Collard greens (Brassica oleracea var. viridis) are productive leafy Brassicas capable of growing under substantial light, but there is no scientifically established PAR, CO₂ and VPD recipe that applies to every greenhouse and every growth stage.
Published collard research gives us useful evidence about:
- Daily Light Integral (DLI)
- photoperiod
- CO₂ enrichment
- light spectrum
- irrigation
- salinity
- photosynthetic acclimation
But the evidence does not support simple rules such as:
high PAR creates tough leaves
or:
high VPD directly causes fiber accumulation.
Leaf texture is influenced by a much larger system involving:
- cultivar
- leaf age
- temperature
- water status
- mineral nutrition
- light
- harvest maturity
For greenhouse collards, the stronger strategy is therefore to measure the actual environment and connect it with crop response.
Quick Reference
| Variable | What It Tells You | What Collard Research Supports |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop now | Collards can use substantial light, but PPFD must be interpreted together with photoperiod |
| DLI | Total photosynthetic light accumulated through the day | Baby-leaf collard research directly tested 6–30 mol/m²/day; yield gains began diminishing above about 24 DLI |
| CO₂ | Carbon available for photosynthesis | Direct collard research compared approximately 350 vs 700 ppm and showed strong growth responses plus photosynthetic acclimation |
| VPD | Atmospheric evaporative demand | Useful for monitoring water demand, but no validated collard stage-specific optimum is established |
| Root-zone water | Water available to support growth | Direct greenhouse research shows irrigation level substantially affects biomass and leaf area |
| Spectrum | Distribution of wavelengths | Red-blue light can increase collard leaf dimensions, while blue light can alter glucosinolate profiles |
| Harvest stage | Determines leaf size and maturity | Baby leaf, microgreen and mature collard results should not be treated as interchangeable |
These are research references rather than universal crop specifications.
1. Collard Greens Are Not Simply “Tougher Kale”
Collards and kale are closely related members of Brassica oleracea, but they are different crop types.
Collards have been selected for characteristics such as:
- large leaves
- broad leaf blades
- relatively smooth leaf surfaces
- repeated leaf harvest
Their morphology, commercial harvest stage and quality objectives differ from many kale cultivars.
Therefore, kale lighting data can provide context, but collard-specific research should be used whenever possible.
2. PAR and DLI Answer Different Questions
PPFD tells you how much photosynthetically active light reaches the crop at one moment.
It is measured in:
µmol/m²/s
It answers:
How much photosynthetic light is reaching the collard canopy right now?
DLI measures the amount of PAR accumulated during the entire day.
It is expressed in:
mol/m²/day
It answers:
How much photosynthetic light did the collards receive today?
For constant lighting:
DLI = PPFD × photoperiod × 0.0036
For example:
200 µmol/m²/s × 16 h
≈ 11.5 mol/m²/day
300 µmol/m²/s × 16 h
≈ 17.3 mol/m²/day
400 µmol/m²/s × 16 h
≈ 23.0 mol/m²/day
In a greenhouse, natural PPFD is constantly changing, so continuous PAR logging is preferable when the real question concerns daily light.
3. Collards Have Direct DLI Research
One useful controlled-environment experiment studied baby-leaf:
Collard ‘Flash’
at five DLIs:
6
12
18
24
and:
30 mol/m²/day.
The photoperiod was fixed at:
20 hours.
Collards were harvested at the baby-leaf stage rather than as mature full-size plants.
This distinction is important.
4. Increasing DLI Strongly Increased Baby-Leaf Yield
At Day 12, estimated collard fresh yield increased approximately as follows:
6 DLI → 0.22 lb/ft²
12 DLI → 0.35 lb/ft²
18 DLI → 0.53 lb/ft²
24 DLI → 0.71 lb/ft²
30 DLI → 0.80 lb/ft²
This is strong direct evidence that daily light can substantially affect collard productivity.
But the response was not perfectly linear.
5. Yield Gains Began to Diminish Above About 24 DLI
Collard yield continued to increase at:
30 DLI
but the increase from:
24 → 30 mol/m²/day
was relatively small compared with earlier increases.
The researchers specifically noted that the economic benefit of lighting above approximately:
24 mol/m²/day
might be limited for baby-leaf collards.
That is a much more useful conclusion than:
“More light always means better collards.”
6. Higher DLI Also Shortened Time to Harvestable Leaf Height
The same experiment measured how quickly collard leaves reached approximately:
4 inches / 10 cm
in height.
