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Paul Dean

Growing Duckweed

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I am wondering if anyone out there knows of specific ways to grow duckweed for maximum production. I've seen numerous references to duckweed in aquaponics circles, and clearly some folks are using it as a feed supplement for fish and animals. I'm not looking to debate the pros and cons of using it for fish feed, just interested in any info anyone has about how to grow it effectively, preferably in a greenhouse setting. I am thinking about the idea of a continuous flow system fed by a couple of tanks of some native N. American cold water fish. Seems like the challenge would be to keep duckweed from entering the fish tanks on recirculation, and to figure out how much duckweed growth and/or additional filtration, would cleanse the water enough for the fish. Or, could fertilize duckweed with compost, but stagnant tanks of water in a greenhouse sounds like a bad idea. I've seen it done in kid swimming pools, but does anybody really know how to do this effectively for sustained production/harvest?

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I grow a little duckweed, but I do it separately from my system, and nothing on a large scale. For the sole reason I don't have room for duck weed tanks. The only person I know who has grown duckweed in quantity would be Gary. But if you think about it, there should not be much to it, duckweed doesn't like a lot of water movement, so you just setup a tank where you have little water movement and a standpipe at the other end, with a guard to keep the duckweed from going into the system. that doesn't have to be any thing much you can use the same housing that you use for the growbeds. Have the water come in under the surface slowly. The duckweed will do its thing.

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Hi Paul,

I wrote an article on duckweed some time back. You'll find it.....here.

I hope to produce a larger piece of work on duckweed......possibly an e-Book.....in the not-too-distant future.

Gary

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Thanks folks for the responses. Gary, I have seen your article, didn't know it was you, but read it sometime back. Just looking to devise a system that works. Am considering making ethanol to heat a greenhouse, which means I will have CO2 to enhance plant growth. I am wondering if pumping CO2 on top of grow beds filled with water and duckweed, would increase the production? It seems like in a greenhouse, the CO2 would settle on the grow beds and stay there, possibly increasing yield dramatically. Any thoughts?

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duckweed is not an oxygenating plant. It is easy to grow. I grow it to feed to my ducks and they will go through 2-3 lbs/day (wet weight).

I took a big rubbermaid plastic tote and filled with water and placed it where it got sunlight. Growth was slow at first, so when cleaning out a solid's settling tank I was experimenting with, I dropped the gathered fish solids in the tub. Growth is more dependent on temperature and light regulation. ~85 degrees is ideal and at least 6 hours sunlight. I have grown it in a tub connected to my aquaponics setup and it will grow well there, it just does not grow that well under flourescent light as it does under sunlight.

Duckweed does a tremendous job and absorbs nutrients out of the water. 2 tubs of it drained so much nutrient out of the water that my tomatoe and okra plants litterally starved to death. I have only qualitative info, so would hazard a guess as one duckweed tub, harvested daily absorbs the nutrient equivalent of about 4 growbeds growing plants such as tomatoes, basil or okra.

As far as keeping the duckweed in the tub, I used a pond basket with a standpipe in the center. The water level just needs to stay below the basket. A small amount seems to make it through from time to time, but I let it drain back to the fish tank and the fish eat it up. I have bluegill and tilapia in the tank which eat it, but not all fish do. You might also consider azolla as it can absorb nitrogen out of the atmosphere.

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Hi Paul,

Some things to think about:

  • At above 1.2kg/sq. metre, duckweed will self-mulch. Below 0.6kg/sq. metre and algae will flourish in the duckweed.
  • Duckweed is capable of taking up ammonia in its raw, unionised state - making it very useful as a plant filter.
  • It doesn't like wind.....so large areas of duckweed will need to be sheltered if wind is not going to hamper production.
  • I use a simple weir arrangement (made from PVC fittings) to keep the duckweed in the tank.
  • Duckweed tanks function as sedimentation tanks, too. We vacuum the sediment from the duckweed tanks at the same time that we harvest the duckweed.

Feel free to ask specific questions as you require.

Gary

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Again, thanks for the feedback. So duckweed will remove nutrients from the fish-waste water at a higher level than grow beds (per equal volume and/or surface area), but will not re-oxygenate the water?

What about a continuous feed, slow flow (entering at the bottom of the duckweed tanks) from crappie or bluegill tanks, into shallow duckweed tanks in a greenhouse, (where there is no wind, and something approaching ideal temperatures) then return the water through shower-head style bubblers into the fish tanks. If I can get the fish-tank size and stocking density, to match the duckweed tank volume, and if I "vacuum" solids off the duckweed tank bottoms, what additional measures might be needed to bring the water quality and oxygen levels back to where they should be for the fish? Also, does anyone see a fatal flaw in this concept that I might not have considered?

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In the previous post I said "If I can get the fish-tank size and stocking density, to match the duckweed tank volume........" I meant......... if I can get the proper ratio of duckweed grow space, to fish tank size and stocking density, so that the proper amount of nutrients are filtered from the water by the duckweed........etc.

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sounds like a good place to start a test... you'll still need some kind of solids removal...

i'm not sure if it's pointed out in Gary's article, but you may find you need to add some duckweed from a different sources occasionally, i read an article a while ago about the way the duckweed reproduces, and the root system get's shorter, it's life span and reproduction capabilities decrease, resulting in "die offs" if all the dw is from the same generation

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Hi Keith,

I haven't read about the shorter roots/lifespan stuff.......and I'd certainly appreciate a link if you can recall where you read it. I know, from experience, that duckweed roots become longer (35 - 50mm) (and the protein level drops if the nutrient content of the water is low.

