Duckweed Invasion - Our Counter Attack!
Light green duckweed takes over! So in our last post (August 26, 2018), we described how our 25 year old, approximately one quarter acre ( 10112 square meters), landlocked pond, had suddenly been completely covered in an inch (2.5 cm) thick green carpet of Duckweed, within a matter of a couple days. Although it had somehow colonized the pond about 6- 7 years after we built it, we never knew how it arrived (the water source is a screened underground spring). The folk lore is that it can arrive clinging to migrating ducks from other waters. This is readily believable as it will cling to your body as you exit the water.
In any event, it never did more than lurk in the recesses of the shore weeds and was never an issue for our swimming or the wildlife the pond attracts. Until this year, when it suddenly grew in a matter of days to cover the entire surface of the pond in a thick green carpet.
We are not sure why this happened this year - whether it was the weather, or new environmental factors we are unaware of, but we could no longer do our daily swims and were concerned about the effects on our wildlife. The bald eagles, blue herons, barn swallows and others, can’t access the fish and insects on the open water surface to feed, and our small mouth bass wouldn’t be able to feed on the surface insects.
In any event, it never did more than lurk in the recesses of the shore weeds and was never an issue for our swimming or the wildlife the pond attracts. Until this year, when it suddenly grew in a matter of days to cover the entire surface of the pond in a thick green carpet.
We are not sure why this happened this year - whether it was the weather, or new environmental factors we are unaware of, but we could no longer do our daily swims and were concerned about the effects on our wildlife. The bald eagles, blue herons, barn swallows and others, can’t access the fish and insects on the open water surface to feed, and our small mouth bass wouldn’t be able to feed on the surface insects.
Duckweed are tiny individual plants Duckweed is not a harmful plant, and in fact can have many positive effects, like improving water quality, shading out algae growth and absorbing environmental toxins. But most people find it an unsightly nuisance. For us, it is more than a nuisance. So we determined we had to control it.
I wasn’t willing to go for the herbicide solution for our pond that we swim in.
So I went to our garden center to ask for advice. You can buy koi fish that eat the duckweed. You need quite a few fish to make the duckweed disappear! Or you can buy chemicals. The disadvantage with them is obvious. In many states - like New York, you need a license to buy and apply it, as it is a poison. You kill the plant, it sinks to the pond floor and decomposes. This robs the oxygen from the pond, which would be bad for the fish. And since we swim in the water and the pond has a slow recharge rate, we were concerned about how long the poison would linger and get into our eyes, mouths and skin.
I wasn’t willing to go for the herbicide solution for our pond that we swim in.
So I went to our garden center to ask for advice. You can buy koi fish that eat the duckweed. You need quite a few fish to make the duckweed disappear! Or you can buy chemicals. The disadvantage with them is obvious. In many states - like New York, you need a license to buy and apply it, as it is a poison. You kill the plant, it sinks to the pond floor and decomposes. This robs the oxygen from the pond, which would be bad for the fish. And since we swim in the water and the pond has a slow recharge rate, we were concerned about how long the poison would linger and get into our eyes, mouths and skin.
individual duck weeds are in millimeters! So perhaps we need a mechanical solution?
This is more easily said than done! My research revealed that doing this is very difficult. People come up with all manner of creative solutions to try to remove the plant. These range from ropes with various barbs attached and dragged across the surface, vacuum contraptions, skimmers, nets, all sorts of ideas. None of which seem to work very well.
There was only one course of action which seemed doable for us, and by harnessing physics and the plant’s anatomy, we felt we could be reasonably successful. We would create a skimmer box and use a trash pump to suck out the duckweed.
Duckweed loosely floats on the surface of the water, and is comprised of thousands of tiny 1-3 leafed plants, which have tendrils that hang down about an inch under the surface. When they become a thickly packed carpet, they tend to cling together, and if you gently sweep your hand across the surface, you’ll see all the surrounding duckweed follow along behind your hand en-mass.
This makes them easy to corral, as they seem to want to follow along as you draw in their brothers and sisters. You can use this to great effect!
This is more easily said than done! My research revealed that doing this is very difficult. People come up with all manner of creative solutions to try to remove the plant. These range from ropes with various barbs attached and dragged across the surface, vacuum contraptions, skimmers, nets, all sorts of ideas. None of which seem to work very well.
There was only one course of action which seemed doable for us, and by harnessing physics and the plant’s anatomy, we felt we could be reasonably successful. We would create a skimmer box and use a trash pump to suck out the duckweed.
Duckweed loosely floats on the surface of the water, and is comprised of thousands of tiny 1-3 leafed plants, which have tendrils that hang down about an inch under the surface. When they become a thickly packed carpet, they tend to cling together, and if you gently sweep your hand across the surface, you’ll see all the surrounding duckweed follow along behind your hand en-mass.
This makes them easy to corral, as they seem to want to follow along as you draw in their brothers and sisters. You can use this to great effect!
But physics could play a role here as well! Even as the plants thin out and become free standing clumps, the natural surface tension of the water, could act as a conveyor belt to continuously carry the now free-floating groups along. Water molecules (H2O - 2 hydrogen and 1 oxygen) have a slight negative charge and therefore create a surface tension that holds the neighboring molecules together. The surface of the water becomes a cohesive film. This is what allows aquatic insects to dance along the surface without sinking and traps other insects on the surface that fish and birds feed upon.
The principle is the same as if you left your book, reading glasses and TV remote scattered across your bed one morning, and then pulled the top linen sheet down onto the floor along one side. They would all be carried down onto the floor, although they weren't touching! A properly placed skimmer box will have the same effect from all four sides. Like the linen sheet, that holds objects on its surface, and will drag them along when pulled, so will surface tension. It will be the conveyor belt that pulls along everything.
