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Controlling Duckweed & Watermeal – Pond & Lake Q & A


Q: I think I have duckweed and watermeal. It’s taking over my pond! I can’t seem to get ahead of it! What do I do?
- Several Customers

Picture of DuckweedA: Duckweed and watermeal are very prolific growers and can cover a pond before you know it. When covering a pond it can look like algae, but up close you can see it’s not (see pictures on the left). You can try to rake the duckweed and watermeal off the pond’s surface but more will be back within the week. The absolute best way to get rid of duckweed and watermeal is to use a product called WhiteCap. WhiteCap works by inhibiting the weed’s ability to produce carotene. Without this ability, chlorophyll is rapidly degraded by sunlight and the weeds die. The only water use restriction is a 30 day irrigation restriction.

WhiteCap will also get rid of many other submerged weeds in the pond and will produce season-long results in as little as 30-45 days.

Picture of WatermealFor more information on Whitecap and the aquatic weeds it will control, click here.

27 Responses

  1. Pond Guy, my 1/4 acre pond gets taken over with watermeal every summer. With the current drought (Missouri) I have drained and cleaned out the sediment from the pond. We received a some rain and the pond now has water about 1 ft deep and the watermeal is starting to grow! Argh. Can I treat the soil to kill the watermeal before the pond fills with water (9ft total depth)? Thanks! Chuck

    • Hi Chuck,

      Unfortunately there is a not a treatment available to apply that will prevent the watermeal from forming. You would need to treat the actively growing watermeal with a product such as RedWing. Keep up on the removal of sediment and organic material however as this will help.

  2. Dear Pond Guy, Good advice on what to use on our pond to kill duckweed, except we live in New York State and the products you suggested are prohibited & cannot be shipped to NY. NOW WHAT??

    It’s not easy bein’ green!

    Sue & Mark

    • You’re right Susan, it’s not easy. For others who may not know, some states like New York and California have strict rules on which chemicals can pass through their borders. Your best bet is to contact your environmental department for some help on which chemicals are acceptable and a bit of insight on who may carry them. Local applicators will be properly licensed to purchase and apply pond chemicals and may also be able to provide some assistance.

  3. I dont know much about watermeal as it has never really been a huge problem for me, I found some information on this site about duckweed but it has been failing so far. I might have to look into one of these more expensive chemical treatments.

    • Duckweed and watermeal are very prolific growers and can cover a pond before you know it. When covering a pond it can look like algae, but up close you can see it’s not. You can try to rake the duckweed and watermeal off the pond’s surface but more will be back within the week. The absolute best way to get rid of watermeal is to use a product called Clipper. Perfect for eliminating persistent floating aquatic weeds like Duckweed and Watermeal, Clipper™ Aquatic Herbicide by Valent® is available in a 5 pound container that is capable of treating ponds up to 5 acres when used in surface applications. Valent® Clipper™ Aquatic Herbicide is formed into quick-dissolve granules that mix effortlessly in water eliminating messy and difficult applications. Valent® Clipper™ can be mixed with water and applied using a chemical sprayer. Valent® Clipper® Aquatic Herbicide can also be used on a wide variety of algae, submerged and floating weeds.

      -Missy

  4. I have a pond about one acre surface that now you can hit golf balls off the watermeal and duckweed. Nver lost a fish but it looks terrible. I want to kill the stuff and still be able to eat the fish. My grandson and I like to fish. I dont have a lot of money. How do I kill the stuff and keep it from coming back every year. It seems to die out in October in Georgia

    • Hi Mark,

      Unfortunetly watermeal is a tough weed to treat. Red Wing or Pond Weed Defense will help remove what you currently see in the pond but won’t be able to keep it from coming back. Aeration and natural bacteria along with Eco-Boost are always helpful. If you are really interested in removing the watermeal we do have a new product called Clipper herbicide. It is more expensive but has thus far produced better results on watermeal then any other treatment.

  5. I’ve got a duckweed infestation that I believe is
    responsible for my fish kill in mid to late June the
    last two years. I’ve already got duckweed visible in
    parts of the pond and the ice isn’t even all off yet.
    The pond is about one acre. Is it too early to try
    Whitecap??

