Kale is a productive cool-season leafy crop, but there is no scientifically established set of PAR, CO₂ and VPD values that applies to every cultivar and every stage of greenhouse production.
Light supplies the photons used in photosynthesis. CO₂ supplies carbon. Temperature and humidity shape the plant-air moisture environment, commonly described using vapor pressure deficit (VPD).
These variables interact.
A kale crop may receive abundant light but respond differently when CO₂ availability changes. Increasing CO₂ can improve photosynthesis and biomass, but recent research also shows that pushing CO₂ higher does not automatically improve nutritional quality.
For VPD, the evidence requires even more caution. Current kale-specific research does not establish a validated stage-by-stage optimum.
This guide therefore uses published research as a reference and focuses on what growers can actually measure.
Quick Reference
| Variable | What It Tells You | Useful Research Context |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop now | Controlled kale studies commonly use approximately 150–300 µmol/m²/s, while greenhouse sunlight can be much higher |
| DLI | Total photosynthetic light accumulated through the day | Recent greenhouse kale studies have operated around 22–29 mol/m²/day under their specific conditions |
| CO₂ | Carbon available for photosynthesis | Direct kale studies show responses at 800 ppm; 1200 ppm may increase growth further but can reduce some nutritional-quality traits |
| VPD | Atmospheric evaporative demand | Useful for environmental monitoring, but no universal kale growth-stage optimum is established |
| Temperature | Strongly modifies light and VPD response | Kale research commonly uses relatively cool production temperatures |
These values describe research environments.
They are not universal kale specifications.
1. Why PAR, CO₂ and VPD Should Be Considered Together
Photosynthesis requires both light and CO₂.
PAR provides the photon energy.
CO₂ provides carbon for carbohydrate production.
Meanwhile, temperature and humidity influence transpiration and stomatal behavior.
That means two kale plants receiving the same PPFD may still experience very different growing conditions.
For example:
Greenhouse Zone A
- strong PAR
- adequate CO₂
- moderate temperature and humidity
Greenhouse Zone B
- the same PAR
- lower CO₂
- warmer and drier air
- greater evaporative demand
A PAR meter alone would show similar light.
The plants may not respond similarly.
This is why environmental measurements become more useful when they are viewed together.
2. PAR and DLI Answer Different Questions
PPFD measures photosynthetic photon flux at one moment.
It is expressed in:
µmol/m²/s
It answers:
How much photosynthetic light is reaching the kale canopy right now?
DLI integrates PPFD over the whole day.
It is expressed in:
mol/m²/day
It answers:
How much photosynthetic light did the crop receive during the entire day?
In a greenhouse, this distinction is especially important because sunlight changes continuously.
A high noon PPFD does not necessarily mean a high daily light total.
3. What DLI Has Been Used in Greenhouse Kale Research?
A 2025 hydroponic greenhouse experiment used:
Brassica oleracea ‘Red Russian’ kale
and recorded an average DLI of approximately:
22.2 ± 1.8 mol/m²/day
during the study.
The crop experienced changing natural greenhouse light rather than one constant artificial PPFD.
This makes the study particularly useful as a real greenhouse reference.
4. Another Greenhouse Study Operated Near 29 DLI
A separate hydroponic experiment involving Red Russian kale reported an average DLI of approximately:
28.7 ± 1.7 mol/m²/day
during the production period.
Again, this was the environmental condition of the experiment.
It was not an experiment designed to determine the optimum DLI.
Therefore, AquaHorti should not convert these numbers into:
“Kale requires 22–29 mol/m²/day.”
A more defensible statement is:
Productive greenhouse kale has been studied under DLIs in the low-to-upper 20s, but the optimum depends on cultivar, temperature, production system and crop goal.
5. Controlled Kale Production Can Use Much Lower DLI
Indoor and plant-factory kale studies often use lower PPFD and DLI than sunlight-driven greenhouse experiments.
For example, one controlled kale experiment used:
200 µmol/m²/s
for:
12 hours per day
This corresponds to approximately:
8.6 mol/m²/day.
Another kale study used:
250 µmol/m²/s
under controlled LED lighting.
Other experiments have used approximately:
180–300 µmol/m²/s
depending on cultivar and production objective.
