Parsley is a leafy culinary herb, but there is no scientifically established PAR, CO₂ and VPD recipe that applies to every cultivar and every stage of greenhouse production.
Light supplies the photons used for photosynthesis. CO₂ supplies carbon. Temperature and humidity shape the plant-air moisture environment, commonly described using vapor pressure deficit (VPD).
For parsley, published research gives us particularly useful information about DLI, temperature and CO₂.
The evidence for a precise parsley-specific VPD optimum is much weaker.
For that reason, a better greenhouse strategy is to use PAR, DLI, CO₂, temperature, humidity and VPD as measurements of the actual growing environment rather than treating them as rigid pass/fail targets.
Quick Reference
| Variable | What It Tells You | Useful Research Context |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop right now | Parsley studies use substantially different PPFD depending on photoperiod and production stage |
| DLI | Total PAR accumulated through the day | Greenhouse research has directly compared about 7 vs. 18 mol/m²/day; other studies span roughly 6–19 mol/m²/day |
| CO₂ | Carbon available for photosynthesis | Direct parsley research found strong responses when increasing from about 378 to 627 ppm |
| VPD | Atmospheric evaporative demand | Useful for environmental monitoring, but no validated parsley stage-specific optimum is established |
| Temperature | Strongly affects parsley growth | Direct greenhouse modeling shows temperature can be as important as, or more important than, DLI |
These numbers describe published experimental conditions.
They are not universal parsley requirements.
Why PAR, CO₂ and VPD Should Be Viewed Together
Photosynthesis requires both light and CO₂.
PAR supplies photon energy.
CO₂ supplies the carbon used to build carbohydrates and biomass.
Temperature and humidity influence transpiration, gas exchange and the plant-air moisture environment.
This means two parsley plants receiving the same PPFD can still experience very different growing conditions.
For example:
Greenhouse Zone A
- adequate PAR
- adequate CO₂
- moderate temperature
- stable humidity
Greenhouse Zone B
- the same PAR
- lower CO₂
- higher temperature
- greater atmospheric water demand
A PAR meter could show similar light in both locations.
The plants may not respond similarly.
That is why environmental measurements become more useful when they are viewed together.
PPFD and DLI Are Not the Same Measurement
PPFD tells you:
How much photosynthetically active light is reaching the parsley canopy right now?
It is measured in:
µmol/m²/s
DLI tells you:
How much photosynthetically active light accumulated during the whole day?
It is measured in:
mol/m²/day
For constant artificial lighting:
DLI = PPFD × light-hours × 0.0036
For example:
150 µmol/m²/s × 16 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
In a greenhouse, sunlight changes continuously, so logging PAR through the entire day is more useful than estimating DLI from one midday reading.
Parsley Has Strong Direct DLI Research
A 2019 greenhouse experiment grew:
Petroselinum crispum ‘Giant of Italy’
in nutrient-film technique hydroponic systems.
Researchers compared approximately:
7 mol/m²/day
with:
18 mol/m²/day
DLI.
After four weeks, parsley grown under the higher DLI produced approximately:
13.3 g more fresh mass
representing about:
120% higher fresh mass
than parsley grown under the lower DLI.
Dry mass also increased.
This is strong evidence that daily light can substantially influence parsley productivity.
Does That Mean Parsley Needs 18 DLI?
No.
A second major parsley study helps explain why.
In 2021, researchers grew hydroponic parsley under greenhouse DLIs ranging approximately from:
6.2 to 16.9 mol/m²/day
while also changing mean daily temperature.
In that experiment, DLI did not significantly affect parsley fresh mass within the tested range.
Temperature had a much stronger effect.
DLI mainly influenced dry matter concentration rather than fresh mass.
This may appear to contradict the 2019 experiment.
It does not.
The two studies used different:
- plant densities
- propagation periods
- environmental conditions
- experimental structures
The authors themselves suggested that these differences could explain why parsley responded differently to DLI.
This is exactly why one DLI experiment should not become a universal crop requirement.
Earlier Research Also Found Strong DLI Responses
The same 2021 paper reviewed earlier parsley studies.
One experiment reported that increasing DLI from approximately:
2 to 19 mol/m²/day
increased parsley fresh mass roughly fourfold after four weeks under its specific conditions.
Another reported the approximately:
120% fresh-mass increase
when DLI increased from 7 to 18 mol/m²/day.
Taken together, these studies strongly support the importance of avoiding very low daily light in productive parsley systems.
They do not identify one universal optimum.
