Duckweed Biomass: Yield, Composition and How to Measure It

Duckweed biomass is the reason anyone grows duckweed. It is the protein, the starch, the nitrogen and phosphorus pulled out of the water, all in one floating mat. So the first question is always how much of it a hectare produces, and the answers you will find disagree by a factor of almost a hundred.

This article puts those numbers side by side, explains why they disagree, and then covers the part almost nobody writes about: how duckweed biomass is actually measured, and how to measure it without harvesting the crop you are trying to grow.

How much biomass duckweed produces, according to whom

The ranges in the literature

Three reviews, three ranges. An early summary of reported yields put duckweed at anywhere from 2 to 183 tonnes of dry matter per hectare per year. A phytoremediation review gives 39 to 105 tonnes of dry weight per hectare per year, and sets it against Miscanthus, a leading bioenergy grass, at 5 to 44. A food and feed review from 2025 uses a far more modest 10 to 30 tonnes per hectare per year.

None of these is wrong. Each one summarises different studies, run in different places, under different conditions. The trouble starts when one of them is quoted on its own, as though duckweed simply yields that much.

What one outdoor pilot actually measured

Field numbers are rarer, which makes a 2025 pilot in Belgium worth reading closely. Three outdoor systems at a pig manure processing facility were monitored over a full growing season. The two that worked averaged 5.5 grams of dry matter per square metre per day and about 8 tonnes of dry biomass per hectare per year.

The same paper notes that warmer pilots have done better, citing 17 tonnes in Spain and 68 in Brazil. And it records something no yield range shows: growth stopped after roughly 100 to 120 days, despite suitable weather, with rising pH and salinity the likely cause. A third system, fed constructed wetland effluent, died off.

Eight tonnes is at the bottom of most published ranges. It is also what a real temperate system delivered. That gap is the whole subject of our work on the duckweed growth rate: a lab rate and a field season are different measurements.

A note on that pilot, because it matters. That biomass was grown on manure. SERAPH treats anything grown on wastewater, manure or contaminated water as remediation biomass, and our published policy is that it never becomes food or feed. We cite the study for its yield and its field behaviour, nothing else.

Why the numbers disagree

Most of the spread comes from five things, and only the first is biology.

1. Climate. Belgium against Brazil is an 8 versus 68 tonne difference in the same paper’s comparison. Temperature and season length dominate.
2. Nutrient supply. The Belgian systems failed on one effluent and worked on two others at the same site.
3. Density. A mat that gets too thick stops behaving like a growing crop. In waste stabilisation ponds, overlap raises mortality and dead fronds sink and degrade, creating new pollution instead of removing it.
4. Harvest regime. Duckweed productivity is defined by what you take off, how often. Change the schedule and you change the number.
5. The unit. Fresh or dry. Per day, or per day extrapolated to a year that the plant may not actually grow through. A figure of grams per square metre per day multiplied by 365 assumes a season that the Belgian pilot did not get.

The fifth is the easiest to miss and the cheapest to fix. Before comparing two duckweed biomass figures, check that they are the same measurement.

What duckweed biomass is made of

Fresh duckweed is mostly water. A study of duckweed grown for nutrient control in municipal wastewater measured it at 94 to 96 percent water, with 21 to 38 percent crude protein and under 10 percent starch in the dry matter, rising to an average of 19 percent starch after nutrient starvation.

That first number explains a lot. A tonne of fresh duckweed holds roughly 40 to 60 kilograms of dry matter. Any yield figure that does not say “dry” is close to meaningless for comparison.

Composition also moves with species and conditions. Across the duckweed family, reported ranges run from 16.0 to 41.7 percent protein, 17.6 to 35.0 percent starch, 8.8 to 29.7 percent crude fibre and 3.5 to 26.0 percent ash. Ask for the species and the growth conditions before you trust any single value. We go deeper on both fractions in duckweed protein content and duckweed starch content.

Three ways to measure duckweed biomass

Fresh weight

Lift the fronds out, blot them, weigh them. It is quick and it does not have to kill the sample. It is also the least reproducible measurement, because how wet the fronds still are depends on how they were drained. Researchers in waste stabilisation ponds describe fresh weight as less destructive but less accurate and less reproducible than dry weight.

Dry weight

Dry the sample, typically at 105 degrees Celsius until the weight stops changing, then weigh it. This is the reference method and the number every yield figure should be in. Its cost is simple: the sample is gone. In a long experiment or a working system, every measurement removes some of the thing being measured.

