How PAR, CO₂ and VPD Affect Growth, Stem Length and Crop Quality
Radish microgreens grow quickly, but their short crop cycle does not make environmental measurement unimportant.
Light intensity, daily light, temperature, humidity and vapor pressure deficit (VPD) can all influence growth. CO₂ is also part of the photosynthetic environment, although current research does not support one universal CO₂ target specifically for commercial radish microgreens.
The important point is that these variables should not be reduced to a precise stage-by-stage recipe.
Published radish-microgreen studies have successfully used very different light intensities and environmental conditions. The crop response also changes with cultivar, spectrum, photoperiod and production goal.
This guide explains what current research actually supports — and how PAR, DLI, CO₂ and VPD can be measured in a greenhouse without treating one number as universally optimal.
Quick Reference
| Variable | What It Tells You | Useful Research Context |
|---|---|---|
| PPFD / PAR | Photosynthetic light reaching the crop right now | Radish studies have tested approximately 67–300 µmol/m²/s |
| DLI | Total PAR accumulated through the day | Successful published systems include roughly 11–16 mol/m²/day and higher |
| CO₂ | Carbon available for photosynthesis | Monitor for depletion; no well-established radish-microgreen optimum |
| VPD | Atmospheric evaporative demand | Direct radish research has tested very low and high VPD, but does not establish one universal optimum |
| Temperature | Crop and water-balance environment | Many controlled studies operate around 20–24 °C |
These values describe experimental conditions, not universal production specifications.
1. Germination and Blackout
Radish microgreens are commonly germinated in darkness.
Published radish studies have used approximately three days of dark germination before exposing seedlings to production lighting.
This means the PAR requirement during blackout can legitimately be:
0 µmol/m²/s
Light becomes important after emergence and uncovering.
During blackout, the main management questions are instead:
- Is germination uniform?
- Is the substrate sufficiently moist?
- Is temperature stable?
- Is condensation excessive?
- Is airflow appropriate once emergence begins?
There is currently no strong evidence supporting a precise blackout target such as:
CO₂ 400–600 ppm + VPD 0.3–0.6 kPa
as a universal requirement for radish microgreens.
Those numbers should not be presented as established crop specifications.
2. After Uncovering: Measure Light at Crop Level
Once the trays are uncovered, cotyledons expand and photosynthesis becomes important.
Measure PPFD at approximately canopy height.
Do not rely only on:
- fixture wattage
- distance from the lamp
- visual brightness
- manufacturer specifications
Published radish-microgreen systems demonstrate that the crop can grow successfully under substantially different PPFD levels.
Examples include approximately:
- 150 µmol/m²/s
- 200 µmol/m²/s
- 230 µmol/m²/s
- 270 µmol/m²/s
- 300 µmol/m²/s
Other recent research has tested even lower intensities.
This range itself demonstrates why a recommendation such as:
“Radish microgreens require 180–300 µmol/m²/s during active growth”
should be treated as a possible production range rather than a universal requirement.
3. PPFD and DLI Are Not the Same Thing
PPFD tells you the instantaneous photosynthetic photon flux reaching the crop.
It is measured in:
µmol/m²/s
DLI tells you how much PAR accumulated over the whole day.
It is measured in:
mol/m²/day
For constant lighting:
DLI = PPFD × light-hours × 0.0036
For example:
200 µmol/m²/s × 16 h
≈ 11.5 mol/m²/day
230 µmol/m²/s × 16 h
≈ 13.2 mol/m²/day
270 µmol/m²/s × 16 h
≈ 15.6 mol/m²/day
300 µmol/m²/s × 12 h
≈ 13.0 mol/m²/day
These examples come close to lighting conditions actually used in published radish-microgreen research.
The important lesson is:
A PPFD number should always be interpreted together with photoperiod.
4. What Does Research Show About Light Intensity?
One particularly useful study compared radish and savoy-cabbage microgreens under:
150 and 300 µmol/m²/s PPFD
together with two very different VPD environments.
The photoperiod was:
12 hours
which corresponds approximately to DLIs of:
6.5 mol/m²/day at 150 PPFD
and:
13.0 mol/m²/day at 300 PPFD
For the crops evaluated, increasing PPFD from 150 to 300:
- increased dry weight
- increased dry matter percentage
- reduced elongation
Fresh weight did not show the same simple overall light response.