A Practical DLI Reference
For productive greenhouse parsley, approximately:
12–18 mol/m²/day
is a reasonable research-based reference region for evaluating the crop environment.
This should be treated as:
a measurement reference
rather than:
a parsley specification.
A 2024 greenhouse study of ‘Giant of Italy’ parsley, for example, used a target DLI of:
13 mol/m²/day
with a 16-hour photoperiod.
This provides another successful greenhouse production example in the low-to-mid teens.
Very Low DLI Can Limit Commercial Production
Parsley has also been studied at DLIs around:
2.9–3.8 mol/m²/day.
Researchers discussing these experiments described those values as very low and unsuitable for efficient commercial production.
That distinction matters.
Parsley can survive under relatively low light.
But:
survival is not the same thing as commercially efficient biomass production.
When evaluating a greenhouse, DLI helps distinguish between those two questions.
Seedlings and Young Parsley
Young parsley should not automatically receive the light environment used for mature harvest-stage plants.
Parsley germination and early growth can also be slow compared with many leafy crops.
A controlled parsley experiment studying microgreens and baby greens germinated seeds in darkness before introducing production lighting.
After emergence, plants were grown at approximately:
300 µmol/m²/s
for:
12 hours
which corresponds to approximately:
13.0 mol/m²/day.
These plants were being produced as microgreens and baby greens.
They were not mature greenhouse parsley.
Therefore, the experiment should not be interpreted as:
“All parsley seedlings need 300 PPFD.”
It demonstrates one successful young-parsley production environment.
What Should You Measure During Early Growth?
Ask:
- Is emergence uniform?
- Are seedlings receiving similar light across the tray?
- Are some plants stretching more than others?
- Are greenhouse structural shadows affecting certain areas?
- Is supplemental lighting also increasing temperature?
Measure PPFD at actual plant height.
Do not rely only on:
- fixture wattage
- lamp distance
- visual brightness
As the crop grows, move the sensor so the measurement continues to represent the canopy.
Vegetative Growth
Parsley is normally harvested for foliage.
As the plant develops:
- leaf number increases
- canopy area expands
- total light interception increases
- water demand changes
- carbon demand increases
At this stage, DLI becomes increasingly useful.
One midday PPFD reading cannot tell you whether a greenhouse location received strong usable light throughout the day.
Measure several locations.
Useful comparisons include:
- greenhouse center vs. edge
- shaded vs. unshaded areas
- different benches
- different times of day
Temperature Is Especially Important for Parsley
One of the clearest findings in parsley research is that light cannot be interpreted without temperature.
The 2021 greenhouse modeling experiment exposed parsley to mean daily temperatures from approximately:
10 to 27 °C
and different DLIs.
Parsley fresh mass increased strongly as temperature increased from approximately:
10 to 22 °C
within the observed range.
The researchers calculated a higher theoretical temperature optimum from their model, while earlier parsley research produced a fresh-mass optimum closer to:
23 °C.
The exact optimum differed between studies because production conditions differed.
The practical lesson is more useful than the exact number:
Parsley response to DLI depends partly on temperature.
Why This Matters in a Greenhouse
Imagine increasing supplemental light during winter.
If the greenhouse is also unusually cold, the crop may not respond to the additional photons in the same way it would at a more favorable temperature.
Conversely, increasing light during warm conditions can increase growth but also change water demand.
This is why:
PAR + DLI + temperature
should be reviewed together.
CO₂ Has Direct Parsley-Specific Evidence
Parsley also has direct evidence for CO₂ enrichment.
A Food Chemistry study compared parsley grown at approximately:
378 ± 25 ppm CO₂
with parsley grown at:
627 ± 24 ppm CO₂.
The plants were grown under controlled environmental conditions.
Elevated CO₂ increased biomass production in parsley and dill to approximately:
1.5 times the ambient-CO₂ treatment.
This is direct evidence that parsley can respond strongly to moderately elevated CO₂.
CO₂ Changed More Than Biomass
The elevated-CO₂ experiment also examined a wide range of metabolites and nutritional compounds.
Elevated CO₂ increased several characteristics including:
- soluble sugars
- starch
- organic acids
- some essential amino acids
- unsaturated fatty acids
- total phenolics
- total flavonoids
- vitamins A and E
The researchers also reported increased total antioxidant capacity.
This is useful because it demonstrates that CO₂ can affect:
crop chemistry as well as crop mass.
Does Parsley Therefore Need 627 ppm CO₂?
No.
The experiment compared approximately:
378 ppm
with:
627 ppm.
