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Duckweed Growing Tips and Tricks

March 7, 2012

I’ve been saying all along how easy it is to grow mega crops of duckweed, IF you know the tricks!

Duckweed grows on benign neglect in the wild. However, to cultivate it in a garden setting, duckweed does have a few definite needs if it is to thrive consistently.

1. Like any vascular plant, duckweed needs a minimum of nitrogen, phosphorus, potash and micro nutrients. Sources can be as simple as a little humus and/or soil or compost tea. To achieve high protein levels of 40% or greater, duckweed needs added nitrogen, preferably in the form of ammonia from animal waste. This is why it thrives so well in fish aquariums.

Nitrogen sources can include fish wastewater, chicken coop drainage, some types of grey water, vermiculture liquor, or aged manure. If a solid, place in a gunny sack and lower into the water column. This releases a steady amount of nitrogen and trace elements for a couple of weeks. Replace if you see duckweed roots grow to an inch or longer. Some people will occasionally spray the duckweed with an organic-based foliar spray as another source of nutrients.

2. Duckweed thrives at a pH of 6.0 to 7.5  If algae is present in large quantities, it can raise the pH to dangerous levels by virtue of CO2 production at night. The trick is to monitor pH, especially if algae is present. Treat pH extremes as described in my prior post. Encourage an adequate surface covering of duckweed at all times to suppress algae production.

3. Harvest duckweed as needed but leave about 1 1/2 to 2 pounds of wet duckweed per square meter on the surface of a pond daily. This insures enough of a cover to slow down algae growth or suppress it altogether. As an added benefit, this much covering also helps with mosquito control, water evaporation and temperature issues. More than 2 1/2 pounds of wet duckweed per square meter will result in its demise, as it self mulches at that concentration.

4. Duckweed prefers water temperatures of 50 to 90 degrees. Above or below that range, duckweed just sort of sits there. Much above 90 degrees, your duckweed crop will crash and it won’t be pretty. Ways to circumvent this is light shading from surrounding trees, plants or shade cloth hung over the growing area.

5. Keep water movement to a minimum. As duckweed floats on the surface, any strong wind will push it to the edge of the pond where it will begin to pile up in layers, effectively self-mulching the layers beneath. Grow taller food crops around the perimeter to shield it from the wind.

Duckweed is an amazing crop that gives much more than it gets, but still needs a few basic “gotta haves” in order to reach its full potential in a garden setting.

33 Comments leave one →
  1. April 21, 2014 3:42 pm

    Amazing, such a handy web page.

    Like this

  2. February 24, 2014 3:55 pm

    Excellent write-up. I certainly love this website. Stick with it!

    Like this

  3. Cindy prindle permalink
    February 22, 2014 9:10 pm

    This article drew me in also about 4-5 mos ago but since then not many of the posts have been about duckweed. Rather disappointed at the moment but hoping for more come warmer weather.

    Like this

    • February 23, 2014 7:06 am

      Cindy, what are you looking for? Yes my ponds have three inches of ice on them. I still work on duckweed projects every day but in an international context.

      Like this

      • Cindy prindle permalink
        February 23, 2014 4:56 pm

        I’m new to aquaponic & want to grow duckweed for my tilapia as well as my chickens & goats so any & all info I can get on the subject is welcome!!

        Like this

  4. February 22, 2014 6:09 pm

    Hey There. I found your blog using msn. This is a very
    well written article. I will make sure to bookmark it and return to read more
    of your useful info. Thanks for the post. I’ll definitely comeback.

    Like this

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  7. October 26, 2013 10:47 am

    My family and I are just starting with Aquaponics and I’ve been doing a lot of research. Today the topic is Duckweed, we want to grow duckweed in our fish tanks, but it doesn’t look like that is very practical, since our grow beds cover most of the fish tank, which will limit the light that the fish tank gets, plus it looks like the the water from the grow beds flowing back into the fishtank will disturb the duckweed growth. So, will duckweed grow at all under these conditions? or would we be better off to make shallow tanks specifically for the duckweed and then feed it to the fish?

    Like this

    • October 26, 2013 11:59 am

      Hi Cindy. Duckweed can grow with only 20% of available sunlight. Of course in the winter, that ratio may need to be bumped up. Your fish will probably out-eat duckweed grown in their tank. Duckweed can handle some water movement though. I’d opt for separate dw tanks with more sun. If you wanted to TRY to throw some duckweed in your fish tanks and watch what happens- cool. Usually I don’t recommend it but if your tanks are well-oxygenated already, it might work. If you want to remove duckweed from a fish tank, I stumbled on a great little DIY project to make that happen easily:
      Watch the video on this page: http://www.wafishbox.com/t8414-you-know-how-i-get-rid-of-duckweed This guy makes it SO… easy.

