Dill is a fast-growing culinary herb, but there is no scientifically established PAR, CO₂ and VPD recipe that applies to every greenhouse, cultivar and stage of production.
Light provides 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 dill, published research gives us particularly useful evidence about the interaction between:
DLI and temperature
and direct evidence that:
moderate CO₂ enrichment can increase biomass and alter nutritional composition.
The evidence for one exact dill-specific VPD optimum is much weaker.
That means greenhouse dill is better managed by measuring these variables together rather than following a rigid stage-by-stage target table.
Quick Reference
| Variable | What It Tells You | Useful Research Context |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop now | Interpret together with photoperiod and temperature |
| DLI | Total PAR accumulated through the day | Direct greenhouse studies span roughly 6–19 mol/m²/day, with other research extending to about 20 |
| CO₂ | Carbon available for photosynthesis | 627 ppm produced higher biomass than about 378 ppm in direct dill research |
| VPD | Atmospheric evaporative demand | Useful for monitoring, but no validated stage-specific dill optimum is established |
| Temperature | Strongly modifies the effect of DLI | Direct research shows DLI can help at warm temperatures but behave differently when the crop is cold |
These values describe experimental conditions.
They are not universal dill requirements.
Why Dill Is a Good Example of Environmental Interaction
Many grow-light guides assume:
More light = more growth.
Dill research shows why that is too simple.
In a greenhouse experiment, researchers grew dill under multiple combinations of DLI and mean daily temperature.
When temperatures were relatively warm, increasing DLI increased fresh mass.
When temperatures were low, increasing DLI could actually reduce fresh mass.
Therefore:
The same DLI can produce different outcomes depending on temperature.
This is exactly why PAR, DLI and crop climate should be interpreted together.
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 photosynthetically active light is reaching the dill canopy right now?
DLI integrates PAR through the day.
It is expressed in:
mol/m²/day
It answers:
How much photosynthetically active light did the dill receive during the whole day?
In a greenhouse, DLI is particularly valuable because sunlight constantly changes with:
- clouds
- greenhouse structure
- shade cloth
- season
- sun angle
- supplemental lighting
A high midday PPFD cannot describe the full-day light environment.
A Direct Greenhouse Study Compared About 7 and 18 DLI
One strong greenhouse experiment grew:
Anethum graveolens ‘Fernleaf’
in nutrient-film technique hydroponic systems.
Researchers compared approximately:
7 mol/m²/day
with:
18 mol/m²/day
DLI.
Four weeks after transplanting, dill fresh mass averaged approximately:
7.1 g under low DLI
versus:
24.2 g under high DLI.
That is an increase of approximately:
17.1 g, or 241%.
Dry mass also increased substantially:
0.72 g → 3.12 g.
Node number increased from approximately:
6.0 → 8.5.
This provides strong evidence that increasing daily light can substantially improve dill production under suitable greenhouse conditions.
Does Dill Therefore Need 18 DLI?
No.
The experiment shows that:
18 DLI performed much better than 7 DLI under that specific production environment.
It does not prove that:
18 mol/m²/day is the universal dill optimum.
Another experiment demonstrates why this distinction matters.
Dill DLI Response Changes With Temperature
A 2021 greenhouse study grew dill across mean daily temperatures of approximately:
9.7–27.2°C
and DLIs of approximately:
6.2–16.9 mol/m²/day.
The researchers found a clear:
DLI × temperature interaction.
This is one of the most useful findings in the dill literature.
At relatively high temperatures, increasing DLI improved fresh mass.
At relatively low temperatures, increasing DLI could reduce it.
What Happened at Warm Temperatures?
At approximately:
27.2°C
increasing DLI from roughly:
6 → 15 mol/m²/day
increased fresh mass by approximately:
57%
or around:
19 g
under the experimental conditions.
This supports the expected pattern:
When temperature supported rapid growth, additional daily light could be converted into additional biomass.
What Happened at Low Temperatures?
The same experiment produced a very different result when dill was cold.
At approximately:
9.7°C
increasing DLI from roughly 8 to 15.5 mol/m²/day reduced fresh mass by approximately:
25%.
At approximately:
13.9°C
the same general increase in DLI reduced fresh mass by approximately:
53%.
This is extremely important.
It means a greenhouse grower should not respond to slow winter dill growth by automatically adding more light.
If the crop is temperature-limited, more photons may not produce the expected yield response.
Why Temperature Changes the Value of Light
Photosynthesis, respiration, leaf development and enzyme activity are all temperature-dependent.