These examples demonstrate why one universal kale DLI cannot be inferred from a single study.
6. Seedlings Should Not Automatically Receive Mature-Crop Conditions
Young kale plants have:
- small leaf area
- smaller root systems
- lower total canopy light interception
- different transplant-quality objectives
Research protocols have successfully established kale seedlings under approximately:
150–200 µmol/m²/s
before plants were transferred into larger production systems.
This does not establish 150 or 200 PPFD as the universal seedling optimum.
It shows that mature greenhouse light conditions should not automatically be applied to small seedlings.
7. What Should You Measure During Seedling Production?
Instead of trying to hit one exact number, ask:
- Is light uniform across the propagation area?
- Are seedlings compact?
- Are petioles stretching?
- Are edge trays receiving less light?
- Does supplemental lighting increase temperature excessively?
Measure PPFD where the leaves actually are.
If comparing different days or lighting schedules, also calculate or log DLI.
8. Vegetative Growth Changes the Light Environment
Kale is grown primarily for its leaves.
As the canopy expands:
- leaf area increases
- light interception increases
- self-shading develops
- water use changes
- total carbon demand increases
One measurement at the brightest upper leaf therefore becomes less representative of the entire crop.
Measure multiple locations.
Useful comparisons include:
- greenhouse center vs. edge
- sunny vs. structurally shaded areas
- upper vs. lower parts of larger plants
- different rows
- morning vs. midday vs. afternoon
9. Light Distribution Matters, Not Only Peak PPFD
A greenhouse zone with:
600 µmol/m²/s
at one exposed location may still contain substantial shaded crop area.
Another zone with:
400 µmol/m²/s
distributed more evenly may produce a different canopy response.
That is why light mapping can be useful.
Do not define a greenhouse by its single highest PAR reading.
10. Kale Can Use Substantially Different Light Intensities
Controlled-environment kale research illustrates broad physiological flexibility.
Studies have used:
- approximately 150 µmol/m²/s
- approximately 180 µmol/m²/s
- approximately 200 µmol/m²/s
- approximately 250 µmol/m²/s
- approximately 300 µmol/m²/s
depending on experiment and photoperiod.
Leaf physiology research has also exposed kale-related Brassica leaves to substantially higher instantaneous PPFD to examine photosynthetic responses.
These treatments should not be converted into a universal commercial PPFD target.
11. Photoperiod Changes the Meaning of PPFD
Consider:
200 µmol/m²/s × 12 h
≈ 8.6 mol/m²/day
200 µmol/m²/s × 16 h
≈ 11.5 mol/m²/day
250 µmol/m²/s × 16 h
≈ 14.4 mol/m²/day
300 µmol/m²/s × 16 h
≈ 17.3 mol/m²/day
The PPFD may appear similar while daily photon exposure changes substantially.
For constant artificial lighting:
DLI = PPFD × light-hours × 0.0036
In a greenhouse, continuous logging is more useful because sunlight is not constant.
12. Light Spectrum Also Matters
PAR and DLI measure light quantity.
They do not describe how photons are distributed by wavelength.
Kale research has demonstrated responses to:
- blue light
- red light
- green light
- far-red light
- white LEDs
- UV-A supplementation
Different spectra can alter:
- plant architecture
- leaf expansion
- biomass
- chlorophyll
- carotenoids
- glucosinolates
- antioxidant compounds
Therefore:
Two grow lights producing the same PPFD and DLI can still produce different kale plants.
13. More Light Is Not Automatically Better Quality
One reason kale lighting recommendations should remain cautious is that production goals differ.
A grower may be optimizing for:
- fresh biomass
- compact morphology
- deep coloration
- carotenoids
- glucosinolates
- vitamin content
- electricity efficiency
The light environment maximizing one characteristic may not maximize another.
For example, supplemental UV-A studies show that moderate UV-A can enhance several kale quality traits, but larger treatments do not necessarily continue improving every response.
The same principle applies to total light.
14. CO₂ Has Strong Kale-Specific Evidence
CO₂ is one area where recent kale evidence is particularly strong.
A 2026 experiment directly compared kale grown at:
400 ppm
800 ppm
and:
1200 ppm CO₂
The study used two cultivars:
Curly Kale
and:
Red Ursa
under controlled artificial lighting.
