Kale is a leafy Brassica crop that can perform well across a relatively wide range of controlled-environment conditions.
That does not mean there is one universal PPFD, CO₂ concentration or VPD that should be used from seedling to harvest.
As kale develops, leaf area increases, canopy structure changes, total photon interception rises and crop water demand changes.
Production goals also matter.
A grower optimizing fresh biomass may not choose exactly the same environment as a researcher studying pigments, glucosinolates or antioxidant compounds.
For that reason, PPFD, DLI, CO₂ and VPD are best treated as measurements that describe the crop environment, not as four independent numbers that must always match an “ideal” chart.
Start With PPFD, Not a “PAR Number”
PAR means Photosynthetically Active Radiation, traditionally describing photons from approximately 400 to 700 nm.
A quantum sensor generally measures:
PPFD — Photosynthetic Photon Flux Density
expressed in:
µmol/m²/s
PPFD tells you how much photosynthetic photon flux reaches a particular location at a particular moment.
So a statement such as:
“Kale needs 400 PAR”
is technically unclear.
A better statement is:
“PPFD at canopy level was 400 µmol/m²/s.”
That still describes only an instant.
To understand light across the full photoperiod, use DLI.
PPFD and DLI Should Be Interpreted Together
DLI — Daily Light Integral — expresses the accumulated photosynthetic photon exposure during a day:
mol/m²/day
For constant PPFD:
DLI = PPFD × photoperiod hours × 0.0036
For example:
200 µmol/m²/s × 16 h ≈ 11.5 mol/m²/day
Recent controlled-environment kale studies illustrate why PPFD should not be interpreted alone.
One study maintained kale at 200 µmol/m²/s for 16 hours, while another kale microgreen experiment compared 150, 200 and 250 µmol/m²/s under controlled conditions. These numbers describe specific experiments, not universal crop requirements.
Another kale experiment used approximately 230 µmol/m²/s with a 16-hour photoperiod, corresponding to a DLI of about 13.2 mol/m²/day.
The useful lesson is:
Always ask how PPFD, photoperiod and DLI fit together.
Stage 1: Seedling Establishment
Young kale plants have relatively little leaf area.
The purpose of lighting at this stage is to support compact, uniform development and establish enough leaf area for subsequent growth.
The objective is not to find the maximum PPFD seedlings can tolerate.
Measure at Seedling Height
PPFD should be measured where the seedlings actually are.
A reading near the fixture does not tell you what reaches the tray.
Measure several positions when evaluating a propagation area.
If one side of a tray receives substantially more photon flux than another, plant uniformity can suffer even if the average value seems acceptable.
Research Values Are Context, Not Targets
Controlled kale research has used quite different lighting conditions.
For example, one study germinated kale under approximately 136 µmol/m²/s for 12 hours before later treatments. Another plant-factory experiment used a PPFD around 160 µmol/m²/s with a 16-hour photoperiod while examining temperature, humidity and CO₂ effects.
These values should not be converted into:
“Kale seedlings require 136–160 PPFD.”
Different studies use different:
- cultivars,
- spectra,
- temperatures,
- nutrient systems,
- plant densities,
- and research objectives.
The more useful question is:
Is the seedling environment providing enough total daily light for uniform growth without unnecessary heat or water demand?
Stage 2: Rapid Vegetative Growth
Once kale begins producing larger leaves, the crop intercepts substantially more light.
At this stage, the relationship between PPFD and DLI becomes more important.
More PPFD Can Change Both Growth and Quality
Light intensity affects more than fresh weight.
It can also influence:
- morphology,
- pigmentation,
- antioxidant compounds,
- and secondary metabolism.
A 2024 kale microgreen study comparing 150, 200 and 250 µmol/m²/s found that changing light intensity affected several quality-related compounds, and responses differed between kale and mustard.
A recent 2026 kale study also showed that spectral composition could significantly change leaf area, biomass, pigments and antioxidant-related compounds even when total PPFD was held approximately constant.
This is important because it means:
two environments with the same PPFD do not necessarily produce identical kale.
Spectrum, photoperiod and crop conditions also matter.
Do Not Interpret One Midday Reading as the Whole Light Environment
In a greenhouse, PPFD changes continuously.
Clouds move.
Sun angle changes.
Shade systems operate.
Supplemental fixtures turn on and off.
So two days can both reach:
500 µmol/m²/s at noon
and still deliver very different DLIs.
That is why DLI becomes useful when comparing:
- seasons,
- benches,
- lighting schedules,
- and cloudy versus sunny production periods.
Stage 3: Canopy Expansion
As leaves become larger, kale develops its own canopy structure.
One PPFD reading may no longer represent every leaf.
Upper leaves can shade lower leaves, particularly as plant spacing becomes tighter.