When the crop is cold, its ability to use additional photons can change.
When temperature rises into a more productive range, additional light may produce a much stronger biomass response.
Therefore, instead of asking:
“What DLI does dill require?”
ask:
“How is dill responding to this DLI at the temperature it is actually experiencing?”
That is a much stronger greenhouse-management question.
Different Studies Report Different Temperature Responses
The 2021 experiment found dill fresh mass continuing to increase as mean daily temperature rose through approximately:
27.2°C
within its tested range.
The experiment did not reach a clearly supra-optimal temperature for fresh mass.
However, other dill research has estimated a fresh-mass temperature optimum closer to:
22.5°C
under a different production system.
These results do not mean one study is wrong.
They show that temperature response can change with:
- cultivar
- production system
- DLI
- container vs. hydroponic culture
- plant density
- crop duration
For AquaHorti, the correct conclusion is:
Temperature strongly modifies dill growth, but there is no universal greenhouse temperature optimum independent of the rest of the environment.
A Practical DLI Reference
Published greenhouse dill research gives us useful production environments roughly spanning:
7–20 mol/m²/day.
Very low DLI can restrict production.
Higher DLIs around the mid-to-upper teens have produced much greater biomass under favorable conditions.
For actively growing greenhouse dill, roughly:
10–18 mol/m²/day
can therefore be used as a practical comparison region when temperature is suitable.
But this should be treated as:
a research-based monitoring reference
not:
a dill specification.
The temperature interaction is too important to ignore.
Seedling and Early Growth
Young dill plants should not automatically receive the same light conditions as mature harvest-stage plants.
Early plants have:
- smaller leaves
- smaller root systems
- lower canopy interception
- different structural goals
In published greenhouse protocols, dill seedlings have been propagated under moderate supplemental lighting before being moved into finishing environments.
The important measurement questions are:
- Is light uniform across the propagation area?
- Are seedlings stretching?
- Are some trays shaded?
- Is supplemental lighting raising temperature?
- Is growth uniform?
Measure PPFD at actual crop height rather than relying on fixture specifications.
Active Vegetative Growth
Dill is normally harvested for its foliage before flowering.
During vegetative production:
- leaf number increases
- canopy size increases
- carbon demand rises
- total light interception increases
This is where DLI becomes increasingly useful.
The 7 vs. 18 DLI experiment showed that higher daily light substantially increased:
- fresh mass
- dry mass
- node number
under its greenhouse conditions.
But again, the later temperature experiment demonstrated that this response changes with crop temperature.
Higher DLI Also Changes Dry Matter
The temperature × DLI study found that dill dry matter concentration increased as DLI increased.
This means higher daily light affected not only fresh weight but the proportion of plant material represented by dry matter.
Meanwhile, increasing temperature tended to reduce dry matter concentration.
This is another example of why:
fresh mass alone does not describe the entire crop response.
A grower may care about:
- fresh yield
- dry matter
- leaf number
- aroma
- harvest time
- postharvest quality
Different environmental combinations may favor different outcomes.
Light Spectrum Matters Too
PAR and DLI measure photon quantity.
They do not describe wavelength distribution.
Dill research has demonstrated that light spectrum can alter:
- growth
- specialized metabolites
- nutritional quality
- phytochemical composition
Therefore, two fixtures producing the same PPFD and DLI can still produce different dill plants.
A complete lighting comparison should consider:
quantity + duration + spectrum.
Does Higher Light Automatically Mean Better Aroma?
No.
Dill aroma comes largely from volatile and essential-oil compounds.
Those compounds can respond to:
- light environment
- temperature
- plant age
- water status
- harvest stage
- cultivar
There is not enough evidence to support simple statements such as:
Higher PAR = stronger dill aroma
or:
Higher VPD = more concentrated aroma.
If aroma matters commercially, it should be assessed directly rather than inferred from one environmental measurement.
CO₂ Has Direct Dill-Specific Evidence
CO₂ enrichment is another area where dill has useful direct research.
One controlled study compared dill grown at approximately:
378 ± 25 ppm CO₂
with dill grown at:
627 ± 24 ppm CO₂.
The elevated-CO₂ treatment increased biomass production to approximately:
1.5 times
the ambient-CO₂ treatment.
That is a substantial response.
CO₂ Also Changed Dill Chemistry
The same experiment measured dozens of metabolites and minerals.
Elevated CO₂ increased several classes of compounds across parsley and dill, including:
- soluble sugars
- starch
- organic acids
- some essential amino acids
- unsaturated fatty acids
- total phenolics
- total flavonoids
- vitamins A and E
Total antioxidant capacity also increased.