Both growth and physiological responses changed substantially with CO₂ concentration.
15. What Happened at 800 ppm?
The researchers concluded that approximately:
800 ppm CO₂
provided the best overall balance among:
- growth
- yield
- phytonutrient content
- soluble protein
- nitrate reduction
- economic feasibility
under the specific controlled-environment system tested.
This is one of the strongest current pieces of evidence for using approximately 800 ppm as a research reference for kale.
But it is still not a universal greenhouse requirement.
16. What Happened at 1200 ppm?
Increasing CO₂ to:
1200 ppm
further enhanced several growth and photosynthetic traits.
However, the study found an important tradeoff.
At 1200 ppm, some nutritional-quality characteristics declined, including important compounds such as:
- vitamin C
- amino acids
The result is a useful warning against the assumption:
More CO₂ is always better.
Growth and nutritional density are not necessarily maximized by the same CO₂ concentration.
17. CO₂ Response Is Also Cultivar-Specific
Curly Kale and Red Ursa did not respond identically.
Their:
- photosynthetic characteristics
- pigment responses
- light adaptation
- biochemical responses
differed.
This reinforces the same principle we have seen with light:
A number measured in one cultivar should not automatically become a universal kale setpoint.
18. A 2026 Meta-Analysis Strengthens the CO₂ Evidence
A recent meta-analysis combined:
13 studies
and:
339 effect sizes
examining elevated CO₂ responses in kale and spinach.
Approximately 79% of the analyzed effect sizes came from kale-related crops and cultivars.
Overall, elevated CO₂ significantly increased kale biomass.
The analysis also reported increases in kale traits including:
- fresh weight
- leaf area
- plant height
- root characteristics
under elevated CO₂.
This confirms that the positive CO₂ growth response is not limited to one isolated experiment.
19. But Elevated CO₂ Can Affect Nutritional Quality
The same meta-analysis also identified nutrient-quality tradeoffs.
For example, elevated CO₂ was associated with reduced protein concentration in kale across the analyzed studies.
Responses varied by:
- cultivar
- CO₂ concentration
- nutrient
- exposure duration
Therefore, a commercial recommendation should not be based on fresh weight alone.
20. Is 800 ppm the “Correct” Greenhouse Kale CO₂ Level?
No.
A better statement is:
Around 800 ppm is a particularly well-supported experimental enrichment level for kale and provided a favorable yield-quality balance in one recent controlled-environment study.
Whether enrichment is appropriate in a greenhouse also depends on:
- natural light
- ventilation
- greenhouse tightness
- temperature
- CO₂ cost
- crop value
If greenhouse vents are fully open, maintaining 800 ppm may not be practical or economical.
21. Why CO₂ Measurement Is Useful Even Without Enrichment
You do not need a CO₂ injection system for CO₂ measurement to be useful.
A sensor can answer:
Does CO₂ remain stable when the kale canopy is actively photosynthesizing?
Watch CO₂ during:
- morning light increase
- peak sunlight
- supplemental-light periods
- ventilation
- closed-greenhouse periods
Repeated measurements can reveal whether the crop environment differs from assumptions based on outdoor air.
22. PAR and CO₂ Should Be Viewed Together
Suppose two kale greenhouse zones both receive:
500 µmol/m²/s PPFD
Zone A
CO₂ remains around ambient outdoor concentration or above.
Zone B
CO₂ declines substantially during active photosynthesis.
The PAR measurement is identical.
The carbon environment is not.
Likewise, supplying elevated CO₂ during very low light may produce less benefit than supplying it when the crop has sufficient photon energy.
This is why light and CO₂ should be interpreted together.
23. What Does Research Say About VPD?
This is where AquaHorti should remain conservative.
Current kale literature contains many studies reporting:
- temperature
- relative humidity
- irrigation
- photosynthetic response
But there is not enough kale-specific controlled research to define one validated stage-by-stage VPD optimum.
Therefore, the old AquaHorti ranges such as:
0.8–1.3 kPa
1.0–1.8 kPa
1.2–1.8 kPa
should not be presented as experimentally verified kale requirements.
