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Duckweed- Great but not Superman

June 22, 2012

                                                                                                I was telling a good friend about my current struggle with an algae invasion in several of my duckweed pools. He said I should blog about it. I said I need to overcome the algae before I blog about it as I am… ahem… a duckweed expert. He said that I needed to blog about it anyhow. So here I am, blogging about algae that has decided to decimate my duckweed population in four of my pools.

Funny thing is, I’ve got two other small bodies of water where duckweed is thriving, even in water that is only 1/2 inch deep. No algae.  The only differentiator is… I don’t know. That’s the point. All have had manure tea added to them. All have had a bit of 20-20-20 added just for a little shot of phosphorus and nitrogen. My larger pools started off with healthy full crops of duckweed. Then production slowed and one day, I noticed a zig zagged line of demarcation running through my largest pool. Day by day, the line of separation widened and I began to see duckweed roots rising green to the surface of the water.
No, wait… duckweed roots are pale to white. What is this? Turns out, under a microscope, the duckweed roots were still white but tangled around them was thin, stringy green algae. Lots of it. A zigzag line has turned into masses of algae crowding out the duckweed.  This has happened to all four pools.

I could use chemicals to kill off all of the above and start over but this is a challenge I can’t turn my back on. How to lend duckweed a hand and let it regain strength in numbers over its arch rival- algae? Will I have to resort to chemicals or is there a more organic approach?

Nothing I love more than a good science experiment. Stay tuned.

16 Comments leave one →
  1. Nkereuwem D. Famous permalink
    June 13, 2022 12:26 pm

    I have a pond of size 30 X 50 feet filled with duckweed. How can I use it to grow rich financially?
    Would you advise or partner with me to make it work?


    • August 25, 2022 9:36 am

      Maybe self sufficient- rich but you would need many hectares under cultivation to make a real world living at it.


  2. Randy K permalink
    June 28, 2012 6:58 am

    Check the pH. Some algae drive it alkaline which is inhospitable to duckweeds. Other algae can drive pH too acidic (less likely).
    Keep a diverse mix of Lemna, Spirodela ( or Landoltia), and Wolffia. If one gets knocked back by current conditions, the other sizes of duckweed maintain coverage and keep the algae from getting a foothold.


    • June 28, 2012 3:11 pm

      Great points, Randy. pH has hovered around 7.5. I’ve seen prolific algae production push the pH over 10.0 in a algae-only situations, so I know what you are talking about. Currently I have a Lemna/Spirodella mix. Right now, Spirodella is undergoing growth acceleration and I see mottling in my ponds of lighter and darker green duckweed species. Another much larger pond I have access to usually goes primarily to Wolffia in July and August. Love Wolffia- a bit harder to harvest due to smaller netting size but easier to solar dry. Need to get a fresh sample of that to add to the above mix.


  3. Susan Munroe permalink
    June 26, 2012 3:20 pm

    Well. I went out with a cup of tea this morning and viewed my new pet duckweed. It seems awfully small. I remember the duckweed I’ve seen in the past as being about 1/8″ long. This stuff is tiny, probably less than 1/16″ long, like seeds. I’ve been looking up some of the duckweeds, and I’m wondering if this is really Wolffia, instead.

    Tamra, could I take you up on your offer of a small amount of duckweed, to compare?


    • June 26, 2012 6:05 pm

      Sounds like Wolffia all right. Would be good to start it growing in a small container before you turn it loose in huge pool. Sometimes small amounts get lost in the shuffle of birds, algae, assorted tiny predators, etc… It’s a rule of thumb in aquatic circles to “grow out” samples of duckweed, algae, etc… in successively larger containers, 10x larger than the last one until you have enough to grow in a large space. With duckweed, this doesn’t take too long because it’ll double every day. Send me your mailing address at Will be happy to help with some Lemna minor.
      I would like to say that Wolffia is a great species for protein. Fun to watch grow.


  4. June 25, 2012 2:24 pm

    Joe, thanks for your experiences.Very helpful! Sue, perhaps you could modify Joe’s remediation process to suit your needs. Retention time in your remediation ponds is going to be the key factor here, and will be dictated by amount of organics in the water etc… I’d go with Joe’s three step approach. If you have an old sump pump, you could incorporate that into your run-off from a sand filter so you have occasional electricity usage and not constant.Will you be blogging about your system?


    • June 25, 2012 6:47 pm

      I’ll only be leaving info here on the results. I don’t have a blog site and am not really interested in creating one. And you’re doing such a nice job, I’ll just add a bit to it, if you don’t mind.

      Yes, I was planning on running it through four ‘ponds’ (wading pools) via gravity (still free, at least this year), from cattails to bulrushes to water hyacinth to duckweed (for finishing). Or maybe other types of plants. Water hyacinth doesn’t seem to be winter-hardy here (Z 7b) although it’s been found a bit south of here.

      I really don’t want to run it through a sand filter — there is a trick with them that you have to keep the top algae layer moist, can’t let it dry out. I’ve read and thought about it for cleansing a human drinking water supply, but I don’t know how to keep the top layer moist. More research needed.

      My $5, 1/4-cup supply of duckweed arrived today! I put some duckpoop-enhanced 8-/ water into a concrete half-sphere and dumped into the duckweed as soon as I opened it. (Didn’t want it to spoil!)



