Duckweed Isn’t Algae And What To Do About It

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It may come as a shock to some but pond algae isn’t the only plant that can take over a pond and make life miserable for the frustrated pond owner.

Of the free-floating plants that show up a lot in the summer, duckweed has to be near the top of the list. This particular growth is not an algae at all but a rooted plant that floats on the surface of a pond and often get’s blown from end to end as the wind dictates.

Many people attempt to treat duckweed with a typical algae control like a copper sulphate based algaecide and find out pretty quickly that the stuff won’t make a dent in the growth. What’s often needed is a form of aquatic herbicide. Even with these it’s best to treat really early in the growth cycle and not wait too long. It get’s harder to control as it goes along.

Since duckweed pulls much of it’s nutritional support directly from the water itself, it’s possible to control it with beneficial bacteria sometimes. It’s really important to begin treating with a bio well before the plant would traditionally show up and see if you can nutrient loads down enough to keep it from blooming with gusto.

To learn more about my take on duckweed and see an up close and personal video on it, just click the play button below.

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10 thoughts on “Duckweed Isn’t Algae And What To Do About It”

  1. I enjoy your e mails. What I would like to share is that I was plagued with duckweed for 28 years. I tried everything and spent a lot of money. Approx. 7 yrs. ago I purchased feeder gold fish (10-$1.00)to be exact 100 of them. It took one season for the duckweed to clear up as the goldfish ate it. and for the past 6 years I have been duckweed free. This past spring I had a flood and the county drain which is in my pond plugged. When they unplugged the drain I lost all my fish and was sick about it. Needless to say I started having duckweed again. I went out and bought 100 feeder gold fish and when I put them in there was about a $.50 cent size of duckweed and those little fish swarmed over and ate it all. Even though I am starting over I do know for a fact that they eat the duckweed. I also have areators in my pond and I put beneficial bacteria in it….have a good day.

    Lois Stamey

  2. My pond is covered in duckweed this year as well. I know that I have a high nutrient load just from the depth of organic matter in the bottom of the pond. I’ve been fighting that battle (bottom sediment) for three years. I’m losing the battle and the war.

  3. Hi Lois,
    It sounds like you’ve found a plan. The aeration and bacteria is really good and if the fish can keep up they are a great solution. I’ve also been told Talapia will work over duckweed pretty good too…however they are a warm water fish and will die off in the winter.

    Hi Dan,
    You hit the nail on the head with the high nutrient loads. In a strange way, the duckweed is helping your pond process these loads even though you may not like to look at it. As Lois pointed out, one way to lower nutrient loads and organic matter is with beneficial bacteria and some forms of aeration help this work better…so that’s the route I would try if you haven’t as yet. And perhaps fish as noted above would help in some ways too. So unless you make the call, the battle is not over, nor is the war:)

    Best wishes,

  4. I got rid of the algae last year, but have duckweed now. Called the University of Maryland top person in weed control and he said diequat would do the job but the problem is finding someone to spray a mist on top of the plants. In our area, applicators must be licensed in the aquatic field to kill off weeds. Called the Dept of Agriculture and their website showed no applicators licensed. The ones that do spray cannot do a fog/mist spraying. Also checked about weedtrine-D and you must be licensed to buy and spray. I’m batting zero. I like the idea of feeder gold fish, but where can you buy them.
    Charlotte Sutton

  5. Hi Charlotte,
    Interesting how the algae is dealt with and duckweed shows up. This happens quite often. Both plants feed off a lot of nutrients in the water…usually duckweed will out-compete the algae for this and you’ll end up with only duckweed but not always.

    Unfortunately most agriculture departments and even universities think the only way to deal with this kind of thing is through chemicals but as you’ve read that’s not always the case.

    You might do a web search and see if anyone comes up in your area that might stock fish. And remember should you need to treat chemically (if you can find someone to do it) I wouldn’t suggest doing it now anyway…do it in the spring when you can get a jump on it.

  6. I too am developing some duckeweed in my backyard pond this summer. I am just scooping some out at the moment as it has not overtaken the pond as yet. My pond is approx. 1500 gallons. I am however having a problem with floating heart taking over the surface of the pond. I wish I had never introduced it. It has along with several other plants ex. rushes and grasses rooted itself in the sludge at the bottom of my pond and is taking over the surface of the water. I would like to eradicate it if possible. It is nice to have the coverage early in the spring before the lilies and hyacinth/lettuce that I add start to grow and give coverage but the downside is it takes over. Perhaps I need to address the sludge in the pond bottom (rubber liner with riverstone on top). Please advise. Thank you.

    P.S. Your emails and videos are very informative and interesting.\

  7. Hi Janine,
    You bring up a very good point. I would be reluctant to use any kind of plant killer in a small pond like this if you have fish.

    But your comment about reducing the sludge is right on. Rooted plants have to have some where to set up and when you take the sludge away, they will have a tougher time growing. The same thing applies in any size of pond from the backyard variety to large multi-acre waters.

    And the best way to reduce this sludge is either through a manual clean out in the small pond, dedging in a large pond, or using some form of beneficial bacteria to break this down over time.

    Thanks for your comments!

  8. Hi Mark,

    Thanks for the timely and informative emails. I really appreciate it.

    I live in NE Ohio & have a pond (app 60’X 80′) and was, frankly, relieved when I learned that what was growing on top was Duckweed and not algae. I’ve let things get a little out of hand but, whenever I get a chance I go out in a canoe and simply scoop out as much of it as I can and use it to fill low spots in my yard!


  9. Duckweed problem
    Our small pond (about an acre in size) is covered with duckweed. When we sit on our deck outside our home, overlooking the pond, I feel like I am in a swamp. The farmer’s corn field across our gravel road may be fertilizing the algae as it flows through the covert into our pond as well. This may account for the high nutrient levels. I asked the county conservation board for help and they sent me information on putting in a buffer strip to plant in the area where I am getting runoff from the neighbor’s field. Great idea but I cannot do this, since we do not own the cornfield or land across the covert where water flows into our pond. Can I do anything about the duckweed now or do I need to wait until next spring? How do we control this duckweed? I loved the goldfish idea, but my husband is against it (we have our pond stocked with fish). Would the gold fish reproduce and cause other problems? Please help!

  10. Hi Lynda,
    As the video suggests I like to wait and treat duckweed in the early spring to catch it very early. This minimizes the amount of chemicals needed to control it and it’s safer for fish to treat at this time while the plant is very limited.

    You’re probably right on about the high nutrient loads coming from the nearby farm…and the buffer strip is a great suggestion…just easier said than done:)

    There are several herbicides available to control duckweed. Weedtrine-D is one. Sonar is another to research.

    I’m not against using fish to help but in your case it might not be worth the marital conflict. I can’t fault your husband if he has game fish introduced to the pond and fish probably won’t help much if the duckweed is as well along as you describe.

    All the best,

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