    • Hi Scott,

      Typically you can add whitecap in early in the season and it is best to begin the process before the weed has overtaken the pond. The bigger concern right now would be fluctuation in water levels. If you receive a lot of spring rainfall or the pond is several feet higher now where you may have runoff to other areas outside of the pond the chemical may become to diluted to work. I would start by adding dye to your pond and monitor the color. If the dye stays in the pond for several weeks and remains dark you should be good to go with the whitecap, however if the dye dilutes quickly there may be to much water movement at this time to have a successful whitecap treatment.

  6. We have a 1 1/2 acre pond that we want to treat with Whitecap since watermeal is once again emerging. We want to use Whitecap but understand it isn’t very effective if the pond has an outlet. What are your thoughts? We want to start treatment soon since last year our pond was full of watermeal all summer…HELP!

    • Hi Deb & Mike,

      Unfortunetly Watermeal is a very difficult weed to treat. Even Whitecap may not always be able to conquer the entire problem. If you choose to give this a try make sure you purchase enough to treat the entire pond, taking depth into consideration as well. If you have an outlet you will need to close it off for awhile in order to maintain the concentration of product needed to have an affect on the watermeal and always use pond shade. You can also use Pond Weed Defense or RedWing for short term relief if you are not able to close off the outlet. Treatment early on though will definetly give you the best advantage.

  7. the the first time in 10 years my pond has watermeal growing in it. We have treated it with AquaShade but it only worked for a couple of days. We have had a maaive fish die off also since whe watermeal appeared. I

    • sorry sent message & was not through! I have horses that i removed from the pond when it started scumming over but would like to move them back to that pasture- have you ever heard of watermeal hurting horses if they drink it and also if we treat the pond with WhiteCap would it be safe for the horses as well? Thanks

      • I haven’t heard of watermeal hurting the horses so I think your okay there, but I have heard of watermeal causing a fish die off. Similar to what I mentioned about duckweed above. What can happen over time is that watermeal can cover your entire pond like a mat. If this is left like this it can cause a summer fish kill by limiting photosynthesis to the submerged plants below. This will deprive the overall pond of oxygen. If this level gets low enough, it will be lethal to fish.

        As for WhiteCap, there are no water use restrictions except for a 30 day irrigation. So for animal drinking/swimming you are good to go.

  8. My husband and I have a 1-acre pond that is currently covered with duckweed. This is the first time since we have lived there (9 years) that the duckweed has taken over so much. We also have some type of moss under the duckweed.

    Have read your replies about WhiteCap. Is WhiteCap toxic in anyway to humans/birds/fish, etc.?
    Is it too late to use it kill off the duckweed and get my beautiful pond back for this year? If we don’t get the duckweed under control, will it harm the fish we’ve stocked in the pond?

    Finally, we used to have a pond that pumped water from the pond and sprayed it back onto the surface.
    We thought this would be beneficial for the pond and helped add oxygen. Wondering your thoughts
    on whether this is a good thing for a pond?

    • Laurie,

      The only water use restriction on WhiteCap is a 30 day irrigation. When it comes to swimming, any wildlife or fish, you will be fine.

      Duckweed is still in the growing season. The best time to treat duckweed is when it is still growing because it will take in the aquatic herbicide much more aggressively.

      What can happen with duckweed over time is that it can cover your entire pond like a mat. If this is left like this it can cause a summer fish kill by limiting photosynthesis to the submerged plants below. This will deprive the overall pond of oxygen. If this level gets low enough, it will be lethal to fish.

      From what you describe with the water spraying into the air sounds like you have a fountain. Fountains work great in ponds that are 6′ or less. A bottom bubbler diffuser such as an Airmax Aeration System works great when the water is 6′ or deeper.

      Here is one of my blog post on aeration. Hopefully it sheds some light for you.

      Thanks for the question. Take Care!

  9. My husband’s family has about a 1 1/2 acre pond in SC. It is totally infested with duckweed. Some advice that they have heard is to get some grass carp the other advice it to get the chemical that will kill it this year. I am thinking that they need to do both, get the WhiteCap to kill it now and the grass carp for future maint, right? please advise.