  5. Joe Shallbetter permalink
    June 24, 2012 2:34 pm

    Tamara Interesting blog. Here are some of my thoughts. I owned a landscaping business and we installed Koi ponds some were 200-300 feet long and up to 3 feet deep..After we filled them with fresh city water they were clean looking and customers were happy. We told them not add fish for a couple of days so the chlorine could dissipate or the fish will die. Then we told them to turn off the pump for a couple of days and the pond will turn green as algae took over. Then as the pool cleared up a little ,naturally, turn on the pump and this will add flow and oxygen to the pond and the PH will stabilize.They can then add fish and frogs and turtles and all sorts of wild life will come. The algae would be controlled by natural cleaning as long as there was flow and oxygen and not to many fish. Flow was critical. We added duck weed at times for the fish and wildlife to eat and it always stayed fresh.
    As for the way to work with bio digestive systems you may want to call or look into the different state rest stations that recirculate their own waste water.I think they use a 3 step approach. The first is to grind and send the product to pond #1. Pond #1 contains bull rushes, and all kinds of marginal plants. the water then flows (gravity) to pond #2, the duck weed pond, again under flow, and then finally to a sand filter where it is pumped into the rinse water system for the rest station. All of them work well as long as there is flow.
    Just thoughts.


  6. June 23, 2012 12:42 pm

    Sue, don’t forget to move your ducks to another place or they’ll eat all you new duckweed in a New York minute.


    • June 23, 2012 4:26 pm

      The only duckweed that my feathered piranhas will get is the handfuls that they are given. The ‘main supply’ will be fenced off. I read enough about it to figure out why it’s called DUCKweed.

      I’ve also been reading about how some municipal wastewater outfits are turning their liquid effluent into lagoons full of water/bog plants like cattails, bulrushes, sedges and water hyacinths, and wondered if I could do something similar with the contents of my duck pond.

      My ducks are almost full-grown and they swim and poop in the wading pool all day. Every day I drain and refill it, and they spend the next day turning it into a sewer. That’s wasting a lot of water.

      So I was thinking that maybe I could create a raised pond that can be drained to a slightly lower pool containing cattails, which drains into another lower pool containing bulrushes, which drains into another pool containing water hyacinths. And the last one could be duckweed. Then the water could be pumped (with some kind of sump pump) back up to the duck’s pool and reused for swimming and pooping in again.

      Theoretically, this would be maybe 5 pools worth of water used over and over, rather than new water every day. Theoretically.

      The time issue is what I can’t seem to find out. Would shifting the water from each pool to the next one every 24 hours be enough? Watch for breaking news as it happens.

      Sue (Queen of Good Ideas that don’t always turn out as expected)


  7. June 23, 2012 12:10 am

    My first thought was ‘nutrient overload’. I suspect that algae can reproduce faster than duckweed, given the proper conditions (excess nutrients, sunlight and warm temps). Did you also add the 20-20-20 to the two shallow ponds?

    The NPK of feedlot manure is about 2.4 – 2.1 – 2.5.
    The NPK of chicken manure is about 6.3 – 6.1 – 5.0.
    Even a little of the 20-20-20 would be a 10-times dose.

    I’ve seen in another of your articles that you add 20-20-20 fertilizer, and I wondered why you would add a chemical fertilizer to an otherwise mostly organic duckweed pond. It seems that it would just confuse the results and make situations like you’ve got now harder to figure out, IMO. Not trying to be nasty or anything, I just don’t understand the ‘why’ of it.

    Be glad you’ve got lots of duckweed. I just had to ORDER a quarter-cup of it from eBay because I couldn’t find any within a 15-mile radius! Disgusting! Darned sandy soil and good drainage!

    Sue (new)


    • June 23, 2012 7:56 am

      Sue, the “why” is that I added 20-20-20 sparingly to my larger ponds because ammonia/nitrate/nitrite levels tested out as non-existent, even with the manure tea. Production had slowed down. (The manure had come from a very old, rain-exposed source- great for color but that’s about it apparently and I did not have access to any that was fresher. The chemical fertilizer was left over from a research project and I figured it would work in a pinch in this situation. This wasn’t a study in organic production of duckweed- rather, it was a study in shading intensities.
      As for feedlot manure ratios vs chemical ratios. If I used a pound of manure vs a pound of chemical fertilizer, then the relative weights of N, P2O5,and K20 would correlate out to 10x or more. I only used 5 grams of 20-20-20 in my 8 ft pool and correspondingly less with the smaller ones. I wanted to give them a nudge, not a heart attack. 🙂
      Thanks for your questions. Sorry you are having a tough time finding duckweed. Send me your address and I’ll mail you some Lemna minor.


      • June 23, 2012 12:19 pm

        Okay, I see.

        I guess when I thought of your manure pond, I was thinking of MY manure pond: a 4-ft kids wading pool with six young ducks spending most of their day in it, pooping profusely in it and adding soil to it as well.

        My duckweed order should be here today or Monday, and I’m sure it will reproduce quickly with the above conditions. Unless, of course, I run into a heavy algae problem. I’ll let you know how it goes.



  8. yassin permalink
    June 22, 2012 6:44 pm

    looking forward to see how you will turn things around 🙂


  9. June 22, 2012 4:45 pm

    In Food Production we face challenges everyday. Growing Duckweed should be no different. It’s how we approach the problem, document the issues, and find a solution. Then we can progress to educate and prosper.


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