Frond count and frond area

Counting fronds is non-destructive but blunt, because it ignores frond size. A large Spirodela frond and a tiny Wolffia frond count the same. Measuring the area the fronds cover is better, and that is where image analysis comes in.

Measuring biomass without harvesting it

Area-to-weight regressions

The simplest non-destructive method photographs the water surface, measures green area, and converts area to weight with a calibration line. A study of three species found dry biomass was linearly proportional to frond area, with a separate line for each: Lemna, Spirodela and Wolffia carry different weight per square centimetre because their bodies are built differently. Dry weight tracked area more closely than wet weight did. Healthy and withered fronds fell on the same line.

For Lemna minor, an earlier study found the relationship between covered area and weight had an R squared of 0.972, and the image results matched weighing.

The practical rule: calibrate per species. Which also means you need to know which species is on the water, the problem we cover in telling duckweed species apart.

Machine learning on images

Thresholding green pixels struggles with reflections, shadows and stressed fronds. Deep learning handles those better. One method combining instance segmentation with a health classifier cut frond area error to 3.1 percent from 8.8 percent for existing methods. Frond number was harder: 17.2 percent error even with the improved model.

That difference is informative. Area is now a well-solved problem in a controlled photo. Counting individuals in a clustered colony is not.

Where 2D images break: the dense mat

Every one of these methods measures a surface. A productive duckweed mat is not one. The three-species study says it plainly: coverage on the water can exceed 100 percent, fronds overlap, and image analysis underestimates the biomass in overlapping layers. Its authors expect drone cameras and machine learning to be necessary for crowded, mixed ponds at scale.

So the method that works best on a calibration tray fails exactly where a production system spends most of its time: at high density, just before harvest.

From surface to canopy volume

This is the gap SERAPH Vision is built around. Rather than reading a flat area, it reconstructs the canopy in 3D from a single capture, using 170,000 surface points drawn from 5.9 million captured, and computes canopy volume. It also distinguishes duckweed species frame by frame on a real tray, which addresses the per-species calibration problem above. Growth and stress reading are still in development.

What we will not do is give you an accuracy number. SERAPH is at TRL 3, proof of concept, and outdoor field validation is still ahead. Volume is a better proxy for a layered mat than area is. Whether it holds outdoors, across seasons and species mixes, is the measurement that comes next. AI proposes, the bench validates.

What duckweed biomass becomes, and the line between streams

Duckweed biomass has more uses than most crops, which is part of why it gets oversold.

• Protein. The headline use, covered in the composition section above.
• Starch and bioethanol. One review reports Spirodela polyrhiza reaching an ethanol yield of 6,420 litres per hectare against 4,310 for corn.
• Biogas. In the municipal wastewater study, anaerobic digestion of the harvested duckweed gave biogas with 62.5 to 67.1 percent methane, and it was the recommended management route for that community. More in biogas from duckweed.
• Remediation. The biomass is the means of removing nutrients and contaminants. See duckweed phytoremediation.

These uses sit in two streams that must never mix. Biomass grown for food or feed comes from clean, controlled water. Biomass grown on wastewater, manure or contaminated water is remediation biomass, and its routes are energy and materials, never food or feed. At SERAPH the two streams are separated from the first sensor reading. Measurement matters here too: you cannot trace a stream you cannot measure, which is one more reason the measurement layer comes before any duckweed harvesting decision.

FAQ

How many tonnes of duckweed can you grow per hectare?

Published figures span 2 to 183 tonnes of dry matter per hectare per year, which tells you more about the studies than the plant. A full-season outdoor pilot in temperate Belgium measured about 8 tonnes. Ask for climate, season length and whether the figure is dry weight.

What is the dry matter content of duckweed?

Roughly 4 to 6 percent. Fresh duckweed was measured at 94 to 96 percent water, so a tonne fresh holds around 40 to 60 kilograms of dry matter.

Can you estimate duckweed biomass from a photo?

Yes, for a single layer. Frond area converts to dry weight along a straight calibration line, but the line differs by species, and flat images underestimate biomass once fronds start to overlap.

Is duckweed biomass grown on wastewater safe for animal feed?

Not under SERAPH’s policy. Anything grown on wastewater, manure or contaminated water is remediation biomass and is never sold or blended as food or feed, whatever its protein content.