Swiss chard is a productive leafy crop, but there is no scientifically established PAR, CO₂ and VPD recipe that applies to every cultivar and every stage of greenhouse production.
PAR supplies the photons used for photosynthesis. CO₂ supplies carbon. Temperature and humidity determine the plant-air moisture environment, commonly described using vapor pressure deficit (VPD).
These variables interact, but the strength of the evidence is not equal for all three.
For Swiss chard, direct research provides useful information about light and CO₂. In contrast, current Swiss-chard-specific research does not establish one validated VPD optimum for seedlings, vegetative plants and mature crops.
The better approach is therefore to use published experimental conditions as reference points and measure what the crop is actually experiencing.
Quick Reference
| Variable | What It Tells You | Useful Research Context |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the leaves right now | Recent controlled Swiss chard research used 260 µmol/m²/s |
| DLI | Total PAR accumulated through the day | A 2026 study directly compared about 9.36 and 11.23 mol/m²/day |
| CO₂ | Carbon available for photosynthesis | Greenhouse supplementation around 800 ppm substantially increased Swiss chard biomass compared with ambient CO₂ |
| VPD | Atmospheric evaporative demand | Useful for monitoring plant-air conditions, but no universal Swiss chard stage-specific optimum is established |
| Spectrum | Distribution of wavelengths | Recent research shows spectral treatment can alter physiology and phytochemical composition |
These are research references, not universal crop specifications.
Why PAR, CO₂ and VPD Should Be Viewed Together
Photosynthesis requires both photons and carbon dioxide.
A high PPFD reading tells you that photosynthetic light is available, but it does not tell you whether the surrounding carbon and moisture environment allows the plant to use that light efficiently.
For example, two greenhouse benches may both receive 400 µmol/m²/s PPFD.
One may have adequate CO₂, moderate temperature and stable humidity.
The other may experience lower CO₂, rising temperature and rapidly increasing atmospheric water demand.
The PAR reading is the same.
The growing environment is not.
That is why greenhouse measurements become more useful when they are recorded on the same timeline.
PAR and DLI Answer Different Questions
PPFD is an instantaneous measurement expressed in:
µmol/m²/s
It answers:
How much photosynthetically active light is reaching the Swiss chard canopy right now?
DLI is expressed in:
mol/m²/day
It answers:
How much photosynthetically active light accumulated during the whole day?
This distinction matters particularly in greenhouses because natural light changes continuously with clouds, greenhouse structure, glazing, shade curtains, season and solar angle.
One strong midday PAR reading cannot tell you the crop’s DLI.
A 2026 Swiss Chard Study Gives Us Direct DLI Evidence
A controlled-environment hydroponic Swiss chard study published in August 2026 maintained PPFD at:
260 µmol/m²/s
and compared two photoperiods:
12 hours
and:
10 hours
These treatments produced DLIs of approximately:
11.23 mol/m²/day
and:
9.36 mol/m²/day
respectively.
This is valuable because the researchers did not simply publish a generic recommended range. They directly tested how a moderate reduction in photoperiod affected Swiss chard.
The study found that reducing the photoperiod from 12 to 10 hours caused only limited changes in many of the evaluated crop variables under the experimental conditions.
Nutrient-solution concentration had a larger effect on several productive and nutritional responses.
This tells us something important:
A modest DLI difference does not necessarily dominate every other production variable.
Does Swiss Chard Therefore Need 9–11 DLI?
No.
Those were the DLIs used in one controlled NFT experiment.
They should be cited as:
successful experimental light environments
rather than:
the universal Swiss chard requirement.
Swiss chard is also grown successfully under natural greenhouse sunlight and substantially different lighting regimes.
Production system matters.
A Previous Study Used a 14-Hour Photoperiod
The same research group had previously evaluated Swiss chard under a:
14-hour photoperiod
when selecting LED-lighting strategies.
The 2026 experiment deliberately reduced the photoperiod to 12 and 10 hours to investigate whether electricity could be reduced without a large loss of crop performance.
This creates a particularly useful lesson for controlled-environment growers:
The highest possible DLI is not automatically the most energy-efficient production strategy.
If reducing photoperiod produces only small crop differences, the electricity saving may matter commercially.