Approximate days required were:
6 DLI → 14.8 days
12 DLI → 14.2 days
18 DLI → 13.0 days
24 DLI → 12.2 days
30 DLI → 12.0 days
So increasing DLI could shorten the production cycle.
But again, most of the benefit had already been achieved by approximately:
24 DLI.
7. This Does Not Mean Mature Collards Require 24 DLI
The experiment used:
very young baby-leaf collards
at very high plant density.
That is not the same as:
- mature field-size collards
- long-cycle greenhouse plants
- repeated outer-leaf harvest
Therefore, approximately 24 DLI should be described as:
a useful baby-leaf research reference
rather than:
the mature collard optimum.
8. Microgreen Collards Give Us Another Useful — but Different — Reference
A separate experiment studied:
Collard Greens ‘Vates’
as microgreens.
Researchers compared:
14 vs 21 mol/m²/day
and:
16-hour vs 24-hour photoperiods.
Fresh weight ranged approximately from:
20.4 g
to:
24.7 g
across treatments.
Both DLI and photoperiod influenced production.
Again:
microgreens are not mature collards.
These data demonstrate collard light responsiveness, but exact values should not be transferred directly to full-size greenhouse plants.
9. Higher DLI Also Increased Dry-Matter Percentage in Microgreens
For Vates collard microgreens, increasing DLI from:
14 → 21 mol/m²/day
increased dry-matter percentage.
For example, under a 16-hour photoperiod:
14 DLI → about 9.9% dry matter
21 DLI → about 12.0% dry matter.
Under continuous light, dry matter increased even further.
This is important because:
fresh weight
and:
dry matter
do not always respond identically.
10. Higher DLI Can Reduce Lighting Resource Efficiency
The same microgreen experiment calculated electrical-use efficiency.
For collard greens, increasing DLI increased electricity cost per gram of fresh biomass.
That means:
maximum biological yield
and:
maximum lighting efficiency
are not necessarily the same production target.
This is especially important for greenhouse supplemental lighting.
11. Do Not Translate Young-Plant Research Into a Mature Leaf Texture Rule
Baby-leaf and microgreen experiments tell us a great deal about:
- biomass
- crop timing
- morphology
- dry matter
They do not prove that mature collards become fibrous at a specific PPFD or DLI.
Therefore, statements such as:
“High PAR causes collard fiber buildup.”
should not be presented as research-established facts.
Leaf toughness needs to be studied directly if texture is the production question.
12. Light Spectrum Also Matters
A recent study directly included:
B. oleracea var. viridis — collard
alongside kale and cabbage.
Plants were grown under:
- red LEDs
- blue LEDs
- red + blue LEDs
- a control treatment
The light spectrum affected both:
plant morphology
and:
glucosinolate composition.
13. Red + Blue Increased Collard Leaf Dimensions
For the collard cultivar studied, combined:
red + blue
lighting increased:
- leaf length
- leaf width
relative to several other spectral treatments.
This demonstrates that leaf expansion cannot be predicted from PPFD alone.
Two fixtures delivering similar PAR can still produce different leaf morphology.
14. Blue Light Changed Collard Glucosinolate Profiles
Blue light strongly affected several glucosinolates in the collard cultivar.
The study found high concentrations of compounds such as:
- gluconapin
- glucobrassicin
- glucoraphanin
- gluconasturtiin
under blue-light conditions.
This matters because Brassica quality involves more than leaf size.
Light environment can also affect:
phytochemical composition.
15. Therefore “Largest Leaf” and “Highest Nutritional Quality” May Be Different Targets
Red-blue treatment favored some growth characteristics.
Blue light strongly influenced several glucosinolates.
That creates the same tradeoff seen in many controlled-environment crops:
the lighting treatment producing the largest leaves does not necessarily maximize every nutritional compound.
Before defining “best light,” determine what you are optimizing.
16. CO₂ Has Direct Collard-Specific Research
Collards have also been studied directly under:
350 ppm
and:
700 ppm CO₂.
In controlled-environment experiments, plants received approximately:
1000 µmol/m²/s PAR
for:
12 hours per day.
Day/night temperatures were approximately:
23/17°C.
This corresponds to an extremely high experimental DLI of approximately:
43.2 mol/m²/day.
That is a physiology experiment — not a commercial greenhouse recommendation.