What about a continuous feed, slow flow (entering at the bottom of the duckweed tanks) from crappie or bluegill tanks, into shallow duckweed tanks in a greenhouse, (where there is no wind, and something approaching ideal temperatures) then return the water through shower-head style bubblers into the fish tanks. If I can get the fish-tank size and stocking density, to match the duckweed tank volume, and if I "vacuum" solids off the duckweed tank bottoms, what additional measures might be needed to bring the water quality and oxygen levels back to where they should be for the fish? Also, does anyone see a fatal flaw in this concept that I might not have considered?

Paul......Depending on its size, the duckweed tank can serve as a settling tank while the duckweed is helping to remove ammonia from water passing through the tank.

You can set things up in the way that you've suggested, but I would still include a means of removing suspended solids and also bio-filter - either a trickling bio-filter or a moving bed bio-filter......both simple to set up and operate.

Gary

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from fao.org;

Vegetative growth in Lemna minor exhibits cycles of senescence and rejuvenation under constant nutrient availability and consistent climatic conditions (Ashbey & Wangermann, 1949). Fronds of Lemna have a definite life span, during which, a set number of daughter fronds are produced; each of these daughter fronds is of smaller mass than the one preceding it and its life span is reduced. The size reduction is due to a change in cell numbers. Late daughter fronds also produce fewer daughters than early daughters.

At the same time as a senescence cycle is occurring an apparent rejuvenation cycle, in which the short lived daughter fronds (with half the life span of the early daughters) produce first daughter fronds that are larger than themselves and their daughter fronds are also larger, and this continues until the largest size is produced and senescence starts again. This has repercussions as there will be cyclical growth pattern if the plants are sourced from a single colony and are all the same age. Under natural conditions it is possible to see a mat of duckweeds, apparently wane and explode in growth patterns.

The cyclic nature of a synchronised duckweed mat (i.e. all the same age) could be over at least 1 month as the life span of fronds from early to late daughters can be 33 or 19d respectively with a 3 fold difference in frond rate production (See Wangermann & Ashby, 1950).

The phenomena of cyclical senescence and rejuvenation may cause considerable errors of interpretation in studies that examine, for example, the response of a few plants to differing nutrient sources over short time periods.

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I have had the duckweed die off happen in my small pond the weed completely covered the surface very quickly and I had to remove it everyday,then after approx. a month it just lost vigour and died off and has not returned, in this country there are no temperature variations to explain it .

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it sounds like the reproduction cycle was the reason for your die off wetauser...with my last batch of dw, i put half in a a 55 gal ft with a close light, and half in a 35 gallon plastic pond with no heat and a far light sources.. seeing if i can create 2 groups..

but dw is pretty easy to get.. this is just a "time filler" experiment

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If I get this, the implication is this cyclical die-off can be prevented by introducing several duckweed strains of different ages, or maybe periodically introducing new strains from different local sources?

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I am having a similar experience to Wetauser - my Duckweed has stopped growing and reproducing. It's kind of deceiving here because it's still quite warm, though we are entering the cool season and it looks like Autumn back home with many of the trees losing all of their leaves. I'm guessing it's the change of seasons, even though it's a subtle temperature drop.

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Paul,

that's what all the reading i've done has led me to believe.. i've got a couple near by sources for dw, but i think if i have 3 systems with duckweed, i can change light cycles/temps to introduce changes and then be able to occasionally mix from each system to keep it going... it makes sense in my head anyways..

right now, i just have 2 sets of dw from the same source, with different light levels and temps

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Hi,

I haven't had quite the same experiences with the duckweed dying off........but my stock may have been a bit more diversified at the time that I put it into the grow tank. I've had duckweed crops that have persisted for over a year and, even then, I dismantled the system rather than the plants dying off.

Duckweed appears to be a bit like rabbits. Their reproduction capacity is legendary......until you deliberately set out to breed them......and that's when you encounter the subtleties in their production.

Gary

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The die off only happened in my small pond,and I was glad to see the back of it,I still have it in the filter channel of my main pond,so it cannot be climatic.

I have tried feeding it to my fish ,but they never seem interested the mosquito fish try it when it first goes in but soon give up,the koi never look at it, and so far the tilapia have never tried to eat it.

or maybe its my cooking they don,t like

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Ha ha - you could be using too many thai chilis in the mix.

That is strange though, because both my Koi and Tilapia will eat it often.

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It could be that my fish are not hungry enough which could mean that duckweed is just not tasty ,anybody actually eaten any? because of course there is nothing wrong with my culinary skills, but I will cut down on the bird pepper chillies.

With the ponds being open there is most likely more choice of natural food, insects and beasties that come to grief,the brown dragonflies are always laying eggs in the water, and the alga growth on the walls that the koi graze like cattle.

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I know my Tilapia love to eat the duckweed I grow.. However I can't speak for other species of aquatic vertebrate.

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Hi,

It's not unusual for fish to become selective about what they'll eat. The key is to get them started on the particular ingredient (like duckweed) early in their growing period. You can also try freezing the duckweed and giving it to them that way - they'll push a frozen chunk of duckweed around the tank until it melts.

Tilapia and koi (carp) will both eat duckweed.

Gary

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Tried the frozen duckweed yesterday Gary,at the normal feeding time,the koi would not look at it but would eat their normal food,so I fished out the frozen offering and took it to the small pond where the mosquito fish who normally eat frozen blocks of blood worm consumed the frozen duckweed in seconds but would not touch the melted weed that floated just at the side.

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