24. VPD Is Still a Useful Measurement
VPD describes atmospheric evaporative demand.
As temperature rises or humidity decreases, VPD generally increases.
This can increase water demand.
Plants can also respond to increasing VPD by changing stomatal conductance.
Therefore, VPD helps growers understand the relationship among:
- temperature
- humidity
- transpiration demand
- crop-water status
Its strongest value for kale today is as a diagnostic and trend variable, not as a rigid pass/fail target.
25. What Should You Watch When VPD Becomes High?
A high VPD reading should prompt questions such as:
- Is the greenhouse getting too warm?
- Has humidity dropped sharply?
- Is the root zone sufficiently wet?
- Are leaves losing turgor?
- Has irrigation demand increased?
- Is the increase happening only briefly or for several hours?
One VPD number cannot answer all of those questions.
26. Very Low VPD Is Not Automatically Better
Very humid conditions lower VPD.
But persistently humid greenhouse conditions can contribute to:
- condensation
- slow canopy drying
- weak evaporative demand
- disease-favorable conditions
Therefore, the objective should not be:
Make VPD as low as possible.
The better goal is environmental stability and avoidance of prolonged extremes.
27. Temperature Matters Especially for Kale
Kale is a cool-season crop.
Recent greenhouse research has successfully grown kale under relatively cool day/night conditions.
One Red Russian greenhouse study recorded average:
23.7 °C daytime
and:
16.6 °C nighttime
temperatures.
Another kale/collard greenhouse experiment averaged approximately:
20.7 °C daytime
and:
13.6 °C nighttime.
These are experimental environments, not universal kale temperature specifications.
But they reinforce the importance of interpreting light and VPD together with temperature.
28. Mature Kale and Pre-Harvest Conditions
As kale approaches harvest, the production objective often shifts toward:
- leaf biomass
- color
- texture
- nutritional quality
- marketable size
There is no strong evidence that mature greenhouse kale universally requires:
450–650 µmol/m²/s
or:
22–25 DLI minimum
or:
500–650 ppm CO₂
as the old article claimed.
What current research supports is a broader framework.
Productive kale has been studied across:
- moderate artificial PPFD
- greenhouse DLIs in the low-to-upper 20s
- ambient and elevated CO₂
- substantially different spectra
The crop environment must be interpreted as a whole.
29. A Practical Greenhouse Measurement Workflow
Step 1 — Measure PAR at Canopy Height
Position the PAR sensor where the kale leaves actually receive light.
Avoid measuring only near the greenhouse roof or lamp.
Step 2 — Check Several Locations
Compare:
- center
- greenhouse edges
- shaded zones
- different rows
- representative canopy positions
Step 3 — Measure DLI
For sunlight-driven production, log PAR through the day.
Compare:
- sunny days
- cloudy days
- seasons
- greenhouse locations
- supplemental-light schedules
Step 4 — Monitor CO₂
Watch CO₂ on the same timeline as PAR.
Especially observe periods when:
PAR rises strongly.
If CO₂ changes at the same time, the relationship may be meaningful.
Step 5 — Track Temperature and Humidity
Use them to follow VPD.
Look at trends rather than only daily averages.
Step 6 — Review the Variables Together
A timeline such as:
PAR rises → temperature rises → VPD rises → CO₂ declines
is much more informative than four isolated numbers.
Step 7 — Compare the Data With the Crop
Record:
- leaf area
- plant height
- fresh weight
- color
- wilting
- harvest day
- cultivar
This connects the environment with actual production.
30. Practical Reference by Growth Stage
Instead of giving kale three unsupported numeric recipes, use stage-specific questions.
Seedling Stage
Ask:
- Is the propagation area uniform?
- Are seedlings stretching?
- Is supplemental lighting adding unnecessary heat?
- Are daily-light conditions suitable for compact transplant growth?
Published kale propagation environments commonly use moderate PPFD rather than mature greenhouse sunlight.
Active Vegetative Growth
Ask:
- Is daily light increasing as the canopy expands?
- Are greenhouse shadows creating uneven growth?
- Does CO₂ change during strong photosynthesis?
- Are temperature and humidity producing large atmospheric swings?