This shows that CO₂ can influence:
crop chemistry as well as biomass.
Does Dill Therefore Require 627 ppm CO₂?
No.
The experiment compared approximately:
378 ppm
with:
627 ppm.
It did not test:
700, 800, 1000 ppm
or every intermediate concentration.
Therefore, the correct conclusion is:
Dill has demonstrated a strong positive response to moderate CO₂ enrichment around 600–650 ppm under controlled conditions.
It does not establish:
627 ppm as the universal optimum.
Why CO₂ and Light Should Be Viewed Together
Light supplies photosynthetic energy.
CO₂ supplies carbon.
If light is very low, adding CO₂ may provide less benefit because photons remain limiting.
If PAR and DLI are high but CO₂ becomes depleted, additional light may not be used as efficiently.
Therefore, in greenhouse production, ask:
What happens to CO₂ when PAR becomes strong?
Monitoring both variables on the same timeline can reveal whether:
- CO₂ remains stable
- CO₂ declines during active photosynthesis
- ventilation changes CO₂
- enrichment is being retained or lost
CO₂ Monitoring Is Useful Even Without Enrichment
You do not need a CO₂ injection system to benefit from CO₂ measurement.
Measure near the crop.
Then observe what happens during:
- morning light increase
- peak sunlight
- supplemental lighting
- greenhouse closure
- ventilation
This helps distinguish assumptions about room air from the environment the dill canopy actually experiences.
VPD: Evidence Requires More Caution
The old AquaHorti article used precise stage-specific VPD ranges.
Current dill-specific research does not provide enough evidence to validate one universal:
seedling VPD
vegetative VPD
and:
pre-harvest VPD
target.
That does not make VPD useless.
It means VPD should be used primarily as:
an environmental diagnostic measurement.
What Does VPD Tell You?
VPD describes atmospheric evaporative demand.
It depends strongly on:
- temperature
- relative humidity
When temperature rises or humidity falls, VPD generally increases.
Higher atmospheric demand can increase:
- transpiration
- water use
- substrate drying
The plant response depends on whether the root zone can keep supplying enough water.
Why VPD Cannot Be Interpreted Alone
Suppose VPD increases.
That could happen because:
- greenhouse temperature rose
- humidity fell
- ventilation opened
- heating turned on
The appropriate response depends on what else changed.
Therefore, compare VPD with:
- PAR
- temperature
- humidity
- irrigation
- crop appearance
rather than simply comparing the number with an unsupported “ideal dill VPD.”
Very High VPD
If atmospheric demand becomes high, check:
- substrate moisture
- irrigation frequency
- leaf condition
- greenhouse temperature
- airflow
A prolonged high-VPD period may increase plant-water demand.
But one brief spike does not automatically mean crop damage.
Duration matters.
Very Low VPD
Very low VPD normally indicates humid conditions.
That may reduce atmospheric water demand.
But persistently humid greenhouse air can also contribute to:
- condensation
- slow canopy drying
- disease-favorable conditions
The objective should therefore not be:
Make VPD as low as possible.
The goal is to understand and avoid prolonged environmental extremes.
Water Availability Also Changes the Response
A greenhouse dill study examining substrate moisture demonstrated that growth responds strongly to root-zone water availability.
That is important when interpreting VPD.
The same atmospheric demand can affect:
a well-watered crop
very differently from:
a crop whose substrate is becoming dry.
This reinforces why VPD should be interpreted together with irrigation and root-zone conditions.
Mature and Pre-Harvest Dill
Dill grown for fresh culinary foliage is usually harvested before flowering.
As harvest approaches, important production traits include:
- fresh biomass
- leaf number
- color
- tenderness
- aroma
- postharvest condition
Current evidence does not justify claiming that mature dill universally requires:
450–650 PPFD
or:
18–22 DLI
or one exact:
CO₂ + VPD combination.
What the evidence supports is more nuanced.
Higher DLI can strongly increase production when temperature supports growth.
Moderate CO₂ enrichment can increase biomass and alter crop chemistry.
Temperature can fundamentally change how the crop responds to DLI.
A Practical Greenhouse Measurement Workflow
1. Measure PAR at Crop Height
Place the sensor where dill leaves actually receive light.
2. Check Several Locations
Measure:
- greenhouse center
- edges
- shaded areas
- different benches
- multiple crop positions
3. Measure DLI
For greenhouse production, record PAR through the entire day.
