He called and said “I feel a need to get out and do some manual labor. Can I come up and clean your pond?”
How could we say no?
My husband, Robert, has been complaining about our pond. It is small, shallow, and lately, covered by a bright green film of water plants. Duckweed, I thought.
Robert hates “duckweed”, but I rather like it. After all, it is green (my favorite color), and it’s photosynthesizing. Photosynthesis is a good thing. It means the plant is making food and oxygen, lots of it!
But that is a naturalist’s viewpoint.
Our friend Guy, my husband, and probably just about everyone else considers a bright green layer of duckweed as “pond scum.”
So we encouraged Guy to put on his hip waders and take a pool skimmer and go for it.
After a couple of hours he had pulled out all the cattails and skimmed off a portion of the scum. Then he mentioned something odd. He said the scum was grainy, rather like cornmeal.
Cornmeal? I had never noticed that about duckweed, so my interest was piqued. I gathered a handful of the stuff and sure enough, it did feel grainy. The leaves were smaller, too, than duckweed’s leaves.
I put a sample of the stuff on my binocular microscope.
A microscope of this type is a wondrous device. It magnifies the object and gives you a 3-dimensional view.
Under the microscope these diminutive plants were glorious. (Well, almost anything is glorious under a microscope!)
Each plant was a tiny leaf. No stems, no roots.
The leaf was only about 1/25th of an inch long. And it was not really flat. If I turned the leaf over I could see its shape. It looked like Noah’s Ark to me.
The hollow bottom of the “ark” held a bubble of air (actually, it was probably oxygen). This bubble keeps the plant afloat and upright. The flattened top “deck” of the ark contained cells full of bright green chloroplasts. On the center of the deck, right where Noah’s cabin would have been, was a tiny, pointed bump.
After an afternoon with my plant books, I concluded that this tiny aquatic plant was Water-meal (Wolffia papulifera).
I discovered that Water-meal is the world’s smallest seed-bearing plant! It was not algae, a moss, or a fern; these plants don’t have seeds. A seed plant has flowers, followed by a fruit with seeds inside it. Where could tiny Water-meal have put a flower?
I read on.
Water-meal bears its flowers, two of them per leaf, from that little bump on the deck. There is one male flower with a single stamen in it and one female flower with a single pistil. No petals, no sepals, nothing colorful at all. They are as basic as flowers can be!
I went back to the microscope to see if I could find one of the Water-meal plants in bloom. And I did find one. The leaf held two little tufts on the top. As I touched them with the needle probe, the tiny white pollen grains floated off. I noticed that many of the surrounding plants had a depression in the top (where the flowers had been). In that hole was a light green ball. I assumed that this was the fruit. It contained the one seed that a Water-meal flower produces. It looked like a miniature pea in a tiny boat!
Now, knowing all this about Water-meal, why would anyone want to get rid of it? It’s a cool plant! But of course, it’s a weed to many folks. Unsightly pond scum, right?
There are poisons that can be used to kill Water-meal but they are expensive. Also, you should keep your kids and pets away from the pond for at least 24 hours. What about the wildlife that uses the pond?
Another possibility is to stock your pond with something that eats the unwanted vegetation. Grass carp are an option, but a permit to keep this exotic fish species is needed.
Currently, another exotic fish, the Mozambique Tilapia, can be introduced into private ponds for personal use, without a special permit.
This species of tilapia is tropical and can’t survive water temperatures below 55 degrees Fahrenheit. This means that you’d probably have to restock every spring, but it also insures that the tilapia are unlikely to become invasive in the wild.
However, the fisheries biologists seem to be of two minds about whether the tilapia will control the weeds. A few think the tilapia won’t eat anything but algae (and maybe duckweed). Some say the fish might eat Water-meal, but not enough of it to really help. Still, it seems worth a try. At least, you can eat the Mozambique Tilapia.
What can you do about the Water-meal? Well, maybe you have a friend like Guy.