Watermeal Duckweed– A Fascinating Problem

This post is about a problem that we faced starting in 2013. This problem damaged the view in our back yard and it even killed some fish in our pond. The name of this problem is Wolffia (otherwise known as Watermeal, a form of duckweed).  No this is not a disease or a wild canine, it is a plant. And after studying this plant I am fascinated, this plant grows out of control on my property and at an alarming rate.

I calculate that if conditions are right on my pond (and they usually are), it can reproduce over 100 pounds of itself in one day.

In 2013 this watermeal plant invaded and quickly covered the pond surface by the end of June. It dies off in the fall but every year by the start of June the pond is typically covered. In some years near the end of July dozens of my large fish die from oxygen depletion in the water in the early morning hours.

This watermeal flourishing on the surface of the water chokes out sunlight at the bottom of the pond. This lack of sunlight underneath hinders and kills underwater plants which provide oxygen to the water.

This plant is a category of duckweed called Wolffia. Below are interesting facts about duckweed breeds:

• There are five breeds of duckweed.

• Wolfia is the smallest plant on earth, the entire plant is only 1 to 1.5mm.

Interesting Facts

• This plant can double in population in 4 days given the right water and wind conditions. Meaning hundreds of pounds of vegetation growth per day on my 2/3 acre pond.
• Watermeal and most duckweed’s thrive in stagnant, warm water with high nutrients.
• This plant has high starch levels which gives it potential for making ethanol.
• Some Asian countries and increasing number of feed produces farm these plants to feed to chickens, sheep, goats, or cattle. Its a high protein feed supplement that is increasingly seen as useful for animal life.
• Some people around the world actually eat duckweed plants.

• Rutgers University in New Jersey has a duckweed stock cooperative  (RDSC) and does research on these interesting plants.
• The RDSC collaborates closely with a second duckweed research project at Rutgers, an international effort led by the Waksman Institute of Microbiology. Under the auspices of the US Department of Energy’s Joint Genome Initiative, to sequence the genome (~150 Mb) of the Greater Duckweed (Spirodela polyrhiza).
• The RDSC was established in 2009 with the acquisition of strains from the Biolex Corporation in NC. In addition to those collected by Todd Michael (then an assistant professor at Rutgers) and later from Professor Klaus Appenroth at the Friedrich-Schiller-Universität in Germany. As of October 2013, the RDSC has over 900 different strains, collected from all continents except Antarctica, and it continues to add to its collection.

Potential and Current Applications of Duckweeds

Several properties of duckweed make it highly attractive for environmental and agricultural applications:

• Prolific growth on municipal sewage and agricultural runoff (low-cost remediation of wastewater).
• Floats on water surface (easily harvested as opposed to algae).
• High protein content in some strains (a low-cost source of animal feed).
• High starch content in some strains (a low-cost feedstock for biofuels and bioplastics).
• Low lignin content (easy-to-process soft tissue).

One reason why it’s such an efficient plant is that it doesn’t expend energy to grow a stem structure–it just floats on the water soaking up sunshine.

• Three small rootlets gather nutrients from the water, and three small leaves convert that into more duckweed. The roots don’t even have to gather water since the leaves lay flat on the water and absorb water directly.

I’m also guessing that this enables duckweed to take up carbon dioxide at an accelerated rate. Since the solubility of carbon dioxide in water is seventy times greater than its solubility in air.

Our pond water is particularly high in nutrients. This is due to four large willow trees which dump leaves and sticks into the pond almost year round.

There are many varieties of duckweed, some with leaves half an inch across, but the variety growing on my pond is the smallest variety.

Research indicates that the smaller varieties of duckweed are more palatable to fish and humans.

This problem has also been a learning opportunity.  This is a fascinating plant and I hope to take advantage of it.

This article is not on a typical topic for me but it caught my interest so I thought I would share.

How to Clear a Green Pond – 2020 update

So its the summer of 2020 and I decided I cannot take it anymore. The look and smell of the pond is just dreadful this time of year. We want to be free of this duckweed. This summer I took action with what I am convinced is the root cause of this duckweed problem. The constant dumping of vegetation into the pond by two massive willow trees.

So I hired a tree contractor and they downed them with massive 3 foot chainsaws. Since it has been a terrible drought in our area the ground is hard and the trucks were able to get back into our yard for the first time in years. Both trees have been down for two weeks so now we can start to make progress on the pond cleaning.

I added eight diploid grass carp and I also added Macro-Zyme bacteria, which I purchased from a pond supply store not far fom me. The carp were to help consume the duckweed and other plantsand the bacteria is to accelerate the decomposition of the vegetation from our local pond store.

The combination of grass carp eating voraciously and pooping and the increase decay from the bacteria is causing oxygen deprivation. Which was not unexpected. But I thought the fountan and aerator would keep the fish alive. We have lost probably 75 -100 bluegill and three bass in the past two weeks. Its been a difficult to watch, another reason to resolve this pond problem. We are trying to skim the dead fish out before they rot further. Been burying them in my garden and my garden compost for fertilizer.

The carp are seen most mornings eating the duckweed. They have doubled in size over the past 6 weeks, it is amazing to see how fast they have grown.

Fish Pond Aerator

The last aerator I made was just an air pump with a hose, very large bubbles. The idea of aeration is to maximize the surface area between air and water, to get more oxygen into teh water. Translation… you want many tiny bubbles.

I went to the pond store and they wanted over $700 for an aeroator, that is not going to work for us this year so, I had an idea for building a better aerator. I had recently purchased porous hose that leaks water along the length for a slow water of my vegetable garden plants. I figured the hose could do this with air as well. So I cut 12 feet of it and secured some adapters and fixtures to connect to my air compressor hose. I also wrapped the hose in metal fencing and keep it on the bottom of my 5-foot pond with a brick. Tiny bubbles, it works great.

I have been running the fountain and/or aerator pump 24/7. Just to add oxygen and speed up decay. We also plan to hire a backhoe contractor to clean out the bottom of the pond and remove some of the rotting vegetation and to restore the pond walls.

There is at least two feet of black rotting willow leaves and branches around the perimeter of the pond. This muck is full of leaches and other unspeakable bugs. The pond is also full of snapping and red-eared turtles. Its actually a very healthy pond in some respects, and I expect things to turn around quickly now that weve resolved the willow issue.

Our hope is that in the summer of 2021 we can swim in the pond off the dock.

Stay tuned for updates as this pond problem resolves over time.