that we don’t have that you would like to see on our site, please let us know! Thanks for the comment!

  8. our goldfish eat the duckweed as soon as we put it in the pond so we never have a problem with it taking over

    • In a small decorative pond, usually the fish can keep up with the duckweed. But in a large pond/lake, the duckweed can spread so fast that the fish cannot keep up with it and usually a treatment of WhiteCap is necessary.

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