      Like this

  8. September 8, 2013 12:46 am

    i plan to grow duckweed or azolla on slightly bigger scale,say 1 hectare to produce feeds for the free range chicken we have in the farm. at present it is a toss up between duckweed or azolla. which do you think is better, duckweed or azolla?

    Like this

    • September 8, 2013 7:24 am

      Hi Glen. I don’t know where you are located, but that might play a role in which plant you choose. Azolla cannot tolerate freezing weather. Duckweed produces turions and sinks to lower, warmer levels- even down into the mud at the bottom of a pond. Duckweed grows faster than azolla and is easier to digest. I am not an azolla expert but have grown it a few times and appreciate its ability to fix nitrogen due to the Anabaena bacteria that form a symbiotic relationship with it. I’ve seen it being fed to pigs and chickens in S. America.
      Just to be fair, I’d say do test trials in smaller ponds or kiddie pools, then decide for yourself. During the hot summers, duckweed outcompetes azolla.. I like azolla but I LOVE duckweed for its versatility, growth habits, robustness, protein content, ease of harvest, and looks. That’s just me. :)

      Like this

  9. Francisco & Monika permalink
    August 24, 2013 9:21 pm

    Hello, My wife and i are thinking about growing DuckWeed indoors during the winter to feed our Ancona Ducks some greens during the winter we live in the Fingerlakes area of Upstate NY. I have never done this and thanks you for the info you posted here. Can you suggest what kind will be better for our Ducks and Chickens? and if you can mail me some i will pay for it.

    Thank you

    Like this

    • August 26, 2013 7:27 am

      Any species will work just fine. The trick is to grow enough in the winter to feed green-deprived farm animals. Do you have a greenhouse where you could set up shallow temporary ponds? If it gets down to freezing, you will need to introduce additional heat, hopefully from a sustainable, free source. Otherwise, roll with nature and wait again until spring.

      Like this

      • Francisco & Monika permalink
        August 26, 2013 9:08 am

        Yes we do We have a 2 1/2 insulated car garage i will set up 4 small kids pool to grow it there and go outdoors in the spring time.they are going on sale soon ( end of the season )

        Like this

        • August 27, 2013 7:30 am

          How are you going to keep light on your kiddie pools in a garage? Do you have sky lights or a south-facing large window?

          Like this

    • August 27, 2013 7:31 am

      Also, if you email me your address and specify how much duckweed you want, I can send it to you. tamraf9@gmail.com

      Like this

  10. August 16, 2013 8:56 pm

    Hi there, just wanted to mention, I enjoyed this blog post.
    It was helpful. Keep on posting!

    Like this

  11. Amanda permalink
    August 4, 2013 10:28 pm

    HI! I am working on a science project for school about duckweed and I stumbled upon your website. It’s awesome and very informative! I started growing my own but I stumbled upon a few problems. I just got mine shipped in from amazon a couple of days ago, but when I started growing them in containers of houseplant fertlizer, they started to turn a very deathly shade of brown. At first, I thought I was killing them because I put them under the sun, so I scooped in some of the surviving ones into another container and put them under the shade. However, they still started turning brown, so I’m starting to worry. When I researched online to see what dead duckweed look like (by the way, I’m using Lemna minor, thought that would be useful information xD) they all said it would turn white. So I have a feeling that they’re not dead, but instead it’s something else. Do you mind helping me a little bit? It would mean so much to me! Thanks!

    Like this

    • August 5, 2013 7:25 am

      Amanda, love your science project. Even fails are great if you learn from them. In this case, the duckweed is dying. Using garden fertilizer alone in the water is a tricky way to grow duckweed. I’ve had some success but usually not as the pH gets out of whack.
      Here is what I recommend these days for transplanted duckweed: Use water that has sat out for a day to release all chlorine. Add a cup of soil per two gallons of water for micro nutrients. Add a pinch of sea salt if you have it. Release new duckweed into this mix and park in semi shade for a week to adjust to its new surroundings. If it is beginning to grow, you can give it a bit of garden fertilizer but just a tiny pinch. (or a tiny bit of aged manure if you can find it) Go easy on fertilizing it overall. The dirt will help buffer the water pH so when you DO add fertilizer, it won’t cause the pH to shift wildly.
      If the water temperature gets in the low 90′s, you will experience die-off. Therefore in the hot summer months, semi-shade is safer. You can always add cooler water from a garden hose to cool it off.
      Send me your address and I’ll mail you some new duckweed. tamraf9@gmail.com All I ask is that you pay me for shipping so I can continue to pay it forward.

      Like this

      • Amanda permalink
        August 9, 2013 11:15 pm

        Hi Tama,
        Thank you so much for this information! And also, thank you for offering me the duckweed, but I already ordered duckweed before I saw this. If my new ones die out, I may take up your offer again :) Actually, I just got my new duckweed yesterday and I just put them in a solution according to your instructions. I hope it will be better now :) So it was the pH that was the problem; I wanted to test the pH of my water but I unfortunately had not gotten pH strips yet. I ordered them on amazon a day ago. And I agree, duckweed that is shipped in isn’t very fresh and most of them die very quickly. Maybe that’s another reason why they turned brown? Again, thank you for everything!