Sensor Position Matters More as Plants Grow
For repeatable measurements:
- keep the sensor near a consistent crop reference plane,
- keep sensor orientation consistent,
- and avoid changing measurement height between comparisons.
If the crop becomes taller but the sensor stays fixed far below or above the canopy, apparent trends may partly reflect sensor position rather than the actual lighting strategy.
Density Changes the Meaning of PPFD
The PPFD above a dense canopy is not the same thing as photon interception by each leaf.
Plant spacing therefore affects how useful a particular light environment is.
This is one reason research results from small experimental plants cannot always be transferred directly to mature commercial kale stands.
Stage 4: Pre-Harvest Growth
Near harvest, kale is producing substantial leaf biomass.
At this stage, higher photon supply may support continued carbon assimilation, but the relationship is not unlimited.
Other variables can increasingly influence how effectively the plant uses light.
These include:
- CO₂,
- temperature,
- root-zone water,
- nutrient availability,
- canopy structure,
- and cultivar.
High PPFD Does Not Automatically Mean Better Quality
Some growers assume:
more PPFD → faster growth → better crop
but kale research shows that quality responses can move differently from biomass responses.
Light treatments may change pigments, phenolics, anthocyanins, carotenoids or glucosinolate-related characteristics independently of fresh weight.
So the production target matters.
Are you optimizing:
yield?
leaf color?
nutritional compounds?
energy efficiency?
The answer may change the preferred environment.
CO₂: Kale Can Respond, but More Is Not Automatically Better
Kale is a C3 crop.
Increasing CO₂ can increase carbon assimilation when light and other conditions allow the crop to use the extra carbon.
But kale-specific research also demonstrates why one fixed CO₂ recommendation is misleading.
Kale Studies Have Tested a Wide Range of CO₂
A controlled plant-factory study compared:
400, 700, 1000, 1300 and 1600 ppm CO₂
and found that the CO₂ conditions associated with strongest vegetative growth were not the same as those associated with highest glucosinolate content.
Under that particular experimental system, growth performed best around 700–1000 ppm, while higher CO₂ favored glucosinolate accumulation.
That is an important result.
It means the correct conclusion is not:
“Kale requires 700–1000 ppm.”
The correct conclusion is:
CO₂ response depends on what you are trying to optimize.
Newer Research Shows the Same Trade-Off
A 2026 study compared 400, 800 and 1200 µmol/mol CO₂ in two kale cultivars and found that elevated CO₂ affected photosynthesis, growth and quality, with cultivar-dependent responses.
A 2026 meta-analysis covering kale and spinach also found that elevated CO₂ generally increased kale biomass, while some nutritional measures, including protein and certain mineral concentrations, could decline.
Again, the lesson is:
CO₂ enrichment is a production trade-off, not a universally positive number that should simply be maximized.
CO₂ Must Be Interpreted Together With Light
CO₂ becomes useful for photosynthesis when sufficient photon energy is available.
If PPFD is very low, increasing CO₂ cannot replace the missing light.
Likewise, if PPFD becomes substantial in a closed growing space, active photosynthesis can draw CO₂ down.
This is why the useful measurement is not simply:
“What is the room CO₂ concentration?”
but:
“What happens to CO₂ while the crop is receiving meaningful PPFD?”
If records show:
PPFD increasing
while:
CO₂ decreases
that may indicate that crop uptake is outpacing CO₂ replacement.
Ventilation, air mixing and supplementation can then be evaluated.
Do Not Assume Outside CO₂ Equals Canopy CO₂
CO₂ can vary spatially inside protected environments.
A sensor beside:
- a vent,
- door,
- CO₂ injector,
- or circulation fan
may not represent the concentration inside the crop canopy.
As plant density increases, sensor placement becomes more important.
For useful comparisons, keep the sensor in a consistent location that reasonably represents the crop environment.
VPD Describes Atmospheric Water Demand
VPD — Vapor Pressure Deficit — combines temperature and humidity into a measure related to atmospheric evaporative demand.
Higher VPD generally represents stronger drying demand.
Lower VPD represents weaker evaporative demand.
But this does not mean:
higher VPD = bad
or:
lower VPD = good.
Plant response depends on the complete water balance.
High VPD
When VPD rises, potential water loss from leaves increases.
If roots and irrigation can supply enough water, the crop may continue transpiring.
If atmospheric demand becomes too great relative to water supply, stomatal conductance may decrease and photosynthesis can become restricted.
This is why strong light on a hot, dry afternoon does not guarantee equally strong carbon assimilation.
Low VPD
Very low VPD reduces transpiration.
That can affect plant cooling and the transport of some nutrients through the transpiration stream.
But it would be scientifically incorrect to write:
“Low VPD stops nutrient uptake.”
Mineral nutrition also depends on:
- root-zone concentration,
- EC,
- pH,
- root health,
- water supply,
- and plant physiology.