This is important.
Higher PPFD made the microgreens more compact and increased dry matter, but the study does not support the claim that stronger light universally increases fresh yield.
5. Higher Light Usually Makes Radish Microgreens More Compact
The same research found shorter microgreens at:
300 PPFD
than at:
150 PPFD
This supports a practical observation often made in microgreen production:
Increasing light can reduce excessive elongation.
But it does not mean that the shortest possible stem is always the commercial goal.
For microgreens, stem length affects:
- appearance
- harvestability
- package volume
- fresh weight
- crop presentation
A very compact radish microgreen may not automatically be more valuable than a somewhat taller one.
The appropriate morphology depends on the market.
6. Recent Research Shows Strong Species-Specific Responses
A 2026 study compared carrot, basil, arugula and radish microgreens under approximately:
67, 100, 140 and 174 µmol/m²/s
Increasing light generally promoted biomass accumulation, but phytochemical responses depended strongly on species.
Intermediate light levels around:
100–140 µmol/m²/s
often produced high concentrations of several phytochemical groups across the tested microgreens.
This reinforces an important principle:
The highest PPFD does not automatically maximize every desirable crop trait.
Yield, morphology, nutritional composition and electricity use can favor different lighting strategies.
7. Light Spectrum Matters Too
PPFD measures photon quantity within the PAR range.
It does not describe how those photons are distributed by wavelength.
Radish-microgreen research has shown that different LED spectra can alter:
- biomass
- pigmentation
- phenolic compounds
- flavonoids
- anthocyanins
- glucosinolates
- antioxidant characteristics
Therefore, two fixtures providing exactly the same PPFD can still produce different crop responses.
When comparing very different grow lights, measure light quantity but also consider spectrum.
8. Does Stronger Light Make Radish Microgreens More Pungent?
This is where the old version of this article was too confident.
Radish pungency is associated partly with glucosinolates and their breakdown products.
Light environment can influence these compounds.
However, current evidence does not justify a simple statement such as:
High PAR = hotter flavor
or:
High VPD = harsher radish microgreens.
Spectrum, light intensity, cultivar, harvest age and growing conditions can all alter phytochemical composition.
Without direct sensory measurements and controlled comparisons, flavor should not be inferred from one PAR or VPD reading.
For practical production, treat flavor as a quality characteristic that must be evaluated alongside environmental data.
9. What Does Research Actually Show About VPD?
VPD describes atmospheric evaporative demand.
It depends on both temperature and humidity.
A direct microgreen experiment tested radish under:
0.14 kPa VPD
and:
1.71 kPa VPD
combined with 150 and 300 µmol/m²/s PPFD.
These conditions represented extremely different atmospheric moisture environments.
The result is useful because it does not support a simplistic VPD recipe.
Across the experiment:
- VPD significantly affected microgreen length
- lower VPD produced greater elongation
- VPD did not significantly change overall fresh weight
- VPD did not significantly change overall dry weight
This means it would be misleading to say:
“VPD 0.8–1.2 kPa produces thicker stems and higher yield.”
The available evidence is more nuanced.
10. VPD and Light Interact
The same experiment found interactions between:
- species
- VPD
- light intensity
for several growth and anatomical traits.
This is exactly why VPD should not be interpreted alone.
A crop under:
150 PPFD + very humid air
is experiencing a different physiological environment from one under:
300 PPFD + much drier air.
Changes in VPD can affect:
- transpiration
- tissue development
- elongation
- nutrient transport
while light simultaneously changes photosynthesis and morphology.
The better approach is to measure both.
11. Should You Target One Exact VPD?
Current radish-microgreen research does not justify one universally optimal VPD number.
Instead, use VPD to detect environmental extremes and instability.
Watch for situations such as:
Persistently Very Low VPD
Possible concerns include:
- excessive humidity
- condensation
- limited evaporative demand
- disease-favorable conditions
Excessively High VPD
Possible concerns include:
- rapid water loss
- substrate drying
- increased irrigation demand
- reduced plant water status
The appropriate range depends on temperature, airflow, irrigation and crop stage.
For that reason, AquaHorti should not publish a rigid growth-stage VPD table unless it is supported by crop-specific experimental evidence.
