When the rainy season ends and you lift the lid on the rain barrel in the corner of your garden, you may find the water gone green in just a few days. Look closer and you'll see duckweed: leaves smaller than a fingernail, floating in clusters of two or three. You reach in to change the water, then stop. Once you know what these tiny leaves are doing down there, dumping the barrel starts to feel like a waste.
It Pulls Pollutants Out of the Water as It Floats
Duckweed is a simple plant. It barely has separate leaves and stems. Its short roots hang in the water, and the underside of each leaf sits against the water all day long. With the right temperature and nutrients, a colony can double in as little as two days. That's a lot of surface in contact with the water, and a lot of fast growth. So duckweed quickly takes up dissolved nitrogen and phosphorus, and it also stores heavy metals like lead and cadmium, along with pesticide residues, in its tissue.
A quick look at the state of the world's water shows why scientists got interested. Roughly 80% of the world's wastewater flows into rivers and oceans untreated. Even water that passes through treatment plants often still carries newer pollutants like pharmaceutical residues and microplastics. Lead and mercury, once they get in, can linger in water for decades. That's why using plants to clean up pollution has drawn attention as a cheaper, lower-maintenance option, and duckweed stands out as one of the easiest plants to work with.
The Leaves Don't Do It Alone
A 2026 review in the journal Biology concludes that duckweed's cleaning power comes from two partners working together: the plant's own detox system and the microbes living around its roots. On the plant's side, it makes metal-binding proteins that lock heavy metals away inside its cells. Meanwhile, bacteria clinging to the roots break pesticides and drug residues down into smaller pieces the plant can handle. Experiment after experiment has found that removal rates drop sharply when only one partner is present.
Still, lab results don't carry over neatly to the outdoors. Real rivers hold a mix of pollutants, not just one, and water temperature and sunlight change with the seasons. It's too early to roll this out across large sites.
What You Can Try in Your Garden Water Barrel
On a smaller scale, you can put the same traits to work in your own garden. Toss a handful of duckweed into a rain barrel or a small stone water basin. Once it covers the surface, it blocks sunlight so less algae grows, and it grabs dissolved nitrogen and phosphorus before anything else can, which keeps the water from going bad.
- Once a week, skim off about half the surface with a small net. If you leave it all, the leaves underneath will rot.
- Shake the water off what you've scooped out and mix it into your compost pile. It's high in nitrogen, so it pairs well with dry fallen leaves.
- If you keep chickens or ducks, you can feed it to them fresh.
There's one condition. Only return duckweed to your soil or your animals if it grew in clean water. Duckweed gathered from a roadside ditch or water near a factory may have heavy metals built up in its tissue. Composting it would simply move the pollutants you pulled out of the water right back into your garden soil. This is also why the idea of filtering wastewater with duckweed and turning the harvest into feed or compost hasn't caught on widely yet. Nobody has settled what to do with the plants once they're pulled out.
Duckweed shows up in Korea's historical records, too. The Dongui Bogam, a classic Korean medical text from the early 1600s, calls this floating plant bupyeong (浮萍) and notes that it was used to bring on sweating and help the body pass urine. That evidence is only as strong as a historical record, so don't treat it as medicine. Read it simply as a sign that people noticed this little plant long ago.
Today, lift the lid on your rain barrel and take a look at the water. If you see green leaves floating on top, don't dump them. Start by skimming them off with a net and carrying them over to the compost pile.