VPD is one part of that system.
Why Fixed Kale VPD Ranges Are Misleading
The old article gave ranges such as:
- 0.8–1.3 kPa,
- 1.0–1.8 kPa,
- 1.2–1.8 kPa
as though kale moves through fixed VPD requirements by growth stage.
There is not enough evidence to support that level of precision.
A kale plant-factory study directly compared relative humidity treatments from 45% to 85% and found that growth and glucosinolate responses differed, again showing that environmental optimization depends on the outcome being measured.
So instead of publishing one “perfect VPD,” it is better to monitor:
- temperature,
- humidity,
- irrigation,
- leaf condition,
- and atmospheric demand together.
Air VPD Is Not Exactly the Same as Leaf VPD
Most greenhouse controllers calculate VPD using:
air temperature + relative humidity
This is useful for routine management.
But the actual water-vapor gradient at a leaf also depends on leaf temperature.
Leaves can be:
warmer than the surrounding air under strong radiation,
or:
cooler because of transpiration.
So air-based VPD is an environmental indicator.
It is not a perfect description of every kale leaf’s microclimate.
PPFD, CO₂ and VPD Interact
The most useful interpretation comes from combining the measurements.
Consider two kale benches.
Both receive:
300 µmol/m²/s PPFD
Bench A
CO₂ remains stable.
Temperature is moderate.
Water supply is adequate.
Atmospheric demand is moderate.
Bench B
CO₂ decreases during peak photosynthesis.
Temperature rises sharply.
Humidity falls.
VPD rises.
The PPFD measurement is the same.
The physiological environment is not.
This is why:
PPFD should describe photon supply, not predicted growth.
DLI Adds the Time Dimension
Now imagine two different days.
Both reach a midday PPFD of:
500 µmol/m²/s
but one remains bright for eight hours.
The other experiences heavy cloud for most of the day.
The crop does not receive the same total photon exposure.
DLI captures that difference.
Therefore:
PPFD tells you intensity now.
DLI tells you accumulated light over the day.
Both can be useful.
How Measurement Priorities Change as Kale Develops
| Growth stage | Light question | CO₂ question | Climate / VPD question |
|---|---|---|---|
| Seedling | Is light uniform and sufficient for establishment? | Is CO₂ being depleted in the propagation area? | Is atmospheric demand excessive for the small root system? |
| Early vegetative | Is DLI supporting steady leaf expansion? | Does CO₂ fall as leaf area increases? | Is the crop maintaining water balance through the day? |
| Canopy expansion | Is PPFD distributed reasonably across the crop? | Is CO₂ uniform through the denser canopy? | Are heat, humidity and airflow becoming less uniform? |
| Pre-harvest | Is additional light still improving useful biomass or quality? | Is CO₂ limiting during the brightest periods? | Are atmospheric conditions supporting continued gas exchange without excessive stress? |
This is more useful than publishing a fixed number for every stage.
A Practical Monitoring Workflow
1. Measure PPFD at crop level
Keep sensor height and orientation consistent.
If plants become substantially taller, move the reference position with the canopy.
2. Track DLI when light varies
This is particularly useful in greenhouses.
Compare sunny and cloudy days rather than relying only on peak PPFD.
3. Measure CO₂ during active photosynthesis
A CO₂ reading taken before sunrise tells you little about whether carbon becomes limiting at midday.
Look at CO₂ while PPFD is high enough for active photosynthesis.
4. Record temperature and humidity
Use them to interpret atmospheric demand and VPD trends.
5. Compare the measurements on the same timeline
For example:
09:00 — PPFD rising / CO₂ stable / VPD low
12:00 — PPFD high / CO₂ falling / VPD increasing
15:00 — PPFD high / CO₂ recovering / VPD high
18:00 — PPFD falling / CO₂ rising / VPD declining
This tells you much more than three unrelated spot readings.
Why Environmental Logging Matters
Protected environments change continuously.
Lighting schedules change PPFD.
Photosynthesis affects CO₂.
Temperature influences humidity and VPD.
Ventilation changes all three.
Irrigation changes plant water status.
If those parameters are logged together, growers can identify patterns that would be invisible from one measurement.
But an important distinction remains:
a pattern is not proof of cause.
If growth slows during a high-VPD period, VPD may be involved.
But the same period may also include:
- higher temperature,
- lower DLI,
- irrigation problems,
- nutrient changes,
- root stress,
- or disease.
Measurement helps narrow the question.
It does not automatically prove the answer.
Kale Quality Adds Another Layer
Kale is unusual among common leafy crops because growers may value both:
biomass
and:
phytochemical composition.
This is important when interpreting environmental research.
