Growing Green Onions in a Greenhouse

What Research Actually Supports About Light, CO₂, Regrowth, Water, Texture and Shelf Life

Green onions, scallions, Welsh onions and Japanese bunching onions are names commonly used for non-bulbing or weakly bulbing Allium fistulosum crops.

They are fast-growing leafy Alliums valued for:

• green leaves
• pseudostems
• repeated harvest potential
• characteristic sulfur-containing flavor compounds

Their rapid growth can make them appear almost impossible to stress.

But there is no scientifically established stage-by-stage combination of:

PAR

CO₂

and:

VPD

that guarantees:

• fastest growth
• thick leaves
• strong pseudostems
• rapid regrowth
• ideal flavor
• minimum yellowing
• long shelf life

Fortunately, green onions have unusually strong crop-specific research on:

• PPFD
• DLI
• CO₂
• light spectrum
• repeated harvest
• salinity
• temperature
• postharvest handling

The strongest evidence supports measuring the environment and understanding interactions rather than following a rigid three-number recipe.

Quick Reference

Variable	What It Tells You	What Green-Onion Research Supports
PPFD / PAR	Photosynthetic light reaching the crop now	Direct hydroponic research tested 150, 300 and 450 µmol/m²/s
DLI	Total photosynthetic light accumulated through the day	The same experiment directly tested approximately 8.6, 17.3 and 25.9 mol/m²/day
CO₂	Carbon available for photosynthesis	Direct factorial research tested 400, 1200 and 4000 ppm; response depended strongly on light
Spectrum	Wavelength distribution	At the same ~300 PPFD, white and blue light produced very different photosynthetic and growth responses
VPD	Atmospheric evaporative demand	Useful for understanding water demand, but no validated green-onion stage-specific optimum exists
Root-zone EC	Salinity and nutrient-solution environment	Direct hydroponic research shows increasing salinity reduces dry matter and nutrient accumulation
Harvest schedule	Frequency of cutting	Direct A. fistulosum work compared 7-day, 14-day and single harvest strategies
Temperature	Thermal environment	Literature synthesis suggests roughly 13–25°C is favorable for effective vegetative growth; severe heat causes measurable stress
Storage	Conditions after harvest	Direct 2024 research found ~1°C storage dramatically extended marketability compared with 20°C

These are research references, not universal greenhouse specifications.

1. Green Onion Is a Leaf-and-Pseudostem Crop

Unlike bulb onion, commercial green onion production emphasizes:

• leaves
• pseudostem
• white basal portion
• repeated or single harvest

before a large storage bulb becomes the main carbon sink.

That distinction matters when interpreting onion research.

Data from mature bulb onion should not automatically become:

green-onion PAR, CO₂ or VPD targets.

2. Allium fistulosum Has Strong Direct Controlled-Environment Evidence

This article can rely heavily on crop-specific evidence.

One particularly valuable study grew:

Allium fistulosum L. cv. ‘Kinka’

hydroponically in controlled-environment chambers.

Researchers simultaneously tested three PPFDs:

150

300

450 µmol/m²/s

and three CO₂ concentrations:

400

1200

4000 ppm.

This 3 × 3 factorial design allows light and CO₂ interactions to be examined directly.

3. The Photoperiod Was 16 Hours

Plants received:

16 h light / 8 h dark

at approximately:

25°C

and:

50% relative humidity.

Because PPFD remained constant during the light period, the three DLIs were approximately:

150 PPFD → 8.6 mol/m²/day

300 PPFD → 17.3 mol/m²/day

450 PPFD → 25.9 mol/m²/day.

This is one of the strongest direct DLI references available for green onions.

4. Increasing PPFD From 150 to 300 Approximately Doubled Biomass

Across all three CO₂ environments, increasing PPFD from:

150 → 300 µmol/m²/s

approximately doubled fresh biomass.

This is strong direct evidence that:

150 PPFD / 8.6 DLI was light-limiting relative to 300 PPFD / 17.3 DLI

under the experimental conditions.

5. But 300 to 450 PPFD Produced Much Smaller Returns Under Ambient CO₂

At approximately:

400 ppm CO₂

fresh biomass was:

150 PPFD → 1.4 g/plant

300 PPFD → 3.4 g/plant

450 PPFD → 4.1 g/plant.

So:

150 → 300 PPFD produced a major increase.

But:

300 → 450 PPFD produced a much smaller increase.

This is a classic diminishing-return response.