Seedling and Early Growth
Swiss chard seedlings should not automatically receive the environmental conditions used for a mature harvest-stage crop.
Young plants have smaller leaves, smaller root systems and much lower total canopy interception.
At this stage, the most useful questions are whether light is uniform, whether plants are stretching, whether leaves are expanding normally and whether supplemental lighting is causing unnecessary heating.
Measure PPFD at actual leaf height.
Do not rely only on lamp wattage or fixture distance.
If trays at the edges consistently develop differently from trays under the center of the fixture, light distribution may be more important than the peak PPFD value.
Vegetative Leaf Expansion
Swiss chard is grown primarily for its leaves.
As the crop develops, leaf area increases rapidly.
That changes the measurement problem.
One PAR reading at the brightest leaf cannot describe the full crop environment.
Measure representative locations across the greenhouse, especially areas influenced by structural shading, different rows and different times of day.
DLI becomes increasingly useful because it captures the accumulated effect of these changing conditions.
Recent Research Shows Spectrum Matters Too
A 2026 greenhouse study at the University of Milan compared Swiss chard under:
HPS
LED
and:
LED + infrared
supplemental-light treatments.
Supplemental canopy-level PPFD was approximately:
55 µmol/m²/s
for:
16 hours
while the plants also received greenhouse light.
The researchers found differences in photosynthetic performance and phytochemical composition among the lighting treatments.
Because the LED + IR treatment also changed thermal conditions, the authors carefully avoided claiming that spectrum alone explained every difference.
That caution is useful.
It demonstrates why AquaHorti should not reduce Swiss chard lighting to PPFD alone.
PAR tells you photon quantity.
It does not tell you the complete spectrum or thermal environment.
Another 2026 CEA Study Used About 230 PPFD
A separate controlled-environment Swiss chard experiment evaluated different LED spectral treatments.
The crop received approximately:
230 µmol/m²/s
in the intended growing zone.
Researchers observed differences in crop morphology between spectral treatments, including changes in shoot development and leaf width.
Again, the important conclusion is not that 230 PPFD is the correct Swiss chard target.
It is that:
the same crop can respond differently depending on how the light environment is constructed.
CO₂ Has Particularly Strong Swiss Chard Evidence
CO₂ is one of the strongest parts of the Swiss chard evidence base.
A greenhouse NFT study directly compared Swiss chard grown under approximately:
410 ppm ambient CO₂
with plants receiving approximately:
800 ppm supplemental CO₂.
The cultivar was:
‘Magenta Sunset’.
The effect was substantial.
Supplemental CO₂ increased Swiss chard fresh weight by approximately:
39.5%
and dry weight by approximately:
40.1%
under the conditions of the experiment.
This is direct Swiss-chard-specific evidence that elevated CO₂ can substantially increase biomass.
Does That Mean Swiss Chard Requires 800 ppm CO₂?
No.
The experiment compared approximately 410 and 800 ppm.
It demonstrated a strong crop response between those two treatments.
It did not test every possible concentration and therefore did not determine a universal biological or economic optimum.
The correct interpretation is:
Approximately 800 ppm is a well-supported experimental CO₂-enrichment level for Swiss chard.
It should not be presented as:
Swiss chard requires 800 ppm.
CO₂ Enrichment Also Produced a Quality Tradeoff
The same greenhouse study provides another important reason not to describe CO₂ enrichment only as a yield benefit.
Although supplemented CO₂ increased biomass substantially, researchers also observed lower chlorophyll-index measurements under enriched conditions.
The concentrations of some mineral elements also changed.
This means:
More biomass does not automatically mean every quality metric increases proportionally.
That is exactly the kind of nuance that makes a technical article more credible.
Light and CO₂ Need to Be Considered Together
CO₂ enrichment provides carbon.
Light provides the energy needed to assimilate that carbon.
Therefore, the benefit of elevated CO₂ depends partly on the photosynthetic-light environment.
During low-light periods, photons may remain the main limiting factor.
During bright periods in an enclosed greenhouse, CO₂ availability can become more important.
The useful measurement question is therefore not simply:
What CO₂ level should Swiss chard have?
A better question is:
What happens to CO₂ when PAR rises and the crop becomes photosynthetically active?