12. What About CO₂?
CO₂ is essential for photosynthesis.
But radish microgreens are a very short-cycle crop, and current published literature does not provide a well-established commercial CO₂ optimum comparable with greenhouse tomato or cucumber research.
Some controlled microgreen experiments that included radish have used approximately:
800–1000 ppm CO₂
as part of their growing environment.
That tells us that radish microgreens can be grown under enriched CO₂.
It does not prove that 800–1000 ppm is the optimum range.
Therefore, do not present:
“Radish microgreens require 800–1000 ppm CO₂.”
as a research-established recommendation.
13. Why CO₂ Monitoring Can Still Be Useful
Even without a universal enrichment target, CO₂ measurement can answer a useful question:
Does CO₂ decline when the lights are on and the crop is actively photosynthesizing?
Dense trays inside:
- enclosed greenhouses
- grow tents
- growth rooms
- vertical farms
can create environmental conditions different from outdoor air.
Monitoring CO₂ can reveal whether concentration remains stable or changes strongly through the photoperiod.
This is especially useful when evaluating a tightly controlled production environment.
14. CO₂ Enrichment Is Different From CO₂ Measurement
These should not be confused.
Measuring CO₂ tells you what the crop environment actually contains.
Enriching CO₂ is an active production decision involving:
- equipment
- ventilation
- crop response
- worker safety
- economics
For radish microgreens, there is currently stronger justification for monitoring CO₂ than for prescribing one universal enrichment target.
15. Pre-Harvest: Do You Need to Reduce Light?
Not necessarily.
The old article recommended reducing PAR and DLI immediately before harvest to improve:
- stem firmness
- moisture retention
- shelf life
Current evidence does not establish this as a general radish-microgreen production requirement.
Pre-harvest light treatments can alter phytochemical composition, but shelf life is strongly influenced by what happens after harvest, including:
- temperature
- packaging
- moisture loss
- respiration
- storage light
Therefore, AquaHorti should not claim that one pre-harvest PAR or VPD range “locks in” storage quality without direct evidence.
16. What Research Says About Radish Microgreen Shelf Life
Postharvest studies provide much stronger evidence.
Research on daikon radish microgreens found that storage temperature and packaging strongly influenced shelf life.
Another experiment comparing light and dark storage found that dark storage helped preserve visual quality and extend shelf life, while light exposure accelerated weight loss and quality deterioration despite maintaining some nutritional compounds.
This means shelf-life management should be discussed primarily in terms of:
- rapid cooling
- storage temperature
- packaging
- moisture loss
- postharvest light exposure
rather than unsupported pre-harvest VPD claims.
17. Greenhouse Light Is Different From Indoor Light
In a greenhouse, PPFD changes continuously because of:
- clouds
- sun angle
- glazing
- greenhouse frames
- shade curtains
- neighboring crops
A reading of:
300 µmol/m²/s at noon
does not tell you how much light the microgreens received during the whole day.
This is where DLI monitoring becomes especially useful.
18. Why DLI Matters for a Short-Cycle Crop
Radish microgreens may be harvested in roughly 8–15 days depending on cultivar and production method.
That means one or two unusually dark days represent a meaningful percentage of the entire production cycle.
Recording DLI can reveal:
- cloudy-day light deficits
- seasonal differences
- bench-to-bench variation
- shading from greenhouse structure
- the contribution of supplemental lighting
For a short-cycle crop, these differences can accumulate quickly.
19. A Practical Greenhouse Measurement Workflow
Step 1 — Germination
During blackout, focus on:
- germination
- moisture
- temperature
- uniformity
PAR may legitimately be zero.
Step 2 — Measure PPFD After Uncovering
Place the PAR sensor at canopy height.
Measure multiple positions across the tray.
Step 3 — Check Uniformity
Measure:
- tray center
- corners
- edges
- areas affected by greenhouse shading
Do not use only the highest reading.
Step 4 — Measure DLI
If sunlight contributes significantly to crop lighting, log PAR through the day.
Step 5 — Track Temperature and Humidity
Use them together to understand VPD.
Pay attention to sudden afternoon drying or prolonged excessive humidity.
Step 6 — Monitor CO₂ Where Useful
In enclosed environments, observe whether CO₂ falls during high-light periods.