It did not test every possible concentration.
Therefore, it cannot tell us whether:
600 ppm, 700 ppm, 800 ppm or another concentration
would provide the best commercial result.
The correct conclusion is:
Parsley has demonstrated positive biomass and metabolic responses to moderate CO₂ enrichment around 600–650 ppm under controlled experimental conditions.
That is different from saying:
Parsley requires 600–650 ppm CO₂.
CO₂ Monitoring Is Useful Even Without Enrichment
A greenhouse does not need a CO₂ injection system for CO₂ monitoring to provide useful information.
Measure CO₂ near the crop.
Then compare it with PAR.
Ask:
Does CO₂ change when the greenhouse becomes bright and the parsley canopy becomes photosynthetically active?
Watch what happens during:
- sunrise
- peak sunlight
- supplemental lighting
- greenhouse closure
- ventilation
This gives a more realistic view of the carbon environment than assuming greenhouse CO₂ always equals outdoor concentration.
CO₂ and PAR Should Be Viewed on the Same Timeline
Suppose the greenhouse records strong PPFD.
If CO₂ is also adequate, the crop has both photon energy and carbon available.
If CO₂ falls substantially during that same period, the physiological environment is different.
Likewise, adding CO₂ during extremely low light may not produce the same response as enrichment during stronger photosynthesis.
This is why simultaneous monitoring is more informative than isolated measurements.
VPD: This Is Where We Should Remain Conservative
The old AquaHorti article assigned parsley stage-specific VPD ranges such as:
0.8–1.3 kPa
1.0–1.8 kPa
and:
1.2–1.8 kPa.
Current parsley-specific literature does not provide enough direct experimental evidence to validate those values as universal stage-specific optima.
For parsley, VPD should therefore be treated primarily as:
an environmental diagnostic variable.
What Does VPD Tell You?
VPD describes atmospheric evaporative demand.
It is determined mainly by:
- temperature
- humidity
As air becomes warmer or drier, VPD generally rises.
That can increase the atmospheric demand for water from the plant.
But the crop response also depends on:
- root-zone moisture
- plant size
- airflow
- temperature
- duration of the condition
This is why one VPD number should not be interpreted without context.
When VPD Becomes High
A rising VPD should prompt questions such as:
- Is greenhouse temperature increasing rapidly?
- Has relative humidity fallen?
- Is substrate moisture sufficient?
- Are leaves losing turgor?
- Is irrigation demand increasing?
- Is the high VPD lasting minutes or hours?
The measurement is valuable because it identifies a change in the atmospheric environment.
It does not automatically diagnose the cause of poor growth.
Very Low VPD Is Not Automatically Ideal
Low VPD normally corresponds to humid air.
That reduces atmospheric evaporative demand.
But persistently high humidity can create other production concerns, including:
- condensation
- slow canopy drying
- disease-favorable conditions
Therefore, the correct strategy is not:
Keep VPD as low as possible.
It is:
monitor atmospheric conditions and avoid prolonged extremes.
Why We Should Not Invent a Parsley VPD Target
Parsley studies frequently report temperature and relative humidity as part of their experimental conditions.
For example, one young-parsley study used approximately:
24/18 °C day/night
and:
70/80% RH day/night.
Those numbers describe the experiment.
They do not prove that the corresponding VPD is optimal.
Converting ordinary experimental conditions into a precise “parsley VPD requirement” would overstate the evidence.
Mature and Pre-Harvest Parsley
The old AquaHorti article stated that mature parsley needed approximately:
450–650 µmol/m²/s PPFD
18–22 mol/m²/day DLI
500–650 ppm CO₂
and:
1.2–1.8 kPa VPD.
Current evidence does not support presenting that combination as a universal mature-parsley requirement.
The evidence is better summarized this way:
- higher DLI has substantially increased parsley biomass in several greenhouse studies
- successful productive systems have operated around the low-to-high teens DLI
- temperature strongly changes parsley growth
- moderate CO₂ enrichment around 600–650 ppm has produced strong biomass and metabolic responses
- no universal parsley-specific VPD optimum is established
That is both more accurate and more useful.
A Practical Greenhouse Monitoring Workflow
Measure PAR at Crop Height
Position the sensor where the parsley leaves actually receive light.
Check Several Locations
Measure:
- center
- edges
- structurally shaded positions
- multiple benches
Measure DLI
When sunlight contributes to crop light, record PAR through the day.
Compare:
- sunny vs. cloudy days
- different seasons
- different greenhouse positions
- supplemental-light schedules
Monitor CO₂
Measure near the crop and compare CO₂ with the same time period as PAR.