        Like this

  12. February 1, 2013 9:32 pm

    I may be late to the conversation, but I am curious about how the duckweed would be used. Human consumption? Animal feed? Would chickens eat duckweed (obviously ducks do…)?

    Like this

    • February 1, 2013 9:39 pm

      Oops. Nevermind, I found the website where this is discussed. Intriguing. I’m thinking we have an ideal setup to utilize this to cut chicken feed costs while recycling their manure…

      Like this

      • February 2, 2013 8:09 am

        Desert Cat, am not sure if you meant my site or another. Try my animal feed posts, or http://www.InternationalLemnaAssociation.org and join our LinkedIn group: International Lemna Association for answers from our duckweed growers and researchers. Your blog pics of your farm are beautiful. Never knew the science behind how to put up a wire fence. (Also, your disclaimer cracks me up.)
        As far as chicken manure, I’d let it decompose or run through a biodigester, then dilute. You could use multiple dilutions to see which one duckweed grows best in. I wouldn’t feed it back to my chickens (cross-pathogen issues perhaps) but instead use it for fish, hogs, cattle, etc… Cheers! Tamra

        Like this

  13. February 1, 2013 12:11 pm

    It is normal. It is prablboy hair algae. Nothing wrong with it and it is actually healthy as long as it does not take over. You might have more problems with pond scum algae or water algae but since your water is clear dont worry.

    Like this

  14. Ken Knight permalink
    September 25, 2012 10:31 pm

    Hi- Love your blog. I’m wondering if there is any way to incorporate duckweed into a closed-loop aquaponics system so that it doesn’t take over the entire system? Frequent changes of water seems like a waste of water and the benefits of the duckweed to an aquaponics system. But how would one keep it from invading the whole enchilada? Is there any way to sequester it? I look forward to any thoughts you might have on this.
    Thanks, Ken

    Like this

    • September 26, 2012 7:17 am

      Ken, I don’t know your set-up, but do know that it is hard to sequester duckweed to only one part of your system as the daughter fronds are small and easily siphoned off, despite your best efforts to screen them out. I do have a few ideas. 1. Go with a larger species of duckweed, such as Lemna gibba. 2. Insure that your outlet pipes are well below the surface of the water so as to not disturb the duckweed layer any more than you have to. 3. Use a series of fine mesh screens for your return water from a duckweed remediation pond. 4. Get a herbivorous fish like tilapia or a grass carp to live in an interim water space as they love duckweed.
      Thanks for reading. Good luck!

      Like this

      • Ken Knight permalink
        September 26, 2012 9:18 am

        Thanks for the ideas. I’ll let you know how it goes.

        Like this

        • February 4, 2013 4:53 am

          we had a barrel pond too and the fish awalys died until we got a pump the person at the water garden store said that the oxygenating plants don’t work when the sun is down. the pump did the trick until the evil racoons came

          Like this

  15. July 12, 2012 4:16 pm

    How thick does duckweed get? How thick can it get and still remain healthy? What happens if it is allowed to go past ‘harvest’ time? Does it rot and stink or anything? Or does it just stop growing?

    Sue

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    • July 12, 2012 6:02 pm

      Hi Susan, the sort of short answer is that you will want to aim for a covering of duckweed all the time to reduce chances of algae taking over. However, how much is too much? The answer is a bit different with each species. Bigger fronded varieties like Spirodella can withstand less layering on itself than say, Wolffia. With Spirodella and Lemna minor, each successive layer gets less and less light and nutrients and a once-thriving stand will self-shade (or self-mulch) within a few days. At first, the lower fronds quit dividing, then the roots lengthen and the undermass starts getting darker and darker to where the lower fronds die and begin not smelling so great. Wolffia on the other hand- I’ve harvested Wolffia that was over an inch deep and all appeared green and healthy. Perhaps as Wolffia has no roots to get entangled, any that die simple drift to the bottom of the pond.
      If I find a batch that is “over-ripe”, I try to harvest just the top, brighter green duckweed and put it aside for a bit. I then harvest the underlayers and use them in my compost or as mulch for something that isn’t up close to the house. It’ll smell for a day or so and you’ve just got to let it air out. The green duckweed is re-released in the pond if I know the water is a healthy environment. This doesn’t happen too often as I use any spare duckweed on a pretty continual basis. However, if you go on vacation in the heat of the summer, it’s a good idea to have whoever feeds your dog to do a bit of duckweed harvesting every couple of days and run a water hose to “rain” on your growing ponds to insure a healthy stand.

      Like this

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