Giant Duckweed: Benefits, Drawbacks, and the Filtration Question

What giant duckweed actually is

Giant duckweed (Spirodela polyrhiza) is a free-floating plant with round fronds that run roughly 4 to 12 millimeters across, about twice the size of common duckweed (Lemna minor), with a reddish-purple underside. The easiest way to tell the two apart is to look underneath: common duckweed hangs a single root from each frond, while giant duckweed hangs a cluster of them, anywhere from 5 to 21. It is also native to North America, which most people do not expect from a plant with this reputation.

A third look-alike is worth knowing, Landoltia punctata. It has fewer roots per frond, two to seven, and it is an introduced plant that some sellers also label "giant duckweed." If you are buying, count the roots.

Why duckweed has a bad name

The reputation is earned, mostly by the small kinds. A thick mat of floating plants blocks light and slows oxygen exchange, and one Italian field study found that light and dissolved oxygen both dropped as duckweed mats got thicker, with oxygen-sensitive animals disappearing from the worst sites. That study was about a small, fast-growing species, Lemna minuta, which is an invasive alien in Europe. Common duckweed also jams into filters and pumps, clings to everything, and takes off in warm, nutrient-rich water.

None of that is made up, and giant duckweed is still a mat-former. NC State Extension calls it aggressive and spreading. So the reputation is fair as a warning, and unfair only if it lumps every duckweed together.

What is different about giant duckweed

It is native, it is bigger, and it is rooted, which in my tubs makes it far easier to manage: it lifts out in sheets with a net or your hand. I want to be careful about the next part, because it is easy to oversell. In the one side-by-side growth comparison I could find, giant duckweed grew slightly slower than common duckweed, not faster, at roughly 0.38 versus 0.41 per day. It is manageable, not slow.

What it does for a medaka tub

Like any floating plant, duckweed shades the water, which helps hold algae back, and it gives fry and shrimp cover. Giant duckweed's root cluster is a useful surface for biofilm and tiny organisms to grow on, which is part of why fry hang around floating roots, though I have not seen a study measure that for duckweed. It also pulls nutrients out of the water, and that is where the claim I hear most deserves a closer look.

Is it really the best plant for filtration?

I used to describe giant duckweed as the best natural nitrate remover there is, and the research does not back the word "best," so I have corrected my listings. Here is what is solid.

• It is genuinely good at taking up nutrients. Duckweeds take up ammonium first, even when nitrate is available, and in wastewater studies they removed large shares of ammonia and phosphate. In one fish-farm study, Lemna and Spirodela duckweeds removed roughly 85 to 89 percent of phosphate.
• The plant only exports nutrients if you remove it. A plant that grows in your tub and then dies in your tub gives the nutrients back. The filtering happens when you scoop out the extra.
• It is not the whole filter. In duckweed lagoons, harvesting recovered only about 15 to 33 percent of the nitrogen coming in, with sediment and bacteria doing much of the rest.
• It is not proven to be the best. In a comparison of four floating plants in tropical ponds, water hyacinth removed the most nutrients, then water lettuce, then common duckweed, then salvinia. I found no head-to-head against frogbit, red root floaters, or rooted plants.

My summary: giant duckweed is an excellent, low-effort nutrient sponge and a real help in a planted tub, working alongside the bacteria and the other plants. Thin it regularly and it earns its place. I would not call it the best, and I would not rely on it alone.

Using it well in a tub

• Thin it on a schedule. Scoop out the extra every week or two. Farms harvest 10 to 35 percent of the standing crop daily, but a tub does not need that pace.
• Leave open water. Keep a part of the surface clear, roughly a third as a rule of thumb, more in hot weather. That figure is a guideline, not a measured threshold. My guide to floating plant cover explains why.
• Do not panic in winter. Giant duckweed makes dense, starchy resting buds called turions in autumn. They sink to the bottom, need several weeks of cold, and sprout again once the water warms, so a tub that looks empty in January may refill by spring.
• Dry the excess. Live duckweed can regrow from fragments in a damp pile, so let the extra dry out before it goes in the garden or the trash, and never dump it in a waterway. You can also dry it as fish food.

Check your state

Giant duckweed is native and is not on the state lists I checked, apart from a listing in Puerto Rico. The look-alike Landoltia punctata is restricted in Louisiana and prohibited in Texas. Rules change, so check your own state before you order any floating plant, and see my floating plant care guide for the plants that are regulated.

Sources

• USGS Nonindigenous Aquatic Species: Landoltia punctata (root counts)
• NC State Extension: Spirodela polyrhiza
• Ceschin et al. 2020, dense duckweed mats and oxygen (Biological Invasions)
• Strzałek and Kufel 2021, growth of Lemna minor and Spirodela polyrhiza (PeerJ)
• FAO: Duckweed, a tiny aquatic plant with enormous potential
• Tripathi et al. 1991, nutrient removal by four aquatic plants
• Zimmo 2003, nitrogen removal in duckweed lagoons
• Ziegler et al. 2024, duckweed turions (Plants)

Common questions

Is duckweed bad for fish?

Not by itself. A thick mat can cut light and lower dissolved oxygen under it, which is the real risk, so thin the plants regularly and leave part of the surface open.

Does giant duckweed filter water?

It takes up ammonium, nitrate, and phosphate and grows fast, so it helps. The nutrients only leave your tub when you remove the plants, and it works alongside bacteria and other plants rather than replacing them. It is not proven to be the best plant for the job.

Is giant duckweed better than common duckweed?

For a small tub, I think so: it is larger, it has a cluster of roots, and it is easier to lift out. It still spreads, so thin it. In the one growth comparison I found, common duckweed grew slightly faster.

Does duckweed die in winter?

Giant duckweed forms dense resting buds that sink in autumn and sprout again when the water warms, so a tub can look empty in winter and refill in spring.

Do medaka eat duckweed?

I have not found any research on medaka eating it, and their mouths are small. Mostly they use the plants for shade and cover. Duckweed is fed to other farmed fish, covered in my guide to drying duckweed as fish food.