6. Higher PPFD Was More Useful When CO₂ Was Also Elevated

At approximately:

1200 ppm CO₂

fresh biomass was:

150 PPFD → 3.4 g/plant

300 PPFD → 6.8 g/plant

450 PPFD → 9.0 g/plant.

Here, moving from:

300 → 450 PPFD

produced a much stronger response than it did under ambient CO₂.

This is direct crop-specific evidence that:

light and CO₂ interact.

7. The Full 3 × 3 Biomass Table Is Especially Useful

Approximate fresh biomass from the experiment was:

PPFD	400 ppm CO₂	1200 ppm CO₂	4000 ppm CO₂
150	1.4 g	3.4 g	4.1 g
300	3.4 g	6.8 g	7.2 g
450	4.1 g	9.0 g	8.4 g

This table demonstrates something far more useful than an isolated “ideal PPFD.”

The biological value of additional light depended strongly on:

CO₂ availability.

8. The Highest CO₂ Was Not the Highest-Yield Treatment

This is one of the most important findings.

At:

450 PPFD

fresh biomass was approximately:

9.0 g/plant at 1200 ppm

but:

8.4 g/plant at 4000 ppm.

Increasing CO₂ beyond 1200 ppm therefore did not continue improving biomass.

The researchers concluded that enrichment beyond approximately:

1200 ppm

provided little or no additional biomass benefit across the tested light range.

9. Therefore “More CO₂ = Faster Green Onions” Is Wrong

The response clearly showed:

diminishing returns.

Going from ambient CO₂ toward 1200 ppm substantially changed growth.

Going from:

1200 → 4000 ppm

did not produce another comparable gain.

So the old-style greenhouse recommendation:

keep increasing CO₂ as growth accelerates

is biologically unsupported.

10. 4000 ppm Was an Experimental Treatment — Not a Production Recommendation

The study investigated environmental conditions relevant partly to controlled life-support systems.

A concentration of:

4000 ppm

should not be interpreted as a normal greenhouse target.

It serves here because it clearly demonstrates:

CO₂ response saturates.

11. Even 1200 ppm Is a Research Reference, Not a Universal Optimum

The experiment’s best biomass occurred around:

450 PPFD + 1200 ppm CO₂.

But it also used:

• hydroponics
• 25°C constant temperature
• 16-hour photoperiod
• 50% RH
• one cultivar
• high plant density

Therefore:

1200 ppm is a well-supported experimental reference.

It is not:

the universal green-onion commercial optimum.

12. Low Light + Elevated CO₂ Could Outperform High Light + Ambient CO₂

One particularly interesting comparison was:

150 PPFD + elevated CO₂

versus:

450 PPFD + ambient CO₂.

The researchers found low light combined with elevated CO₂ could produce yield similar to or slightly greater than high light under ambient CO₂.

This demonstrates why greenhouse inputs cannot be interpreted independently.

13. PAR and CO₂ Must Be Considered Together

Photosynthesis requires:

• photons
• carbon dioxide

If CO₂ is strongly limiting, more photons may produce smaller returns.

If light is strongly limiting, additional CO₂ may also have diminishing value.

This direct green-onion experiment provides unusually clear evidence for the interaction.

14. DLI Gives Us Another Way to Read the Same Experiment

Because the photoperiod was fixed at 16 hours:

150 PPFD

≈ 8.6 DLI

300 PPFD

≈ 17.3 DLI

450 PPFD

≈ 25.9 DLI

The strongest response occurred between:

8.6 → 17.3 DLI.

The response from:

17.3 → 25.9 DLI

was more dependent on CO₂.

15. Does That Mean 17.3 DLI Is the Green-Onion Optimum?

No.

The experiment tested only three photon levels.

It did not test:

10 / 12 / 14 / 16 / 18 / 20 / 22 DLI

to identify a precise commercial maximum.

What it does show is:

about 8.6 DLI was substantially more light-limited than 17.3 DLI under these conditions.

16. Is 25.9 DLI Too High?

Not automatically.

At:

1200 ppm CO₂

the 25.9-DLI treatment produced the highest biomass.

At ambient CO₂, the additional gain was much smaller.

Therefore the value of a high DLI depends partly on:

• carbon availability
• temperature
• cultivar
• other resources

This is why DLI should not be interpreted alone.

17. Light Also Had a Different Effect on Antioxidant Quality

The same experiment measured:

total antioxidant activity.

Elevated CO₂ generally reduced antioxidant activity when expressed per unit dry mass.

Increasing light intensity, by contrast, increased biomass without the same dilution pattern in antioxidant production.

The authors concluded that light was a more effective stimulus than CO₂ for maintaining antioxidant production relative to biomass.

18. Bigger Plants Were Not Automatically More Antioxidant-Dense

This is another important production tradeoff.

