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My pond is covered in duckweed. What can I do to treat this stuff? – Ponds & Lakes Q & A


Q. My pond is covered in duckweed. What can I do to treat this stuff? – Tony in Indiana

Friend or Fowl
A: For the lucky pond guys and gals out there who have never experienced duckweed in their pond, or those of you who think you may have it but are unsure; duckweed is a very small floating plant with kidney shaped leaves and a small hair-like root hanging below. It is approximately the size of a pencil eraser and is frequently misidentified as algae. It can cut off sunlight to submersed plants and cut off oxygen to fish and other wildlife. Duckweed is an extremely prolific grower and can quickly cover an entire pond making it frustrating to treat and maintain. More often than not, duckweed is introduced into your pond by hitching a ride on the feet of waterfowl.

Don’t Go Daffy Over Duckweed
There are two great options available to you in your fight against duckweed. Choosing the product that is right for you depends on the time frame you have to treat your pond. For fast acting, short term results, you can use a contact herbicide like Ultra PondWeed Defense® . Treatments with these types of herbicides work best on mature aquatic plants that are in a contained environment. Multiple treatments are typically required for effective short term control. It is important to remember that whenever you are treating large amounts of weed growth, only treat one third of the pond at a time and implement aeration whenever possible to ensure safe oxygen levels. When treating with Ultra PondWeed Defense®, we suggest that you use a tank sprayer to apply the herbicide directly onto the weeds. Ultra PondWeed Defense® has some water use restrictions, see label for details.

For long term treatment of duckweed, we suggest using Sonar™ A.S.. By adding Sonar™ A.S. to your pond in early Spring, you will inhibit 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. Sonar™ A.S. must stay in your pond for up to 90 days for maximum results, so if your pond has a constant overflow or you are experiencing heavy rains, you may need to include additional treatments. Sonar™ A.S. is degraded by sunlight, so when applying, make sure you spray the product directly into the water and not onto the plant foliage itself. Also, adding Nature’s Blue™ Pond Dye after treatment will help prevent sun degradation as well as track water dilution from heavy rains. Sonar™ A.S. has a 30 day irrigation restriction.

POND TALK: Have you ever experienced duckweed in your pond? What did you use to treat it?

11 Responses

  1. I’m amazed that your instant response is chemicals! This is the 21st century; chemicals should be a last resort. As was mentioned above, fish are an obvious solution, and if that’s not appropriate for your pond, aeation bacteria or simple hard work are all possibilities.

    Most chemicals have a down side, if only the balancing act of getting the dose right, the expense of endless repetition, and the sterile environment you might end up with.

    I’m no hippy; chemicals have a place – but surely not as first line management?

    • Hi Andrew – We agree the proactive approach is the way to go for pond management. Many times, pond owners wait too long and the weed problem has overtaken their pond. Chemicals are needed to regain control of these prolific growers. Grass carp can be added but they take up to 2 years before they will make a dent in weed control. Additionally, they are prohibited in many states and big fines can be issued if a pond owner is caught with one.

      Here’s an additional article on reviving overgrown ponds: I’ve inherited an overgrown pond. How can I whip it back into shape?

  2. Both Pond Weed Defense and Whitecap are prohibited in NY state (and others). What would be a good alternative to these products for duckweed problems?

    • Hi Christian,

      Unfortunetly we can only offer a long term approach by promotiong a healthy pond through aeration and the use of natural bacteria to reduce the muck accumulation in the pond. You may want to contact your local environmental agency for a liscensed applicator that may be able to treat your weed problem if you are looking for quick results.

  3. We used hybrid grass carp to control our duckweed. Now that the weeds are gone, we have to feed the fish!

    • We found that our very common goldfish love duckweed. They clean up a pond in no time.

      • Hi, Did the goldfish really work? I have been told that goldfish are not good for natural ponds as they stir the bottom too much and make the pond very cloudy. Have you noticed this? I loathe duckweed!!! It’s been a never ending battle every year, and I am not one to throw a ton of chemicals on everything!

  4. To: Jason
    SUBJECT: Muck Away and fish. This past w/e I tried to use Muck Away for its intended purpose. in shallow water. I hoped to greatly reduce the muc, there-by deepenng the warer. A half hour after distributing it I walked past the area of concern and could not find any MuckAway tablets however there were 2 schools of carp actively feeding on the site. Question: Did the tablets dissolve before the carp attack? If not, how do you avoid having your very expensive MuckAway Stolen by the fish?

    • Hi Dennis,

      It will take a little time for the tablets to dissolve but depending on the amount of muck present the carp may also have just stirred it up enough for the tablets to sink a little making them unnoticable. The fish may pick at them at first but they are not a very tasty treat and will most likely move on and leave them to do their job.

  5. the problem is once the blanket of duckweed dies it just stays there , i need to eradicate it before it blooms

    • You can add whitecap a few weeks before you would normally see the duckweed appear in your pond. This is the best way to get ahead as far as chemical prevention. We would always also suggest aeration and the use of natural bacteria to reduce the organic material in the pond and also to break down any duckweed that has formed and been killed off.

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