Duckweed vs Giant Duckweed: Lemna vs Spirodela Compared
Duckweed vs giant duckweed: how to tell Lemna minor from Spirodela polyrhiza, and which grows faster, overwinters, holds more metal or digests better.
Duckweed vs giant duckweed is usually asked by someone looking at two different green dots on the same pond. “Duckweed” in that sentence almost always means common duckweed, Lemna minor. “Giant duckweed” is Spirodela polyrhiza, also called greater duckweed or duckmeat. Flip a frond over: one pale root and a pale underside is Lemna, a tuft of roots and a red to purple underside is Spirodela.
That is where every identification guide stops. This one keeps going, because the two plants behave differently in a system. They differ in how fast they grow, how they get through winter, how much metal they hold, how well an animal digests them and how big their genomes are. Each difference below comes with its source. And each comes with the same warning the literature keeps repeating: the clone in your water decides more than the name on the label.
The short answer
- Giant duckweed is bigger and more complex. Spirodela is the largest and the most structurally complex duckweed genus, with many roots per frond, according to the Aquatic Plant Management Society chapter on duckweeds.
- Common duckweed multiplies faster in most head to head tests and holds its rate when light changes.
- Giant duckweed forms true turions that sink for winter. Lemna minor does not.
- On metals the result depends on the metal. Spirodela held more cadmium per kilo in one comparison, Lemna more lead in another.
- On protein, Lemna digests far better in the one study that measured both through a single protocol.
How to tell them apart
The side-by-side table
| Feature | Common duckweed (Lemna minor) | Giant duckweed (Spirodela polyrhiza) |
|---|---|---|
| Frond length | About 2 to 7 mm | About 4 to 12 mm |
| Shape | Oval, roughly twice as long as wide | Rounder, nearly as wide as long, often tear-shaped |
| Upper surface | Bright to yellow-green | Dark, glossy green, often with a red dot near the node |
| Underside | Pale | Red to purple |
| Roots per frond | One | Several, from 4 to more than 15 depending on the source |
| Veins | 3 or more, one primary | 5 to 12 |
The sizes come from the UK OPAL water survey guide, which gives 2 to 7 mm for Lemna minor and 4 to 12 mm for Spirodela. The Texas A&M extension sheet puts Lemna minor fronds at about 0.14 inch long and 0.08 inch wide, with one root, against 0.12 to 0.39 inch and five or more roots for giant duckweed, plus the single red to purple dot near the narrow tip.
Root counts are where sources disagree, and the disagreement is informative. Washington’s aquatic plant manual counts 4 to 16 roots and 5 to 12 veins on a 4 to 10 mm thallus. An identification guide aimed at pond owners counts 7 to 21 roots and notes the two species often grow mixed, which makes size the quickest separator. Roots get eaten and broken, so “more than one” is the safe test, not an exact number.
The Somerset Rare Plants Group adds two details. Lemna minor is ovate with a length to width ratio of 1.2 to 2.0 and carries no red pigmentation, while Spirodela turns red in autumn. For the full key across all five genera, see duckweed species identification.
The look-alike that fools people: Landoltia
Most “giant duckweed” misidentifications are Landoltia punctata, dotted duckweed, which used to sit inside Spirodela. The University of Florida describes it as smaller, more shoe-shaped, with no red dot on top and usually up to four roots, against up to nine and more for the native giant duckweed. It matters beyond botany. Texas A&M notes Landoltia is non-native and illegal to possess or transport in Texas.
Growth: which one is faster
Lemna minor, in the one direct comparison that varied conditions. Grown across three light intensities, both species grew fastest at the middle intensity of 236 micromoles of photons per square metre per second. Spirodela grew slower than Lemna minor throughout and slowed further at the highest intensity, while Lemna held its rate. Spirodela answered poor light by enlarging its fronds. The authors frame it as a trade-off between size and number.
That is a species-level pattern, and it is weaker than it looks. The reference screen of 39 clones across all five genera found relative growth rates from 0.153 to 0.519 per day, and concluded that growth does not vary primarily by genus or species but by the clone’s adaptation to its origin. Published Spirodela figures alone span dry-weight growth rates from 0.149 to more than 0.600 per day and doubling times from 1.12 to 9 days. A fast Spirodela clone beats a slow Lemna clone. The full ranking, and why it does not travel outdoors, is in fastest growing duckweed species.
Winter: turions vs resting fronds
This is the cleanest difference between the two, and the one that matters most for an outdoor system.
Spirodela polyrhiza makes true turions: small, dense, starch-filled resting fronds that sink and stay dormant until cold releases them. The 2023 survival review lists true turions in S. polyrhiza, Lemna turionifera, some clones of Lemna aequinoctialis and many Wolffia species, while Lemna minor overwinters as resting fronds that stay capable of growth. In Spirodela, a mother frond of about 8 by 6 mm buds off turions of about 2 by 3 mm.