17. Elevated CO₂ Strongly Increased Collard Biomass
When plants were approximately:
23–27 days old
shoot dry mass was roughly:
10 g/plant
under about 350 ppm CO₂
versus approximately:
20 g/plant
under about 700 ppm CO₂
in the controlled experiments.
This demonstrates a strong growth response to elevated CO₂ under abundant light.
But the physiological response became more complicated over time.
18. Elevated CO₂ Also Caused Photosynthetic Down-Regulation
Collards continuously grown at approximately:
700 ppm CO₂
eventually showed lower photosynthetic capacity per unit leaf area when compared under standardized measurement conditions.
Researchers linked this response to the accumulation of:
- starch
- soluble sugars
in leaves.
In other words:
the plant had more carbon than it could immediately use or export.
19. This Is a Critical CO₂ Lesson
Elevated CO₂ can increase growth.
But the leaf’s photosynthetic machinery can also acclimate to prolonged enrichment.
That means:
CO₂ response is dynamic.
It cannot be summarized as:
“700 ppm is always better than 350 ppm.”
The plant’s ability to use additional carbon depends on:
- light
- growth rate
- sink demand
- developmental stage
- nutrient status
20. Daily Light Changed the CO₂ Acclimation Response
The USDA collard experiments found that photosynthetic down-regulation under elevated CO₂ was related to the amount of PAR received previously.
After higher-light days, sugar accumulation and photosynthetic feedback inhibition became more apparent.
This directly demonstrates an important greenhouse principle:
PAR and CO₂ interact.
CO₂ should not be interpreted independently from daily irradiance.
21. Does That Mean 700 ppm Is the Best Collard CO₂ Level?
No.
The research shows that:
350 vs 700 ppm produces important physiological and growth differences.
It does not establish:
700 ppm as the universal commercial optimum.
The experiment used unusually strong controlled lighting and was designed to study photosynthetic acclimation.
Use 700 ppm as:
a collard-specific research reference
rather than:
a greenhouse setpoint.
22. CO₂ Monitoring Is Useful Even Without Enrichment
A CO₂ sensor can still answer a useful question:
Does crop-zone CO₂ change when light becomes strong?
Monitor CO₂ during:
- morning light increase
- peak sunlight
- supplemental-light periods
- greenhouse closure
- ventilation
Then compare those values with PAR.
A timeline can reveal whether the carbon environment changes when photosynthetic demand rises.
23. What About Water Availability?
Collard-specific greenhouse research strongly supports the importance of irrigation.
One experiment with:
Collard ‘Georgia’
tested irrigation equivalent to:
55%
70%
85%
and:
100%
of pot water capacity
combined with several irrigation-water salinity levels.
The treatments affected:
- plant height
- stem diameter
- leaf number
- fresh mass
- dry mass
- leaf area
24. Full Water Replacement Produced High Biomass in the Irrigation Study
One of the strongest biomass treatments combined:
100% pot water capacity
with one of the tested irrigation-water salinity conditions.
It produced approximately:
206.7 g fresh matter
25.5 g dry matter
and:
3044 cm² leaf area.
Meanwhile, maximum water-use efficiency occurred under another treatment around:
85% pot capacity.
Again, we see two different production targets:
maximum biomass
versus:
maximum water-use efficiency.
25. This Does Not Support “Dry Air Makes Collard Midribs Fibrous”
The irrigation study directly demonstrates that:
water availability affects collard growth.
But it did not establish:
high VPD → fibrous midribs
or:
early dryness permanently determines leaf texture.
Those are much stronger causal claims.
Without direct texture, lignin or fiber measurements, they should not be published as scientific conclusions.
26. Salinity Is Also Part of Root-Zone Stress
The same research showed that irrigation-water salinity interacted with irrigation volume.
Higher salinity affected several collard morphological characteristics.
This means poor crop performance under an apparently adequate PAR environment could originate from:
- insufficient water
- excessive EC
- salinity
- root-zone stress
rather than light itself.
27. What Does VPD Tell Us?
VPD describes atmospheric evaporative demand.
It depends mainly on:
- temperature
- relative humidity
When temperature rises or humidity falls, VPD generally increases.
That can increase:
- transpiration demand
- root-zone water demand
- substrate drying
VPD therefore provides useful environmental context.
28. But There Is No Validated Collard VPD Target Table
The old article gave precise targets such as:
0.5–0.8 kPa
0.7–1.1 kPa
0.9–1.3 kPa
for different stages.