This allows comparison between:
- sunny days
- cloudy days
- different seasons
- supplemental-light schedules
4. Record Temperature With DLI
This is especially important for dill.
A DLI number without temperature can be misleading because the two variables interact strongly.
5. Monitor CO₂
Observe CO₂ during active photosynthesis.
Compare it with the PAR timeline.
6. Track Humidity and VPD
Look for environmental trends rather than one isolated value.
7. Include Root-Zone Water
When VPD rises, check whether irrigation and substrate moisture remain adequate.
8. Compare With the Crop
Record:
- fresh mass
- leaf number
- plant height
- color
- harvest timing
- aroma if commercially important
This turns environmental measurements into production data.
Practical Research-Based Reference Points
DLI
Published dill greenhouse experiments cover approximately:
6–20 mol/m²/day.
Direct research showed a very large biomass increase when DLI increased from roughly:
7 → 18 mol/m²/day
under favorable greenhouse conditions.
However, another experiment demonstrated that higher DLI could reduce fresh mass at low temperatures.
Therefore, approximately:
10–18 mol/m²/day
can be used as a practical comparison region for actively growing dill when temperature is supportive.
It is not a universal target.
CO₂
Approximately:
600–650 ppm
is a useful dill-specific research reference.
Increasing CO₂ from approximately:
378 → 627 ppm
increased biomass to roughly:
1.5 times
the ambient treatment and altered numerous metabolic and nutritional traits.
Again, this is an experimental reference rather than an exact optimum.
VPD
There is currently insufficient dill-specific evidence to publish one universal stage-specific VPD target.
Use VPD primarily to understand atmospheric water demand and identify persistent environmental extremes.
Temperature
Temperature is essential when interpreting dill DLI.
One greenhouse experiment showed:
higher DLI helped at warm temperatures but could reduce fresh mass at low temperatures.
This makes dill an especially strong example of why one-variable growing charts are misleading.
A Better Way to Think About Dill Measurements
Instead of asking:
What PPFD should dill have?
ask:
How much light reaches the canopy right now?
Instead of:
What is the perfect DLI?
ask:
How much accumulated today, and at what temperature?
Instead of:
What CO₂ number does dill need?
ask:
Does CO₂ remain available during periods of strong photosynthesis?
Instead of:
What is the ideal VPD?
ask:
How is atmospheric water demand changing, and can the root zone support it?
This creates a much stronger greenhouse measurement framework.
Final Takeaway
Greenhouse dill does not have one scientifically established PAR, CO₂ and VPD recipe for every growth stage.
The strongest published evidence supports several important conclusions.
Daily light can strongly increase dill production.
A direct greenhouse experiment found fresh mass increased from approximately:
7.1 g at ~7 DLI
to:
24.2 g at ~18 DLI
— an increase of approximately 241%.
But DLI must be interpreted together with temperature.
Another greenhouse experiment demonstrated that increasing DLI from roughly 6–8 to around 15 mol/m²/day:
increased fresh mass by about 57% near 27°C
but:
reduced fresh mass at approximately 10–14°C.
That interaction is more important than any single “ideal DLI” number.
CO₂ also has direct dill-specific evidence.
Increasing CO₂ from approximately:
378 to 627 ppm
raised biomass to approximately:
1.5 times
the ambient treatment and increased several nutritional and phytochemical compounds.
For VPD, current dill-specific evidence does not justify one precise stage-specific range.
The better greenhouse strategy is therefore:
Measure PAR at crop height.
Record DLI across the whole day.
Always interpret DLI together with temperature.
Monitor CO₂ during periods of active photosynthesis.
Track temperature, humidity, VPD and root-zone water together.
Then compare those environmental trends with actual dill growth and crop quality.
That provides a much stronger basis for greenhouse dill production than unsupported stage-by-stage target tables.
References
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.
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.
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.
Currey et al. Substrate Volumetric Water Content Controls Growth and Development of Containerized Culinary Herbs. Agronomy, 2019.
Light Spectra Manipulation Stimulates Growth, Specialized Metabolites and Nutritional Quality in Anethum graveolens. 2023.
Related AquaHorti Tools
For instantaneous PAR / PPFD measurements and greenhouse light-distribution checks, see AquaHorti AH-Quantuv.
For recording changing greenhouse PAR and measuring DLI across the day, see AquaHorti AH-PARDLI.
For greenhouse monitoring where PAR, DLI, CO₂, temperature, humidity and VPD need to be reviewed together, see AquaHorti AH-200.