A treatment that produces the greatest fresh weight does not necessarily produce the highest:
- glucosinolate concentration,
- anthocyanins,
- phenolics,
- or antioxidant activity.
Research on kale repeatedly shows trade-offs between environmental treatments, biomass and nutritional quality.
Therefore, the phrase:
“optimal growing condition”
should always raise the question:
optimal for what?
Frequently Asked Questions
What PPFD does greenhouse kale need?
There is no universal value.
Controlled-environment kale studies have used PPFD values ranging from roughly 100 µmol/m²/s to several hundred µmol/m²/s depending on cultivar, plant age, spectrum and research objective.
Treat those values as experimental conditions, not universal prescriptions.
What DLI should kale receive?
There is no single DLI suitable for every kale crop.
Studies using 16-hour photoperiods at around 200–230 µmol/m²/s correspond to DLIs of roughly 11.5–13.2 mol/m²/day, while other production systems may operate differently.
Monitor crop response rather than converting one experiment into a universal target.
Does kale benefit from elevated CO₂?
Yes, kale can increase photosynthesis and biomass under elevated CO₂ when other resources are adequate.
However, the response varies with concentration, cultivar, temperature, humidity and production objective.
Is 1000 ppm CO₂ ideal for kale?
Not universally.
Some studies have found strong growth around 700–1000 ppm under specific plant-factory conditions, while higher concentrations affected nutritional composition differently.
That makes 700–1000 ppm a research context, not a universal kale specification.
Can more CO₂ compensate for low PPFD?
No.
CO₂ supplies carbon, but photons supply the energy required for photosynthesis.
Very low light cannot simply be replaced by higher CO₂.
Is high VPD always bad for kale?
No.
VPD represents atmospheric demand.
Problems arise when that demand becomes incompatible with crop water supply or physiological capacity.
Is low VPD always good?
No.
Very humid conditions can suppress transpiration and alter crop microclimate.
Temperature, airflow and water availability must also be considered.
Should I monitor relative humidity or VPD?
Both can be useful.
Relative humidity is a direct environmental measurement.
VPD combines humidity and temperature to provide more context about atmospheric evaporative demand.
Where should PPFD be measured?
At a crop-relevant position, normally close to canopy level.
Keep position and orientation consistent when comparing readings.
Where should CO₂ be measured?
Place the sensor where it reasonably represents the crop environment rather than immediately beside a vent or CO₂ source.
The Main Takeaway
Kale does not follow one universal:
PPFD + CO₂ + VPD recipe.
PPFD describes photon flux at a moment.
DLI describes accumulated photosynthetic light across the day.
CO₂ describes carbon availability.
VPD helps describe atmospheric water demand.
As kale develops, leaf area and canopy structure change.
The crop’s ability to use light and carbon changes.
And production priorities may shift between:
biomass, appearance, energy efficiency and nutritional quality.
So instead of asking:
“What are the perfect kale numbers?”
ask:
“What environment is the crop actually experiencing, and which variable may be limiting the production goal?”
Measure consistently.
Compare conditions over time.
Use research as context.
And avoid turning one experimental treatment into a universal growing rule.
Measuring Kale Light and Environment Together
For growers who need several measurements on the same timeline, AquaHorti AH-200 can record:
- PPFD / PAR
- DLI
- CO₂
- temperature
- humidity
- VPD
The purpose of multi-parameter monitoring is not to label one number automatically “good” or “bad.”
It is to show what happened to the crop environment at the same time, making it easier to compare lighting, ventilation and climate conditions across different periods.
AH-200 → /ah-200
References
Effects of Temperature, Relative Humidity, and Carbon Dioxide Concentration on Growth and Glucosinolate Content of Kale Grown in a Plant Factory — evaluated kale responses to temperature, humidity and CO₂ across multiple treatment levels.
Effects of Elevated CO₂ on Yield and Nutritional Quality of Kale and Spinach: A Meta-Analysis — found increased kale biomass under elevated CO₂ together with potential nutritional trade-offs.
Optimizing Carbon Dioxide Enrichment to Balance Yield, Functional Food Quality, and Economic Feasibility in Plant-Factory-Cultivated Kale — compared 400, 800 and 1200 µmol/mol CO₂ in two kale cultivars.
The Effect of Light Intensity During Cultivation and Postharvest Storage on Mustard and Kale Microgreen Quality — compared 150, 200 and 250 µmol/m²/s PPFD.
Morphological and Photosynthetic Parameters of Green and Red Kale Microgreens Cultivated Under Different Light Spectra — used approximately 230 µmol/m²/s and a DLI near 13.2 mol/m²/day.
Supplemental Red and Far-Red Light in White LED Systems Regulates Growth, Phytochemicals, and Antioxidant Accumulation in Indoor-Grown Lettuce and Kale — demonstrates that spectral composition can alter kale morphology and quality even at similar PPFD.