Lemna reproduces mainly by budding and is spread easily by wind, water, birds, and animals. Since it can double in biomass every day and is transported by clinging to waterbirds, it is a wonder that every body of water everywhere is not covered by this green carpet.
But it is picky. It is a calm, nutrient-rich water specialist. Thus it thrives in the C&O Canal and farm ponds but is rarely seen on whitewater streams. We might carry it to new bodies of water on our paddling gear, but unless the next stop is calm, sunny, and nutrient rich, we are unlikely to start a new population. The birds were probably there first anyway. Because it floats, it is easily moved by wind across a water surface. On a stretch of the towpath where I walk, it is not uncommon to see a level of the canal that looks like a putting green one day, then open water the next after a windy afternoon.
What are the risks? Not many, actually. In very dense populations, it causes swings in dissolved gases in the water column, releasing O2 in the daytime and CO2 at night. The plants that die during the season will decay underwater and reduce dissolved oxygen levels. That anaerobic decay of plant matter on the bottom is the source of the methane bubbles you can see surfacing through the ice on the canal in the winter. A dense carpet of duckweed foliage can interfere with gas exchange at the water's surface, which confounds wildlife. These are problems created largely by excessive nutrient runoff, not by a particular plant, so Lemna is not a villain, it is just responding to conditions we create. Don't overfertilize your lawn.
The benefits might outweigh the risks. Because it grows aggressively, Lemna effectively pulls excess nutrients from the water below. The plant itself is high in protein; it's good animal forage and the high levels of N, P, and K make it a good green manure. Take some home for your compost pile. Work it in around your tomatoes. Try it in a smoothie.
The abundant protein and biomass are also food for a lot of aquatic wildlife. The plant is not harmful to humans or to our pets. As it covers the surface of the water, Lemna reduces light levels for other aquatic plants. It suppresses the stringy algae that is the bane of pond owners. Some of the blue green algae that it suppresses are toxic to mammals—there were warnings in the news during Summer 2024, so duckweed can be an ally. Dense carpets of Lemna can shade out the submerged aquatic vegetation that has made paddling up the canal a misery in past years. Would you rather paddle through floating duckweed or well-rooted Hydrilla?
My best personal experience with duckweed happened when I was a wee nipper, and the family was staying at a cute country inn with a duckpond. I ran down to greet the ducks, thought nothing of the big, flat lawn-like area in the foreground, and splatted right into a gloppy pond covered with an emerald layer of duckweed. It turned out the Lemna was prolific due to the abundant nutrient contributions of the ducks and geese that washed off the lawn back into the pond every rainstorm. It was a lesson in eutrophication. Type II fun. I don't remember what I was wearing, but I don't think I ever saw that outfit again.