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Common Myths about Duckweed

September 14, 2012

I’m copying text right out of articles I am finding on the Internet about duckweed “invasions.”  Here’s the real scoop:

1. Duckweed is a problem because large growths of it can use up the oxygen in the water, and block the sunlight.    Fact: Living duckweed takes CO2 from the air and some from the water column. It produces oxygen. It doesn’t use it up. When it dies, it does use oxygen in the process of bacterial decomposition. Maybe that is what the writer is referring to. A solid covering of duckweed does block sunlight which inhibits algae growth. Algae produce oxygen and a sudden loss of algae means that fish will become stressed and potentially die off. Still, for the record, as a living plant, duckweed does not use up the oxygen in the water

2. Boaters can help reduce the growth of duckweed by using environmentally friendly cleaning products, such as Ecover, which do not contain the nutrients that the weed feeds on.  Fact: the grey water itself is an issue as well. What food scraps or human waste is in the grey water, above and beyond cleaning supplies? Duckweed can thrive on any additions of nutrients.

3. Of course, while duckweed may have its place in the laboratory or in the feeding trough, it doesn’t belong in your small pond. (an advertisement for a herbicide) Fact: Nature thinks it belongs there and Nature is rarely, if ever wrong. Would you rather have a  pond that is clear and loaded with chemicals or a pond covered with duckweed that remediates the water and ready to offer you great composting material, animal feed, etc.?

4. We’re closely monitoring the weed and it should disperse itself naturally. This isn’t a myth in and of itself, but it cracked me up as it speaks to humans thinking that they are in charge of a natural process.

5. Duckweed plants can aggressively invade ponds and bodies of water often disrupting marine ecosystems, according to Texas A&M University.  Fact: Duckweed dies in marine ecosystems due to the salt content. (marine equals oceans, estuaries, etc…)

6. Using duckweed for water cleansing is certainly a low tech and time-consuming process. In addition, duckweed cannot get rid of pesticides, heavy metals or toxic substances. Fact: Duckweed is a low tech process but that is a positive rather than a negative for developing countries.  It does take 9-28 days for a duckweed wastewater remediation process, but the resulting water quality is MUCH better than the current effluent being dumped in our creeks and rivers. Duckweed remediation of wastewater doesn’t use chemicals and has a quality end product of biomass with which to use as a raw ingredient in over 70 broad applications. Duckweed is affected by pesticides, but it does a superb job pulling heavy metals and toxins out of a water column with a process known as “water scrubbing.”

Myths abound, but it’s getting better out there in media land. Thanks for reading. Next time someone tells you a “fact” about duckweed that is wrong, feel free to speak up and explain the real facts.

19 Comments leave one →
  1. December 6, 2014 10:53 pm

    How long can duckweed survive without water I removed all visible duckweed from my shrimp tank but I don’t know how long till it’s safe to drop my guard on it

    Like

    • January 31, 2015 11:46 am

      If you put it out to dry in the sun in a 1 inch thick layer, turning once or twice, it’ll take roughly 2-3 days to dry out enough to where it won’t survive.

      Like

  2. November 7, 2014 11:49 am

    This is a great website and you have found many great uses for duckweed, but I wanted to clear up a few issues that you say are myths. I’m not doing this to be contrary, but rather to bring out the truth.

    1. Duckweed is a problem because large growths of it can use up the oxygen in the water, and block the sunlight.

    This is absolutely true. You are correct that duckweed absorbs CO2 and releases oxygen during photosynthesis. Photosynthesis is what plants use to make sugars and other molecules that store energy, but to use that energy for growth, duckweed (and all other plants) must respire. By respire, I am referring to using oxygen to break down sugars and other molecules for respiration, to respiration in the context of using lungs etc. to breathe. Under normal conditions, in the daylight the net release of O2 from photosynthesis is greater than the release of CO2 from respiration, so the duckweed is not going to sue up the O2 in the pond. However, at night photosynthesis stops (because it requires light), but plant growth and tissue repair, which both require respiration to release energy, can continue. If there are enough plants that are actively growing the levels of O2 in the water can drop drastically, sometimes even to levels that are harmful to other life (This is well documented in aquarium literature if you want a more detailed explantation). I have worked as an aquatic ecologist for years and often we see the creates fluctuation in O2 levels in systems that are heavily vegetated. In addition, duckweed floats on the surface, so it can get it’s CO2 and O2 from the air, but hat the same time slows gases such as CO2 and O2 from dissolving into or out of the water. So, duckweed using up oxygen in the water is not a myth. Additionally, that combined with the mat blocking out sunlight can result in the death of submerged vegetation which can have cascading effects on the rest of pond life. If the sole person of your pond is goring duckweed for use as food, green mulch, etc., none of this will be a problem, but I just wanted your readers to be aware of the issue in case their pond is multi use (e.g. they have fish and duckweed).

    2. Boaters can help reduce the growth of duckweed by using environmentally friendly cleaning products, such as Ecover, which do not contain the nutrients that the weed feeds on.

    You made a good point about grey water here, but I still think it is good to encourage the use of ecologically friendly cleaners as well. Both grey water and cleaners can introduce nutrients that can throw an ecosystem out of balance, but in aquatic systems an increase in the limiting nutrient is what leads to algal and plant blooms. In freshwater systems, the limiting nutrient is usually phosphorous (sometimes nitrogen) and many soaps and cleaners have phosphorous. So, it is best to use eco-friendly cleaners which have reduced phosphorous. I do agree though, that grey water is still a problem.