In practice, a Spirodela mat can vanish from the surface in autumn and reappear in spring. North Carolina’s extension describes the turion detaching, sinking to the bottom and rising again as spring approaches. Lemna minor tends to linger at the surface as cold, slow fronds. Neither is better in the abstract. If you need cover through a mild winter, a species that sinks is a liability. If your winter is hard, a species that banks itself on the bottom is insurance. More in duckweed turions.
Metals and nutrients: remediation duty
Both are used to strip metals and nutrients from water, and the comparison flips with the metal.
Cadmium. Over 22 days at 0.5 to 3.0 mg/L, Lemna minor removed 42 to 78 percent and Spirodela polyrhiza 52 to 75 percent. Tissue loads peaked at 4,734.56 mg/kg in Lemna and 7,711 mg/kg in Spirodela. Spirodela held more per kilo.
Lead. Exposed to up to 50 mg/L for 7 days, Lemna minor accumulated 561 mg/g dry weight and Spirodela 330 mg/g, and the authors judged Lemna the more effective of the two.
So “which duckweed removes more metal” has no single answer. It is a per-metal, per-clone question, and laboratory concentrations are not effluent. The wider picture is in duckweed and heavy metals.
One rule holds whichever species you run. Biomass grown on wastewater, manure or contaminated water is remediation biomass. Under SERAPH’s published firewall policy it is never sold or blended as food or feed. The comparisons in the next section concern clean-stream biomass only.
Protein, starch and biogas: production duty
Protein. On clean-grown biomass, the one study that ran crude protein and digestibility through a single protocol found Lemna at 221.87 and Spirodela at 179.64 g/kg of dry matter. Standardized ileal digestibility then widened the gap: 72 percent for Lemna, 39 percent for Spirodela, which also carried the most ash and crude fibre. For a feed buyer that is the most decisive number in this article, and it is one study. See duckweed crude protein content for how the deductions stack.
Starch. Spirodela has the higher ceiling. Across published studies, Spirodela ranged from 0.7 to 49 percent starch against 0.2 to 10 percent for Lemna of biomass as sampled. That fits the turion story: a plant that banks starch for winter can be pushed to bank it on demand.
Biogas. In a four-species comparison on wastewater effluents, Spirodela returned 340 to 413 NL of methane per kg of volatile solids, higher than both Lemna species in the same design. Digestion is one of the routes open to remediation biomass, because nothing from it goes to food or feed.
Under the hood: two very different genomes
The two plants look like scaled versions of each other. Their genomes say otherwise. Spirodela polyrhiza has a genome of 158 Mb, the smallest monocot genome when it was sequenced, similar in size to Arabidopsis. That small size, and its basal position in the family, is why it became the reference duckweed genome. Lemna minor’s draft assembly runs to 481 Mb, with 61.5 percent repetitive sequence against 15.79 percent in Spirodela. Repeats explain 94.5 percent of the size difference, while the gene counts are close: 22,382 against 19,623.
For anyone doing genetic work, that makes Spirodela the easier genome to read and Lemna minor the better-studied plant in ecotoxicology. The second point is why so much of the literature rests on Lemna minor.
Which one to choose
Start from what you want the system to do, then let your own water overrule the shortlist.
- Steady multiplication across changing light: Lemna minor, on the evidence of the head to head light study.
- Clean-stream protein for feed: Lemna, because digestibility, not crude protein, is what an animal gets.
- Starch or a winter survival bank: Spirodela, with the turion caveat for winter cover.
- Metals: test both, metal by metal. The published comparisons split.
Then screen several clones of each shortlisted species in your own water, at your own light, through a bad week rather than a good one. Every comparison above was run indoors, on particular clones. That gap between lab and field is the one SERAPH exists to close. We are at TRL 3, proof of concept, and outdoor validation is still ahead. Our SERAPH Vision system already tells duckweed species apart on a real cultivation tray, frame by frame, which matters when a mixed Lemna and Spirodela mat drifts away from the species you planted.
For the far end of the size range, see largest duckweed species. For the whole family in one place, see duckweed species.
FAQ
Is giant duckweed invasive?
Not where it is native. Spirodela polyrhiza is native to Florida and much of the world. The plant that causes legal trouble is its look-alike, dotted duckweed (Landoltia punctata), which is non-native and illegal to possess or transport in Texas. Check which one you have before you move any.
Is giant duckweed the same as greater duckweed?
Yes. Giant duckweed, greater duckweed and duckmeat are all common names for Spirodela polyrhiza. Older references file it as Lemna polyrrhiza, which is part of why the names get confused.
Can I grow common duckweed and giant duckweed together?
They often grow mixed in the wild, and nothing stops you mixing them in a tank. Expect the balance to drift. Light, temperature and season favour one or the other, and in autumn the Spirodela fraction can drop to the bottom as turions while Lemna stays up.
Why is the underside of my duckweed red?
A red to purple underside is the classic sign of Spirodela polyrhiza. Some Lemna species redden too, notably Lemna turionifera, where colour starts at the node. Lemna minor carries no red pigmentation, so a red underside rules it out.