Mature Production
Ask:
- Is DLI sufficient for the intended growth rate?
- Is supplemental light providing a real benefit?
- Would CO₂ enrichment be useful under the available light?
- Are temperature and VPD compatible with water availability?
- Is higher growth compromising any quality objective?
31. Practical Research-Based Reference Points
DLI
Recent greenhouse kale experiments have operated around:
22–29 mol/m²/day
under their particular conditions.
Controlled indoor systems successfully operate at substantially lower DLIs.
Therefore:
Do not use 22–29 as a universal kale requirement.
Use it as evidence that productive mature greenhouse kale can operate in that range.
CO₂
Approximately:
800 ppm
is currently a particularly useful research-based enrichment reference.
A 2026 study found it provided a strong balance of yield, nutritional quality and economic performance.
But greenhouse enrichment strategy depends strongly on ventilation and light.
VPD
There is currently no sufficiently validated universal stage-specific VPD target for greenhouse kale.
Use VPD to understand atmospheric demand and identify environmental extremes.
32. A Better Way to Interpret Kale Measurements
Instead of searching for:
one PAR number
one CO₂ number
one VPD number
ask:
PAR
How much usable light reaches the crop now?
DLI
How much accumulated during the day?
CO₂
Is enough carbon available during active photosynthesis?
VPD
How demanding is the atmospheric moisture environment?
Crop Response
What is the plant actually doing under those conditions?
That is a much stronger production framework.
Final Takeaway
Greenhouse kale does not have one scientifically established PAR, CO₂ and VPD recipe for each stage of growth.
Current research supports a more evidence-based interpretation.
Productive greenhouse Red Russian kale has been studied under average DLIs around 22–29 mol/m²/day, but those values are experimental environments rather than universal requirements.
Controlled kale systems also grow successfully at substantially lower DLIs, demonstrating the importance of photoperiod, spectrum, cultivar and production system.
Elevated CO₂ can increase kale photosynthesis and biomass.
A recent controlled study found approximately 800 ppm CO₂ provided a strong balance between yield, crop quality and economic feasibility, while 1200 ppm increased growth further but reduced some important nutritional-quality traits.
A 2026 meta-analysis also confirms an overall positive kale biomass response to elevated CO₂ while identifying nutritional tradeoffs.
Current evidence does not support one universal stage-specific kale VPD range.
The better strategy is:
Measure PAR at crop height.
Measure DLI across the whole day.
Track CO₂ during active photosynthesis.
Measure temperature and humidity and use VPD to understand atmospheric demand.
Then compare those trends with cultivar-specific crop growth and quality.
That provides a much stronger basis for greenhouse kale production than unsupported stage-by-stage target tables.
References
Modified Nutrient Management Protocol for Optimum Biomass Production, Nutritional Quality, and Flavor-Related Phytochemical Properties of Hydroponic-Grown Kale (Brassica oleracea). Frontiers in Plant Science, 2025.
Optimizing Carbon Dioxide Enrichment to Balance Yield, Functional Food Quality, and Economic Feasibility in Plant-Factory-Cultivated Kale. Horticulturae, 2026.
Effects of Elevated CO₂ on Yield and Nutritional Quality of Kale and Spinach: A Meta-Analysis. Biology, 2026.
Influence of Electrical Conductivity on Plant Growth, Nutritional Quality, and Phytochemical Properties of Kale and Collard Grown Using Hydroponics. Agronomy, 2024.
Influence of Blue/Red vs. White LED Light Treatments on Biomass, Shoot Morphology, and Quality Parameters of Hydroponically Grown Kale. Scientia Horticulturae, 2018.
Substituting Green or Far-Red Radiation for Blue Radiation Induces Shade Avoidance and Promotes Growth in Lettuce and Kale. Environmental and Experimental Botany, 2019.
Effect of Supplemental UV-A Intensity on Growth and Quality of Kale under Red and Blue Light. International Journal of Molecular Sciences, 2022.
Pre-Harvest UV-A Supplementation in Plant Factory with Artificial Lighting Improves Growth, Photosynthesis, and Phytonutrients in Kale. Horticulturae, 2024.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and checking greenhouse light distribution, see AquaHorti AH-Quantuv.
For recording changing 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.