Elevated CO₂ strongly increased biomass.

But the antioxidant concentration response did not scale identically.

Therefore:

maximum fresh biomass

and:

maximum antioxidant concentration per gram

are different objectives.

19. This Is Why Green-Onion Quality Cannot Be Judged From Yield Alone

Commercial quality can include:

• fresh biomass
• pseudostem size
• color
• sulfur compounds
• antioxidants
• flavor
• storage performance

One environment may maximize one trait without maximizing the others.

20. Spectrum Provides Another Layer Beyond PPFD

A separate 2020 Allium fistulosum experiment kept PPFD approximately constant at:

301.6 ± 12.7 µmol/m²/s

while changing LED wavelength.

Treatments included:

• white
• blue
• green
• yellow
• red

The photoperiod was:

12 hours

with approximately:

25/18°C day/night

and:

65% RH.

21. That Experiment Delivered About 13 DLI

At:

~300 PPFD × 12 h

DLI was approximately:

13.0 mol/m²/day.

Because photon quantity was held similar, differences among treatments largely reflected:

light quality rather than total PPFD.

22. White Light Produced the Strongest Overall Growth

After approximately:

30 days

white light produced significantly greater:

• leaf number
• leaf area
• plant height
• pseudostem diameter
• fresh weight

than the monochromatic treatments.

This directly shows that:

wavelength composition matters.

23. Blue Light Was the Best Monochromatic Treatment

Among single-color treatments:

blue light

produced stronger growth than:

• red
• green
• yellow

and improved several photosynthetic characteristics.

Blue treatment also produced the most complete leaf/chloroplast structural development among the monochromatic treatments.

24. Blue Light Strongly Changed Stomatal Behavior

The blue-light treatment produced the highest:

• transpiration rate
• stomatal conductance

among the monochromatic treatments.

This is direct evidence that:

spectrum can change gas exchange even when PPFD stays similar.

25. Therefore VPD Alone Cannot Explain Stomatal Conductance

The old AquaHorti article says:

VPD quietly controls stomatal behavior and structure.

But the direct green-onion experiment held environmental conditions similar while changing spectrum.

Stomatal conductance still changed strongly.

That means:

VPD is one factor — not the sole control.

26. Another Direct Study Found White + Blue Was Particularly Effective

A 2021 Allium fistulosum experiment compared several LED combinations.

A:

white + blue combination at approximately 3:1

was particularly beneficial for:

• plant growth
• nutrient accumulation

and produced higher levels of several:

sulfur-containing compounds.

This is especially relevant because sulfur compounds contribute to green-onion flavor.

27. Flavor Cannot Be Predicted From PPFD Alone

The 2021 study directly measured volatile/flavor-related compounds.

Light quality changed:

• growth
• nutrients
• sulfur compounds

Therefore, the old claim:

“higher PAR changes texture and flavor in one predictable direction”

is too simple.

Spectrum can change flavor chemistry even without changing the basic PAR concept.

28. A 2022 Study Confirmed Organ-Specific Nutritional Responses

A later study examined:

leaves

and:

pseudostems

separately under different LED combinations.

White + blue lighting again favored accumulation of several nutritional components and antioxidant capacity.

But the leaves and pseudostems did not respond identically.

This is important because green onions are often sold as the:

whole edible shoot.

29. Leaf and Pseudostem Quality Should Not Be Assumed to Move Together

A treatment that improves:

• leaf antioxidants

may not produce the same proportional response in:

• pseudostem nutrients
• texture
• sulfur compounds

So if the whole plant is the commercial product, measure the parts that matter.

30. “Stronger Light Makes Leaves Thin” Is Not Supported as a Universal Rule

The old page claims that increasing light caused leaves to become:

thinner and less resilient.

Current direct research demonstrates clear effects of light on morphology.

But leaf thickness and mechanical resilience depend on:

• spectrum
• PPFD
• cultivar
• plant density
• water status
• age

There is no universal:

PPFD → thin leaf

response curve supporting that statement.

31. “High PAR Makes Hollow Stems” Is Also Unsupported

The old page says higher PAR and warm air created:

hollow stems.

Current crop-specific literature does not establish a controlled:

PAR → hollow pseudostem

relationship.

If hollow or structurally weak tissue appears, investigate separately:

• cultivar
• growth rate
• temperature
• water
• mineral nutrition
• harvest age

Do not diagnose the cause from PPFD alone.

32. Repeated Harvest Has Direct Green-Onion Evidence

Green onions can regrow after cutting.