Do not assume that a specific enrichment number is required.
Step 7 — Record Crop Response
Compare environmental measurements with:
- stem length
- fresh weight
- dry matter
- coloration
- harvest timing
- uniformity
This turns environmental measurement into useful production data.
20. Useful Research-Based Light References
Published radish-microgreen studies provide several useful examples.
Moderate Indoor Production
Approximately:
200 µmol/m²/s × 16 h
= 11.5 mol/m²/day
has been used successfully.
Another Controlled Radish Study
Approximately:
230 µmol/m²/s × 16 h
= 13.2 mol/m²/day
was used through microgreen production.
Light × VPD Research
The direct radish VPD study compared:
150 vs 300 µmol/m²/s
for:
12 hours
equivalent to approximately:
6.5 vs 13.0 mol/m²/day
Brassicaceae Continuous-Light Research
Radish has also been studied around:
270 µmol/m²/s × 16 h
≈ 15.6 mol/m²/day
and under experimental continuous lighting at substantially higher DLI.
These are useful reference environments.
None should be interpreted as the universal radish-microgreen optimum.
21. So What Should You Start With?
For a greenhouse radish-microgreen crop, a reasonable approach is to begin by measuring what the plants actually receive rather than programming an unsupported recipe.
A practical light environment around the low-to-mid teens DLI is well represented in published radish research.
Then adjust according to:
- cultivar
- stem length
- biomass
- color
- crop cycle
- greenhouse sunlight
- electricity cost
If morphology is excessively stretched, examine both PPFD and overall environmental conditions.
If trays are compact but biomass or quality is disappointing, do not automatically assume that still more PPFD is the solution.
Final Takeaway
Radish microgreens respond to PAR, DLI and atmospheric conditions, but current research does not support a rigid four-stage PAR / CO₂ / VPD recipe.
The strongest evidence supports these conclusions:
Dark germination is commonly used.
Published radish-microgreen systems successfully use a broad range of PPFD.
Increasing light from 150 to 300 µmol/m²/s can increase dry matter and reduce elongation under controlled conditions.
VPD influences morphology, but direct research does not show that one narrow VPD range universally maximizes fresh weight or stem quality.
There is not yet a well-established universal CO₂ enrichment target specifically for radish microgreens.
Flavor and shelf life should not be predicted from PAR or VPD alone.
The better production strategy is:
Measure PPFD at crop height.
Measure DLI when sunlight varies.
Track temperature, humidity and VPD.
Monitor CO₂ when the production environment is enclosed.
Then compare those measurements with crop morphology, yield and harvest quality.
That provides a stronger basis for greenhouse radish-microgreen decisions than unsupported stage-by-stage target tables.
References
Amitrano et al. Defining Growth Requirements of Microgreens in Space Cultivation via Biomass Production, Morpho-Anatomical and Nutritional Traits Analysis. Frontiers in Plant Science, 2023.
Shibaeva et al. Continuous LED Lighting Enhances Yield and Nutritional Value of Four Genotypes of Brassicaceae Microgreens. Plants, 2022.
Light Intensity Drives Species-Specific Growth and Phytochemical Accumulation in Microgreens. Horticulturae, 2026.
Comparative Nutrient Study of Raphanus sativus L. Sprouts, Microgreens, and Roots. Agronomy, 2025.
Effects of Hot Air Drying on the Nutritional and Phytochemical Composition of Radish Microgreens. 2025.
Xiao et al. Effect of Light Exposure on Sensorial Quality, Concentrations of Bioactive Compounds and Antioxidant Capacity of Radish Microgreens During Low Temperature Storage. Food Chemistry, 2014.
Xiao et al. Postharvest Quality and Shelf Life of Radish Microgreens as Impacted by Storage Temperature, Packaging Film, and Chlorine Wash Treatment. LWT – Food Science and Technology, 2014.
Related AquaHorti Tools
For measuring instantaneous PAR / PPFD and checking tray uniformity, see AquaHorti AH-Quantuv.
For recording changing greenhouse PAR through the day and measuring DLI, see AquaHorti AH-PARDLI.
For environments where PAR needs to be evaluated together with CO₂, temperature, humidity and VPD, see AquaHorti AH-200.