Track Temperature and Humidity
Use them to understand VPD and the changing plant-air environment.
Compare the Measurements Together
A pattern such as:
PAR rises → temperature rises → VPD rises → CO₂ falls
contains much more useful information than four unrelated measurements.
Compare With the Crop
Record:
- fresh biomass
- leaf number
- plant height
- foliage density
- color
- harvest timing
This allows environmental data to become production data.
Practical Research-Based Reference Points
DLI
For productive greenhouse parsley:
approximately 12–18 mol/m²/day
is a useful literature-based reference region.
Direct greenhouse research found approximately 120% greater fresh mass at 18 DLI than at 7 DLI, while other successful parsley studies have used targets around 10–13 DLI.
Do not treat 12–18 as a universal requirement.
CO₂
Moderate enrichment around:
600–650 ppm
is a useful parsley-specific research reference.
A direct study comparing approximately 378 with 627 ppm found strong biomass and metabolic responses.
Do not treat 627 ppm as an exact optimum.
VPD
There is currently insufficient parsley-specific evidence to publish one universal stage-specific VPD target.
Use VPD to monitor atmospheric water demand and identify environmental changes or extremes.
Temperature
Parsley growth responds strongly to temperature.
Research shows that temperature can sometimes explain more variation in fresh mass than DLI within a moderate light range.
Always interpret light alongside temperature.
A Better Way to Think About Parsley Measurements
Instead of asking:
What is the perfect PAR number?
ask:
How much usable light reaches the crop now?
Instead of asking:
What is the perfect DLI?
ask:
How much usable light accumulated today?
Instead of asking:
What exact CO₂ number does parsley require?
ask:
Is carbon availability changing during active photosynthesis?
Instead of asking:
What is the perfect VPD?
ask:
How is atmospheric water demand changing, and how is the crop responding?
That is a much stronger greenhouse measurement framework.
Final Takeaway
Greenhouse parsley does not have one scientifically established PAR, CO₂ and VPD recipe for every growth stage.
Published evidence supports several important conclusions.
DLI can strongly influence parsley biomass.
A direct greenhouse experiment found approximately 120% greater fresh mass at 18 mol/m²/day than at 7 mol/m²/day.
However, another greenhouse study covering approximately 6.2–16.9 mol/m²/day found that temperature had a stronger influence on parsley fresh mass than DLI within its particular production system.
That tells us that:
light and temperature must be interpreted together.
CO₂ also has strong parsley-specific evidence.
Increasing CO₂ from approximately 378 to 627 ppm increased biomass to about 1.5 times the ambient treatment and altered several nutritional and phytochemical characteristics.
This supports CO₂ as a meaningful parsley environmental variable without proving one universal enrichment target.
For VPD, current parsley-specific evidence is not strong enough to justify a rigid growth-stage table.
The better greenhouse strategy is therefore:
Measure PAR at crop height.
Record DLI across the whole day.
Monitor CO₂ during active photosynthesis.
Track temperature, humidity and VPD together.
Then compare these measurements with actual parsley growth and harvest quality.
That provides a much stronger basis for greenhouse parsley production than unsupported stage-by-stage target numbers.
References
Currey, Walters & Flax. Nutrient Solution Strength Does Not Interact with the Daily Light Integral to Affect Hydroponic Cilantro, Dill, and Parsley Growth and Tissue Mineral Nutrient Concentrations. Agronomy, 2019.
Walters & Lopez. Modeling Growth and Development of Hydroponically Grown Dill, Parsley, and Watercress in Response to Photosynthetic Daily Light Integral and Mean Daily Temperature. PLOS ONE, 2021.
Saleh et al. CO₂ Enrichment Can Enhance the Nutritional and Health Benefits of Parsley (Petroselinum crispum L.) and Dill (Anethum graveolens L.). Food Chemistry, 2018.
Li et al. Optimizing Sowing Density for Parsley, Cilantro, and Sage in Controlled Environment Production: Balancing Productivity and Plant Quality. HortTechnology, 2024.
El-Nakhel et al. Mineral and Antioxidant Attributes of Petroselinum crispum at Different Stages of Ontogeny: Microgreens vs. Baby Greens. Agronomy, 2021.
Currey et al. Substrate Volumetric Water Content Controls Growth and Development of Containerized Culinary Herbs. Agronomy, 2019.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and checking greenhouse light distribution, see AquaHorti AH-Quantuv.
For recording 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.