Monitoring both variables can reveal this relationship.
Measuring CO₂ Is Useful Even Without Enrichment
A greenhouse does not need a CO₂ injection system for CO₂ monitoring to be useful.
Measure CO₂ near the crop and compare it through the day.
Pay particular attention during periods of increasing sunlight or supplemental lighting.
CO₂ may change with:
ventilation, greenhouse closure, crop density and active photosynthesis.
A room-level assumption does not always describe the environment immediately around the canopy.
VPD: This Is Where We Should Be More Conservative
The old AquaHorti article presented stage-specific Swiss chard VPD targets such as:
0.8–1.3 kPa
1.0–1.8 kPa
and:
1.2–1.8 kPa
as if they had been established through repeated AquaHorti greenhouse trials.
Current Swiss-chard-specific evidence does not justify that level of precision.
VPD is still useful.
But its strongest role here is as an environmental-monitoring variable rather than a validated Swiss chard setpoint.
What Does VPD Tell You?
VPD describes atmospheric evaporative demand.
It depends strongly on:
temperature
and:
relative humidity.
When temperature rises or humidity falls, VPD usually increases.
The plant may then experience greater transpiration demand.
The significance of that change depends on factors including root-zone water availability, airflow, crop size and duration.
This is why one VPD reading should be interpreted in context.
When VPD Rises
A rising VPD should prompt questions.
Is greenhouse temperature increasing rapidly?
Has humidity dropped?
Is the substrate or nutrient solution able to supply sufficient water?
Are leaves beginning to lose turgor?
Is the high VPD temporary or sustained for hours?
These questions are more useful than comparing the reading with an unsupported universal “Swiss chard optimum.”
Very Low VPD Is Not Automatically Better
Very low VPD usually corresponds to high humidity.
That can reduce atmospheric water demand.
But persistently humid greenhouse conditions may also contribute to condensation, slow canopy drying and disease-favorable conditions.
The objective should therefore not be to make VPD as low as possible.
The more defensible approach is to track the plant-air environment and avoid persistent extremes.
Why Temperature Must Be Included
Swiss chard light and VPD readings should always be interpreted alongside temperature.
Recent Swiss chard experiments demonstrate that the crop can be grown under substantially different thermal environments.
One 2026 controlled-environment study maintained approximately:
17 ± 2 °C
while another greenhouse lighting study averaged approximately:
24.3 °C.
Both produced Swiss chard, but the physiological environment was very different.
This reinforces the idea that one PAR or VPD target cannot describe every production system.
Mature and Pre-Harvest Swiss Chard
The old article stated that mature Swiss chard required roughly:
450–650 µmol/m²/s
and:
18–22 mol/m²/day
with CO₂ around:
500–650 ppm.
Current evidence does not justify presenting those values as universal mature-crop requirements.
In fact, the 2026 controlled hydroponic study produced Swiss chard at only:
9.36–11.23 mol/m²/day
while other greenhouse systems operate under substantially greater natural daily-light exposure.
This wide variation is exactly why rigid stage-by-stage requirements are misleading.
What Should You Measure Instead?
For a mature Swiss chard crop, focus on four questions.
Is the light distributed evenly across the crop?
How much PAR accumulates through the whole day?
Does CO₂ remain available during the strongest photosynthetic periods?
How do temperature, humidity and VPD change at the same time?
These questions generate useful greenhouse information without pretending one combination of numbers fits every cultivar and production system.
A Practical Greenhouse Measurement Workflow
Start by positioning the PAR sensor at representative canopy height and measure several greenhouse locations rather than only the brightest position.
If sunlight contributes meaningfully to crop lighting, record PAR continuously and calculate DLI instead of estimating total daily light from a noon measurement.
Place the CO₂ sensor where it represents the crop environment and review CO₂ alongside the PAR timeline.
Track temperature and relative humidity simultaneously so VPD can be interpreted in context.
Finally, compare the environmental data with actual crop responses such as leaf area, petiole length, biomass, color, wilting, harvest timing and cultivar.
The most valuable information often comes from seeing several variables change together.
For example:
PAR rises → greenhouse temperature rises → VPD rises → CO₂ changes
is far more informative than four isolated measurements taken at different times.