    • Michelle,

      Once duckweed begins to take over a pond, it doesn’t take long before it takes the whole thing over. Since the duckweed are prolific growers, you may find that the Grass Carp have a hard time keeping up with the growth. You may not see much of a decline once the duckweed gets a hold for the season.

      Grass Carp do have some drawbacks. Some states, like us here in Michigan, restrict the use of Grass Carp for the sake of The Great Lakes. Grass Carp can really destroy a ecosystem over time.

      I would highly recommend the WhiteCap. Its highly effective and simple to use.

  10. We have an acre stocked pond that is poluted with watermeal. We bought Reward and tried to rig a garden hose to spray it, it did not work. How do most people spray this on their pond?

    • Malenda,

      The sprayer we use to spray on any type of chemical is called our Airmax Pond Sprayer. You mix the product into the sprayer, build pressur by pumping the handle and then spraying by pulling the trigger. Just remember that Reward/RedWing is a short-term control of watermeal. If you are looking for season-long treatment of watermeal, I would suggest a href=”http://www.thepondguy.com/product/1585/212″>WhiteCap.

  11. Cheryl B,

    Great question! During the Spring, you can use WhiteCap approximately 2 to 3 weeks before you would normally see the watermeal floating on your pond’s surface. If you can treat it a couple weeks earlier, you will have a better shot at keeping it bay. To give you a time frame, you would probably see some watermeal forming on your pond’s surface mid-April to Early May. I would apply WhiteCap around mid-April for best results.

    Again, great question, hope this helps. I apologize for the late reply.

    • Along these same lines – we treated our 1.5 acre pond (has small island in middle) for duckweed this spring – used what Sonar recommended – their new product (pour in pond rather than spray). Worked well – then watermeal took over. We have never had that before. At this point it is covering the entire pond except the 20′ spot where our bubbler is for our fish.

      My question – I know you recommend WhiteCap – is it too late to apply it since it is starting Fall now? When should be apply it in the Spring. Lastly, will it also kill duckweed or should we use the Sonar product again next Spring?

      Thanks much –

      C

      • Hi Cindy, Whitecap has the same active ingredient as Sonar, so it will kill both Duckweed and Watermeal. Watermeal is a tougher weed to control, so it will require a higher dosage rate. You can still treat for the weed this year, but it may take 30–90 days to see full results. Early Spring is the best time for the treatment, usually about 2 weeks before you would normally see the weed develop. In either case, just be sure to use the liquid product as the granular does not release fast enough to be effective. When treating by pouring or spraying, you will want to make sure the product gets underneath the floating weed for best results. Duckweed and Watermeal can produce thick mats on the surface and the chemical will just dissolve in the sun before it penetrates the weed (if the chemical is just spraying over the surface of the water). Please let us know if you have any further questions.

  12. Hi Pond guy — I do have a bad case of watermeal (it was sent in for analysis) and would like to use Whitecap. It is now late March and I was wondering if it is too early to apply. Should I wait until I see the first signs? I have a spring-fed pond in Brighton, MI, approx. 100×200 feet.

    Thanks.

  13. Freddie Fay,

    I totally feel your pain when it comes to watermeal. This is an extremely fast growing and hard to kill aquatic plant. The absolute best way to treat watermeal is to use WhiteCap. When using the products you mentioned, was there any die off at all? If not, I guess we should go back and look at first the size of your pond as well as the depth of your pond. The other thing you can do is to send us a weed sample as well, just to make sure we are dealing with watermeal and not another aquatic plan or a form of pollen or oil slick. You can send a sample to: The Pond Guy, ATTN: Weed ID, 6135 King Road, Marine City, MI 48039. Include your pond information as well and we can see what we can do to help you.

  14. I have tried everything. Is there not any home remedy or something that will KILL watermeal for good. I do not want to control it, I want to kill it. I have used reward, sonar, diquat, etc. draining the pond, nothing has helped…any suggestions? I am desperate.

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