Current collard-specific evidence does not validate these as universal optimum ranges.
For collards, the research base is much stronger for:
- irrigation
- salinity
- CO₂
- light
than for experimentally identifying an ideal VPD.
Therefore, the most scientifically defensible role for VPD is:
environmental monitoring.
29. High VPD Does Not Automatically Mean Tough Leaves
A high VPD tells you atmospheric water demand is relatively strong.
It does not directly measure:
- leaf fiber
- lignin
- midrib toughness
- sweetness
Those characteristics depend on multiple variables.
If high VPD coincides with tough leaves, you still need to determine whether other factors changed, such as:
- temperature
- leaf age
- root-zone moisture
- cultivar
- harvest maturity
Correlation should not be presented as causation.
30. Very Low VPD Is Not Automatically Better
Very humid air lowers VPD.
But persistently humid greenhouse conditions may also contribute to:
- condensation
- slow canopy drying
- disease-favorable conditions
Therefore, the objective should not be:
minimum VPD.
The better goal is:
stable water relations without prolonged atmospheric extremes.
31. Temperature Must Be Considered
Collards are cool-season Brassicas.
Temperature changes:
- growth rate
- respiration
- transpiration
- crop timing
- leaf development
and directly changes VPD.
Therefore:
PPFD + VPD without temperature context is incomplete.
The same 500 µmol/m²/s PPFD can create very different plant conditions at:
18°C
and:
30°C.
32. Seedling and Early Establishment
During early growth, focus on:
- uniform germination
- light distribution
- compact morphology
- root-zone moisture
- temperature
Do not automatically apply mature-plant lighting.
Young collards intercept much less total light than a mature canopy.
Baby-leaf studies demonstrate strong responses to DLI, but those values should remain specific to that production stage.
33. Active Leaf Expansion
As collards develop:
- leaf area increases
- canopy photosynthetic demand increases
- self-shading develops
- water use increases
At this stage, measure multiple points across the crop.
Use:
PPFD
for instantaneous mapping
and:
DLI
for total daily exposure.
Monitor CO₂ during the same periods.
34. Mature Leaf Production
For mature collards, the production target may include:
- leaf area
- fresh biomass
- marketable leaf count
- leaf tenderness
- color
- nutritional quality
- regrowth after harvest
Current research does not justify one mature-stage combination such as:
350–550 PPFD + 800–1200 ppm CO₂ + 0.9–1.3 kPa VPD.
Each of those parameters needs context.
35. Repeated Harvest Changes the Problem
Collards are often harvested by removing outer leaves while allowing the plant to continue growing.
That means:
- plant age continues increasing
- canopy structure changes
- sink demand changes
- root system becomes larger
- new leaves develop under a different canopy environment
A long-cycle plant should therefore be evaluated through:
environmental trends over time
rather than one fixed stage chart.
36. Leaf Texture Should Be Measured Directly
If tenderness is commercially important, record it.
Possible practical comparisons include:
- leaf age
- harvest position
- petiole/midrib thickness
- sensory tenderness
- cooked texture
Then compare those observations with:
- DLI
- temperature
- water status
- harvest age
This is much more defensible than assuming one VPD number determines fiber.
37. A Practical Greenhouse Measurement Workflow
Step 1 — Measure PAR at Canopy Height
Measure where collard leaves actually receive light.
Step 2 — Map Several Locations
Check:
- greenhouse center
- edges
- structural shadows
- upper canopy
- inner shaded leaves
Step 3 — Record DLI
Compare:
- sunny vs. cloudy days
- seasons
- greenhouse zones
- supplemental-light schedules
Step 4 — Monitor CO₂
Compare CO₂ with PAR during active photosynthesis.
Step 5 — Track Temperature and Humidity
Use these to understand VPD.
Step 6 — Track Root-Zone Water
Record:
- irrigation
- substrate moisture
- nutrient-solution EC where relevant
Step 7 — Record the Crop Response
Track:
- fresh biomass
- leaf area
- marketable leaf count
- harvest interval
- regrowth
If tenderness matters, measure it directly.
38. Practical Research-Based Reference Points
Baby-Leaf DLI
Direct collard research tested:
6–30 mol/m²/day.
Yield increased strongly through:
24 mol/m²/day
with smaller additional gain from:
24 → 30.
This is useful for baby-leaf systems.
It is not a universal mature-collard requirement.