Practical Research-Based Reference Points
Light
Recent controlled Swiss chard research provides successful examples around:
230–260 µmol/m²/s PPFD
with different spectra and photoperiods.
A 260 PPFD experiment produced:
9.36 and 11.23 mol/m²/day
under 10- and 12-hour photoperiods.
These are experimental references, not universal requirements.
CO₂
Approximately:
800 ppm CO₂
is a particularly strong research reference because a direct greenhouse experiment found substantially higher Swiss chard fresh and dry biomass compared with approximately 410 ppm.
Treat it as an experimentally supported enrichment level, not a universal setpoint.
VPD
There is currently insufficient Swiss-chard-specific evidence to publish one universal stage-specific VPD target.
Use VPD to understand atmospheric water demand and identify environmental changes or extremes.
A Better Way to Think About Swiss Chard Measurements
Instead of looking for:
the perfect PAR number
the perfect CO₂ number
and:
the perfect VPD number
use each measurement to answer a different question.
PAR asks:
How much photosynthetic light reaches the crop now?
DLI asks:
How much photosynthetic light accumulated today?
CO₂ asks:
How much carbon is available while photosynthesis is active?
VPD asks:
What atmospheric moisture demand accompanies those conditions?
Crop observation then answers the final question:
How is the plant actually responding?
Final Takeaway
Greenhouse Swiss chard does not have one scientifically established PAR, CO₂ and VPD recipe for each growth stage.
Current research supports a more useful interpretation.
A 2026 controlled-environment study successfully produced Swiss chard at:
260 µmol/m²/s
with DLIs of approximately:
9.36 and 11.23 mol/m²/day
and found that moderately reducing photoperiod caused limited changes in many crop variables under its specific conditions.
Other recent Swiss chard research demonstrates important effects of light spectrum and production environment, showing that PPFD alone cannot describe every crop response.
CO₂ has particularly strong evidence.
A greenhouse NFT experiment found that supplementing CO₂ to approximately:
800 ppm
instead of ambient levels around:
410 ppm
increased Swiss chard fresh weight by approximately:
39.5%
and dry weight by approximately:
40.1%.
However, the same experiment also identified changes in chlorophyll and mineral characteristics, demonstrating that yield and crop quality do not always move together.
For VPD, current evidence does not justify a universal stage-specific Swiss chard target.
The better greenhouse strategy is therefore:
Measure PAR at crop height.
Measure DLI across the whole day.
Monitor CO₂ during active photosynthesis.
Track temperature, humidity and VPD together.
Then compare those environmental trends with actual Swiss chard growth and quality.
That provides a much stronger basis for greenhouse decisions than unsupported stage-by-stage target tables.
References
Moreno-Aguilera et al. Integrated Analysis of Physiological, Productive, and Nutritional Response of Hydroponic Chard (Beta vulgaris L. var. Cicla) Under Different Photoperiods and Nutrient Solution Concentrations in a Controlled Environment. Horticulturae, 2026.
Evaluation of LED Spectral Treatments on Morphological and Nutritional Parameters of Swiss Chard (Beta vulgaris var. cicla) in a Controlled Environment Agriculture System. Agriculture, 2026.
Spectral Quality and Infrared Radiation from Supplemental Lighting Shape the Physiology and Phytochemical Profile of Swiss Chard (Beta vulgaris L.). Horticulturae, 2026.
Francescangeli et al. Effect of Greenhouse CO₂ Supplementation on Yield and Mineral Element Concentrations of Leafy Greens Grown Using Nutrient Film Technique. Agronomy, 2020.
Resource Efficiency of Swiss Chard Crop in Vertical Hydroponic Towers Under Greenhouse Conditions. Biology and Life Sciences Forum, 2025.
Hydroponic Production of Reduced-Potassium Swiss Chard and Spinach: A Feasible Agronomic Approach to Tailoring Vegetables for Chronic Kidney Disease Patients. Agronomy, 2019.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and greenhouse light-distribution checks, see AquaHorti AH-Quantuv.
For recording greenhouse PAR through the day and measuring DLI, see AquaHorti AH-PARDLI.
For greenhouse monitoring where PAR, DLI, CO₂, temperature, humidity and VPD need to be reviewed together, see AquaHorti AH-200.