    3. Of course, while duckweed may have its place in the laboratory or in the feeding trough, it doesn’t belong in your small pond.

    I agree with you here. Duckweed is naturally occurring and we shouldn’t fight it (unless it is taking over a water to the detriment of other plant and animal species, but that is usually a result of humans adding nutrients and disturbing natures delicate balance).

    4. We’re closely monitoring the weed and it should disperse itself naturally.

    Unfortunately, governments spend a lot of money monitoring and trying to control weeds. Duckweed has many great uses but it can be bed in certain systems, so I would encourage your readers not to introduce it to any natural systems that it doesn’t already occur in.

    5. Duckweed plants can aggressively invade ponds and bodies of water often disrupting marine ecosystems, according to Texas A&M University.

    Duckweed can invade ponds and bodies of water. It usually occurs when the ponds are receiving excessive nutrient runoff, and the duckweed can absorb nutrients, but if the duckweed persist, it can be a problem. Furthermore, duckweed can affect marine ecosystems. In fact, you alluded to it yourself, “Fact: Duckweed dies in marine ecosystems due to the salt content. (marine equals oceans, estuaries, etc…).” In tidal systems, duckweed that is flushed out to coastal and estuarine areas does die, but massive amounts of decomposing duckweed can decrease water quality. Most of America’s rivers drain into coastal ecosystems, so anything that affects the watershed will eventually affect the marine ecosystem.

    6. Using duckweed for water cleansing is certainly a low tech and time-consuming process. In addition, duckweed cannot get rid of pesticides, heavy metals or toxic substances.

    I agree with you here. Low-tech solutions are great and you have done a lot of work showing the many uses of duckweed. Keep up the good work. The only problem I can see with using duckweed to clean up excessive nutrients is (pardon my repetitiveness, but there is a common theme here) when they utilize a lot of nutrients in systems that are naturally low nutrient systems, because even though the duckweed are removing nutrients, they also harm other native plants. This is especially true in clearwater systems with lots of submerged vegetation. If the duckweed, which grows much faster, takes over the surface of the water, the submerged vegetation don’t get light and die. Their death results in more decomposition and increased CO2 levels. In support of duckweed I will re-emphasize that duckweed does not take over like this if people take care of waterways in the first place. What would be great is if cities, counties etc. diverted water into ponds full of duckweed and then allowed it to percolate through the soil and into rivers. That way the duckweed could filter the nutrients out before the water entered back into natural systems, and there wouldn’t be as much of a risk of duckweed taking over the natural systems.

    Anyway, I think you are doing a great job with your work, but some of what your said in this post was a little misleading. I, like you, think duckweed is a great tool, but it can be a problem. Most of these statements seem like myths to you because they don’t really apply to your objectives of raising duckweed in ponds, but that does not make them myths. In fact, they are very real when managing duckweed in natural systems.

    Like

    • January 31, 2015 11:49 am

      Great comments from the perspective of wanting to keep waterways clear. Farming duckweed or wastewater remediation with it is my focus. I do not advocate adding it to natural ponds as it shifts the ecosystem balance. We’re on the same page. Would love to have a phone conversation with you. Thanks for your thoughts.

      Like

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  10. March 29, 2013 10:16 am

    All I know is I have been battling duckweed in my one acre pond for five years. It takes over the pond and makes what should be a beautiful sight from my deck an ugly one. It seems I will never rid myself of this pesty plant.

    Like

    • April 8, 2013 1:07 pm

      Wade, I WISH you were my next-door neighbor because I would take all your duckweed all summer long and be thankful for it. Apparently you’ve got some nutrient inputs that the duckweed are responding to, either from foliage or leaching waste nutrients from nearby fields, etc… Stop the inputs and let the existing duckweed remediate your pond. Pull off the duckweed and use in your garden as compost material. Keep doing this repeatedly all this summer. When your duckweed slows its growth due to lack of nutrients, harvest the heck out of it and then introduce some Koi or tripolid carp (depending on where you live and can get permission on the carp) They’ll eat up the rest of your duckweed, as long as the above path is followed. You’ll have a really clean pond, a beautiful garden (or happy chickens) and can rest assured that your pond is art. chemical-free.

      BTW: Beauty is in the eyes of the beholder. 🙂

      Like

  11. September 14, 2012 11:50 pm

    Right. Suitably and safely options include starting with a working knowledge of the heavy metals involved and their concentration in the duckweed biomass. Then three possible options are to compost it (depending on the existing heavy metal concentrations of the land where it will be applied) use for for bioenergy, or bioplastics.

    Like

  12. Susan Munroe permalink
    September 14, 2012 12:27 pm

    Two additional thoughts…

    Remember to keep in mind that all water and bog plants don’t take up all heavy metals.

    And if duckweed (or any other plant filter) is used for bioremediation of water containing heavy metals, it is no longer suitable as feed for animals (or humans), and harvested plants need to be disposed of suitably and safely.

    Sue

    Like

  13. Susan Munroe permalink
    September 14, 2012 12:00 pm

    Nice post, Tam! We are subjected to so much ‘truth’ that has been altered to make a sale of some product that a person really has to read carefully. And think.

    The link didn’t show up as shown, so I went to the main site and found it at http://dspace.nehu.ac.in/handle/1/2035 Apparently, you can’t link directly to the article itself, but only to the lead-in, and then click on View/Open in the gray box.

    Sue

    Like

  14. September 14, 2012 9:23 am

    Great post! I need to look more into the fact that duckweed can pull heavy metals and toxins from the water column. Seems like I previously read that it couldn’t do that.

    Like

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