Microgreen DLI
‘Vates’ collard microgreens were successfully studied at:
14 and 21 mol/m²/day.
Higher DLI increased fresh weight and dry-matter percentage but reduced electrical-use efficiency.
Again, this applies to microgreens.
CO₂
Direct collard research at:
350 vs 700 ppm
shows substantial effects on biomass and photosynthetic physiology.
Approximately 700 ppm is therefore a useful experimental reference.
It is not a validated commercial optimum.
VPD
There is currently insufficient collard-specific evidence to publish a universal growth-stage VPD target.
Use VPD to understand atmospheric water demand.
Water
Direct greenhouse research confirms that irrigation amount and water quality can strongly affect:
- biomass
- leaf area
- morphology
Interpret VPD together with actual root-zone water availability.
39. A Better Way to Think About Collard Measurements
Instead of asking:
What PPFD makes collards tender?
ask:
How much light is the crop receiving, and how does it affect biomass and crop timing?
Instead of:
What VPD prevents fiber?
ask:
How strong is atmospheric water demand, and is the root zone supplying sufficient water?
Instead of:
What CO₂ number makes larger leaves?
ask:
How does CO₂ change photosynthesis and biomass under the actual light environment?
Then measure tenderness separately if tenderness is the commercial goal.
Final Takeaway
Greenhouse collard greens do not have one scientifically established PAR, CO₂ and VPD recipe for every stage of growth.
But collards have several strong crop-specific research findings.
DLI strongly affects young collard productivity.
A direct baby-leaf experiment tested:
6, 12, 18, 24 and 30 mol/m²/day.
Estimated fresh yield increased from approximately:
0.22 lb/ft² at 6 DLI
to:
0.71 lb/ft² at 24 DLI
and:
0.80 lb/ft² at 30 DLI.
The smaller gain above 24 DLI suggests diminishing biological and potentially economic returns.
Higher DLI also shortened the time required to reach approximately 4-inch leaf height from:
14.8 days at 6 DLI
to:
12.2 days at 24 DLI.
CO₂ also matters.
Direct collard research comparing approximately:
350 and 700 ppm
showed strong biomass responses under high light.
But long-term exposure to elevated CO₂ also caused photosynthetic down-regulation associated with carbohydrate accumulation.
That demonstrates why:
more CO₂ is not simply a linear photosynthesis switch.
Spectrum matters.
Recent collard research found red + blue lighting increased leaf dimensions, while blue light substantially altered glucosinolate profiles.
Water matters.
Direct greenhouse irrigation research shows that root-zone water availability and water quality can strongly affect collard biomass and leaf area.
But current research does not justify claims that:
high VPD directly causes fibrous leaves
or:
early dryness permanently determines collard texture.
For VPD, the stronger approach is to use it as a measure of atmospheric water demand and interpret it together with:
- temperature
- irrigation
- root-zone moisture
- crop response
The better greenhouse strategy is therefore:
Measure PAR at the actual canopy.
Use DLI to understand total daily light.
Monitor CO₂ during active photosynthesis.
Track temperature, humidity and VPD together.
Measure root-zone water conditions.
Then compare those measurements with the production trait that actually matters:
yield, crop timing, leaf area, nutritional quality or tenderness.
That provides a much stronger basis for greenhouse collard production than unsupported stage-by-stage environmental targets.
References
Gagne, C. & Mattson, N. Increasing Yields of Baby Leaf Vegetables. Greenhouse Product News, 2019.
Gagne, C. The Effects of Daily Light Integral on the Growth and Development of Baby Leaf Vegetables. Cornell University.
Continuous Lighting Can Improve Yield and Reduce Energy Costs While Increasing or Maintaining Nutritional Contents of Microgreens. Frontiers in Plant Science, 2022.
Bunce, J.A. & Sicher, R.C. Daily Irradiance and Feedback Inhibition of Photosynthesis at Elevated Carbon Dioxide Concentration in Brassica oleracea. Photosynthetica, 2004.
The Nutritional Value, Biochemical Traits, and Growth of Brassica oleracea Grown Under Red, Blue, and Combined Red–Blue LED Lighting. Plants, 2025.
Viana et al. Saline Irrigation Water Indices Affect Morphophysiological Characteristics of Collard. Horticultura Brasileira, 2021.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and checking greenhouse light distribution, see AquaHorti AH-Quantuv.
For recording changing greenhouse PAR throughout 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 over time, see AquaHorti AH-200.