Duckweed has many uses but brings many problems

That green stuff all over our still waters is probably not algae like most people think. It’s a floating flowering plant without stems called duckweed (Lemna sp.). Its growth rate can be phenomenal. Under good conditions duckweed can double its biomass in less than 24 hours. This means it can cover an acre of water in less than a month and a half.

As one might suspect this aquatic invader is eaten by many species of ducks. The problem

is that the birds don’t really control it. They spread it to other waters. These floating plants cover the surface and eliminate light to submerged aquatic plants, causing their demise.

Anything that dies, whether it is plant or animal, must decompose. This generally requires oxygen, which in aquatic environments is less available than in terrestrial ones. The result is a decrease in dissolved oxygen of the water and that causes problems for many aquatic animals including fish.

But duckweed has many redeeming qualities. It helps purify water by extracting nutrients from it. It has the potential to remove fertilizer nutrients from hog and other animal waste. When skimmed off lagoons and processed it has great feed value for fish and livestock. It’s high in protein and highly digestible.

It’s probably not a great human food source as high oxalate content could pose a problem for kidney stone formers. Small quantities are fine in survival situations as it’s not poisonous and is easily digestible. Bacterial contamination might concern me though. I don’t eat it.

Use of dried duckweed as a fertilizer has potential. It decomposes quickly and provides organic matter, nitrogen and other nutrients to the soil. Skimming it off the surface is a simple process, but removing all of it this way is impossible. Don’t expect to cure your duckweed problem simply by mechanical removal.

Duckweed research is ongoing. Its potential as a biofuel is well documented. Phenomenal growth rate make it better suited for the production of carbon-based fuels than any of our agricultural crops. Several sources claim gasoline produced from duckweed would be competitive with a $72-$100 per barrel crude oil price.

It tolerates a wide range of water conditions too. Duckweed can thrive at a pH range of 4.5 to 7.5. Fertilization is unnecessary. In fact, duckweed removes runoff fertilizer from our waterways, which cleans the water.

All this talk about livestock feed, fertilizer and biofuels mean nothing to you if you have a farm pond and don’t want it overrun with duckweed. There is hope. Several approved herbicides including Diquat will kill it. Proper pesticide certification is necessary, so if you don’t have it please hire a professional. He can also deal with algae, which could result from killing the duckweed.

One problem still remains. If duckweed wants to grow prolifically it means that the water has a high nutrient content. Duckweed may grow, but it doesn’t thrive in clean water. Fertilizer nutrients are coming from somewhere and eliminating the source is the only way to truly eliminate the problem.


Duckweed also often grows on my greenhouse floor.

Duckweed also often grows on my greenhouse floor.

Ted Manzer teaches agriculture at Northeastern High School.

About tedmanzer

I grew up in Old Town Maine and got a B.S. at the University of Maine in Plant Sciences/ minor in Botany. From there I moved to West Virginia and earned a M.S. in Agronomy at WVU. I also met my wife there. She grew up in rural WV as the daughter of tenant farmers who raised cattle and hogs. Their lifestyle at times was one of subsistence and I learned a lot from them. I've always been a foraging buff, but combining my formal botanical knowledge with their practical 'Foxfire-type' background opened up my eyes a little more. I recently retired from teaching high school agriculture after 25 years teaching with my wife. Until recently I wrote a weekly nature/foraging column for the local paper ( I also have written several Christian nature/adventure novels that can be purchased on Amazon in Kindle format. One is a five book family saga I call the 'Forgotten Virtues' series. In the first book, Never Alone (presently out of print), a young boy comes of age after his father dies in a plane crash, and he has to make it alone. The second book, Strange Courage, takes Carl from his High School graduation to his recovery from a nasty divorce. The third book, Second Chances, takes Carl from his ex-wife's death and the custody of his son to his heroic death at age 59. The fourth book, Promises Kept, depicts how his grandchildren react and adjust to his death (this one is not yet published). In the final book, Grandfather's Way, his youngest and most timid granddaughter emerges from the shadow of her overachieving family and accomplishes more in four months than most do in a lifetime. I use many foraging references with a lot of the plants I profile in these articles in those books. I also wrote a romance novel titled Virginia. It is available on Amazon and is a different type of romance from a man's perspective.
This entry was posted in foraging and tagged , , , , , , , , . Bookmark the permalink.

6 Responses to Duckweed has many uses but brings many problems

  1. nkereuwem Famous says:

    I have duckweed growing all over the stagnant water on my piece of land measuring 30 feet wide X 50 feet long.
    How can I benefit from it?
    What are my advantages having it?

    • tedmanzer says:

      Unless you have a way of harvesting it and using it as compost or forage, it’s probably more of a liability than an asset. It will, however, sequester nutrients that could otherwise make their way into waterways and reduce overall water quality.

    • Bardo says:

      This is the best piece I’ve read about duckweed on my initial internet searching about control nutrition uses composting growth rates, etc. I have a large nature reserve pond adjacent to my house which is overrun with duckweed brought over by ducks during covid lockdown. Annexe it conditions on making a pond smell really bad can you help?

      • tedmanzer says:

        There are several commercial formulations that can control it. Most contain the herbicide diquat. These are totally acceptable except in aquaculture situations. There is an underlying problem though. Duckweed is problematic when levels of fertilizer nutrients in the water are high, so this is also an area that should be addresses. You need to find out where the nitrogen and phosphorus are coming from and rectify that problem to keep it from becoming a longterm situation. Good luck.

  2. Nkereuwem Famous says:

    I have a large collection of duckweed in my 30 X 50m earthen pond which has African mud fish in it.
    What is the advantage and disadvantage of duckweed in the pond?

    • tedmanzer says:

      duckweed provides a food source for many aquatic invertebrates ad even some fish. However, it blocks light transmission leading to high BOD which depletes dissolved oxygen and causes the eventual demise of pond life.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s