Pond's surface looking green? It might be covered with duckweed, not algae

It is a common mistake for folks to think shallow ponds are green because algae is covering the surface of the water.

While algae certainly is present, not all green plants near the surface are algae. I'd like to draw your attention to duckweed, an aquatic plant often confused with algae.

Known as one of the world's smallest plants, there are nine species of duckweed to be found across the continent.

How small is this perennial herb? What look like tiny leaves, more accurately called fronds, are only 1.5 to 10 millimeters long, depending on the species.

Some duckweed species have no tiny rootlets, while others have more. Based on the number of rootlets we have on our duckweed at Stillman Nature Center, it is greater duckweed, a classic oxymoron if there ever was one.

While it may be the world's smallest flowering plant, it rarely uses its microscopic flowers to produce seeds for future duckweeds. Typically, duckweed reproduces asexually by forming chains of new stems from vegetative buds. Fair enough.

A frog amid duckweed. Courtesy of Lara Sviatko

So, we have a photosynthetic carpet of duckweed expanding to fill the water surface that it is floating on. Let's think about what this might mean.

Algae also needs sunlight to live. Some argue that the shade caused by a blanket of duckweed will actually reduce the algae in a pond by blocking its access to light. Perhaps.

What about insects that require water to procreate, such as the omnipresent mosquito? It's hard for a female mosquito to lay her eggs in the required water if the best she can do is crawl across an uninterrupted coating of duckweed plants.

It would be especially difficult should the mosquito encounter a hungry spider that was scooting across the duckweed looking for lunch.

Aptly named

Of course there are animals that eat a few holes in this duckweed carpet, not the least of which are ducks. Mallards, blue-winged teal, green-winged teal, wood ducks, and Canada geese all frequent the duckweed diner. To this list you can add coots, rails, muskrats and a couple of insects such as the duckweed weevil.

Not surprisingly, some fish species thrive on a diet of protein-rich duckweed. This includes ornamental species of fish such as goldfish and Koi, which you might have seen in water garden pools.

Since water gardening is something of an industry, there is a lot of information available on the nutritional value of duckweed. For example, duckweed provides essential vitamins and minerals for fish.

In particular, duckweed has high concentrations of lysine and methionine, critical amino acids. In addition, this tiny plant contains important trace elements and proteins such as beta carotene and xanthophyll.

Mallard ducks feeding on duckweed. Courtesy of Leah Kmiecik

Filter and fuel

Any plant this chock full of nutrients must be good at absorbing them, and duckweed certainly is. Duckweed thrives on typical fertilizer components such as nitrogen and phosphorous. Given the right conditions, duckweed can double its mass in one to two days. So duckweed could be the world's fastest growing plant as well as its smallest.

Keep in mind that one person's fertilizer is another person's sewage. Duckweed also grabs hold of heavy metals. Can you see where I'm going with this?

The fancy college word is bioremediation. In other words, species of duckweed are being used, around the world, to filter out pollutants from water reservoirs and other waterways. These hardworking tiny plants restore the quality of the water they are floating on top of.

But if you need more fresh duckweed to stay on the job, what do you do with the duckweed that is satiated with fertilizer, I mean, nutrients? Well, you have a few choices.

First, put it down in an area that needs to be fertilized. Why use synthetic fertilizer; stick with the organic stuff.

Second, feed it to domestic livestock. In addition to fish and ducks, duckweed can be fed to cattle, sheep, swine, goats, rabbits, and chickens.

Third, serve yourself. In some Asian cultures, duckweed is used as human food. Overseas, an alternative name for duckweed is water lentils. By September, we could make a giant vat of "lentil soup" with the pond water here at Stillman Nature Center.

Fourth, keep some in the medicine cabinet. OK, not actual duckweed - that could get messy. In fact, researchers are looking at medical uses of genetically modified duckweed. As plants, duckweeds are immune to animal viruses. Duckweed may even be used to synthesize insulin.

Fifth, use it as an alternative energy source. Duckweed can be utilized to produce ethanol, butanol, and biogas. Talk about renewable energy!

Mallard ducks feeding on - what else?- duckweed. Courtesy of Leah Kmiecik

Turion tour

Since starting the research for this column, I was plagued by the idea that duckweed was a perennial, a plant that lives for many years. How can a plant that shares a space with ice every year be a perennial?

While a terrestrial perennial stays put, a floating aquatic one can move around. Duckweed moves up and down with the help of a turion, a specialized vegetative bud. As autumn takes hold, the turion breaks off the parental stem and sinks to the bottom of the pond, where it quietly spends the winter.

In spring, the starch-filled turion starts to swiftly metabolize. In the process, it floats up to the surface. There the turion grows back into a regular duckweed, genetically identical to its floating precursor.

Not always algae

When visiting Stillman's pond or another nearby shallow waterhole this summer, you'll probably spend some time on a dock looking for the pond's stars; frogs and turtles. With some luck, you might see a turtle's head sticking out of the water or a pair of frog eyes, draped in duckweed, looking up.

As you enjoy these spottings, please remember, not all aquatic plants are algae!

• Mark Spreyer is the executive director of the Stillman Nature Center in Barrington. Email him at

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