The principle is the same as if you left your book, reading glasses and TV remote scattered across your bed one morning, and then pulled the top linen sheet down onto the floor along one side. They would all be carried down onto the floor, although they weren't touching! A properly placed skimmer box will have the same effect from all four sides. Like the linen sheet, that holds objects on its surface, and will drag them along when pulled, so will surface tension. It will be the conveyor belt that pulls along everything.
The set up:
Re-purposed plastic storage bin You’ll need to create an appropriately sized skimmer box and an attachment point for a water (trash) pump and the volume of water it will move; as well as a place to catch the discharging duckweed-filled water. If the discharging rate of the pump is greater than the recharge (replacement) rate of the water in the pond, then you’ll need to set it up so that the duckweed is being filtered out of the discharging water, and that the cleaned water flows back into the pond.
For us, this was easily accomplished by discharging the water onto an uphill grassy slope and letting the lawn catch the duckweed as the water flowed back downhill to the pond. Discharge the strongly pressurized water across a tarpaulin to spread it out and avoid erosion.
We used an old plastic 30 gallon (120 litre) storage container. Use one that has some contours or decorative ridges in the walls. This will increase rigidity and prevent ‘oil-canning’ (collapsing walls from the surrounding water pressure). Cut out a 2 ¾ inch or larger hole for attaching the intake hose fittings.
For us, this was easily accomplished by discharging the water onto an uphill grassy slope and letting the lawn catch the duckweed as the water flowed back downhill to the pond. Discharge the strongly pressurized water across a tarpaulin to spread it out and avoid erosion.
We used an old plastic 30 gallon (120 litre) storage container. Use one that has some contours or decorative ridges in the walls. This will increase rigidity and prevent ‘oil-canning’ (collapsing walls from the surrounding water pressure). Cut out a 2 ¾ inch or larger hole for attaching the intake hose fittings.
Strainer Basket We used a 2 inch (the size of the intake hose) trash pump, capable of handling up to 9/16 inch solids. Commonly used in disasters for pumping out homes, they are built to survive ingesting solids of certain sizes. You can rent them. The intake hose comes with an attached strainer basket, but you will have to remove it to attach it to the skimmer box.
Threaded lug pin coupler- bulkhead fitting We bought the appropriate fittings at a Tractor Supply. We bought a threaded bulkhead coupler and installed it in one wall of the skimmer box. Instead of the strainer basket, we installed a threaded lug pin coupler on the intake hose. This allowed us to pre-position the skimmer box correctly before attaching the intake hose. The intake hose is very rigid, so it is difficult to situate the skimmer box properly and level, while it is attached. It is easier to first secure the skimmer box position, then the pump location, and then attach them together.
The skimmer box has to be perfectly level, with the top edges about one inch below the surface of the water for it to work efficiently. When the pump starts to evacuate the water, the box will want to float up. So you have to first weight it down inside with some heavy things (we used tow chains, but anything that doesn’t block the intake will do. (***Note: we did not reattach the strainer basket, as the pump size we were using would easily handle just the clumps of duckweed). Without a strainer, you have to make sure that the pump will not ingest anything larger than it was designed to handle - or create a screen inside the box.
The skimmer box has to be perfectly level, with the top edges about one inch below the surface of the water for it to work efficiently. When the pump starts to evacuate the water, the box will want to float up. So you have to first weight it down inside with some heavy things (we used tow chains, but anything that doesn’t block the intake will do. (***Note: we did not reattach the strainer basket, as the pump size we were using would easily handle just the clumps of duckweed). Without a strainer, you have to make sure that the pump will not ingest anything larger than it was designed to handle - or create a screen inside the box.
We put the trash pump near the water’s edge. The output hose we put uphill on a tarpaulin and started the pump. Here is the video of what happened:
The duckweed was sucked in and came out in a forceful stream of water. The water rushed back to the pond, but the duckweed was caught by the grass and weeds around the pond. We had mountains of duckweed after many hours, and the clear water surface appeared in more and more areas, as the carpet was slowly drawn in from every direction. As you can see in this video, the 'surface water tension effect' described above, continued to draw along even clumps of duckweed that were not touching any other clumps. The 'surface water tension effect' was acting as a multi-directional conveyor belt, drawing in even the disconnected clumps of duckweed, eventually down to the bottom of the box, to be expelled out onto our lawn.
We pumped for more than eight hours, but in the end, we had cleared over 95 percent of the duckweed. We wanted to pick up the duckweed right away, but left it for the next day, figuring that after it dried out we could mow over it and turn it into lawn food. But the next morning I saw deer at our duckweed piles munching away. They really liked it, eventually eating most of it.
I had read in my research that the duckweed is high in protein and is grown commercially for live stock. They also mentioned that duckweed seems to be a delicacy in some Asian countries.
Now a couple of weeks later, there is still some duckweed, but it is not green anymore; it is kind of yellow and each day I see less of it.
This year, on a flight over the East coast of the U.S. you can see most ponds covered with the duckweed. I guess this summer everything was right for this delicate water plant to conquer all freshwater bodies!
I had read in my research that the duckweed is high in protein and is grown commercially for live stock. They also mentioned that duckweed seems to be a delicacy in some Asian countries.
Now a couple of weeks later, there is still some duckweed, but it is not green anymore; it is kind of yellow and each day I see less of it.
This year, on a flight over the East coast of the U.S. you can see most ponds covered with the duckweed. I guess this summer everything was right for this delicate water plant to conquer all freshwater bodies!
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