A direct controlled-environment study included:

Japanese bunching onion — Allium fistulosum

and compared:

• weekly harvest
• approximately 14-day harvest
• one harvest at 70 days

over a:

70-day crop cycle.

CO₂ treatments were approximately:

400

and:

1200 ppm.

33. Plants Were Cut Above the Root Zone

Repeated shoot harvest began approximately:

28 days after planting.

Shoots were removed around:

50 mm above the growing-medium surface.

This allowed plants to continue producing new edible shoots.

So cut-and-regrow is not merely anecdotal.

It has been directly studied in A. fistulosum.

34. Harvest Schedule Strongly Changed Cumulative Yield

The study showed a large difference among:

• single harvest
• 14-day harvests
• weekly harvests

in cumulative shoot biomass.

That means regrowth performance depends strongly on:

how often the crop is cut.

35. Elevated CO₂ Did Not Consistently Improve Repeated-Harvest Yield

This is particularly important.

Although other single-harvest Allium studies showed clear biomass benefits from elevated CO₂, the repeated-harvest experiment did not show a consistent additional benefit from 1200 ppm during weekly and biweekly harvest systems.

The response depended on:

harvest strategy.

36. Therefore CO₂ Response Depends on Source–Sink Management

When shoots are repeatedly removed:

• photosynthetic leaf area is removed
• carbohydrate pools are altered
• new sinks must develop
• regrowth uses stored resources

That creates a different carbon economy from a plant left untouched until one final harvest.

This is why an elevated-CO₂ response from a single-harvest crop cannot automatically predict repeated-harvest performance.

37. “VPD Determines Regrowth Speed” Is Therefore Much Too Simple

The direct regrowth study manipulated:

• harvest interval
• CO₂
• spacing

not VPD.

The crop’s regrowth depended on harvest management itself.

Therefore, the old statement:

“VPD quietly controls regrowth speed”

should be removed.

38. Regrowth Depends on Multiple Variables

After cutting, green-onion regrowth can depend on:

• cutting height
• remaining leaf area
• root reserves
• harvest frequency
• light
• CO₂
• temperature
• nutrition
• root-zone water

There is no evidence that one atmospheric kPa value dominates all of those factors.

39. CO₂ Can Even Change Visual Preference With Crop Age

The repeated-harvest research also included sensory evaluation of Japanese bunching onions grown under:

400

1200

and:

2000 ppm CO₂.

At an early harvest, panelists preferred the visual appearance of elevated-CO₂ plants.

Later in the crop, ambient-CO₂ plants received better visual rankings.

This illustrates another important point:

the same environmental treatment can look beneficial at one crop age and less beneficial later.

40. Crop Age Therefore Matters

A green onion at:

28 days

is not physiologically identical to one at:

70 days.

When comparing:

• color
• diameter
• texture
• flavor
• regrowth

always record:

days after planting and harvest history.

41. Root-Zone Salinity Has Strong Direct Hydroponic Evidence

A 2020 study grew:

Allium fistulosum ‘Todo Ano Evergreen – Nebuka’

hydroponically under nutrient-solution salinity levels of:

1.5

3.0

4.5

6.0

7.5

and:

9.0 dS/m.

It also compared nutrient-solution circulation frequencies.

42. Increasing Salinity Reduced Dry-Matter Production

As nutrient-solution salinity increased:

• dry-matter production decreased
• nutrient accumulation decreased
• Na⁺ and Cl⁻ stress increased

The negative effects were especially pronounced when brackish water was continually used to replace evapotranspired water.

This is direct green-onion root-zone evidence.

43. Better Solution Management Reduced Some Salinity Damage

Increasing nutrient-solution circulation from:

two to three times per day

helped mitigate some salinity effects.

It reduced Na⁺ and Cl⁻ accumulation and improved accumulation of:

• nitrogen
• phosphorus
• potassium
• calcium
• magnesium
• sulfur

under the study conditions.

44. This Is a Real Example of “Hidden Stress”

A crop with acceptable:

• PAR
• CO₂
• room humidity

can still perform poorly because of:

root-zone EC and ion accumulation.

Therefore:

measuring only the air environment cannot diagnose every problem.

45. High Root-Zone EC and High VPD Are Not the Same Stress

This distinction is critical.

High VPD

Atmospheric demand for water is stronger.

High root-zone EC

Roots have greater osmotic difficulty taking up water and may accumulate harmful ions.

A crop can experience:

moderate VPD + severe salinity

or:

high VPD + low salinity and abundant water.

The plant responses will differ.

46. There Is No Validated Green-Onion VPD Stage Table

The old page gives:

• 0.4–0.8 kPa
• 0.6–1.0
• 0.8–1.2
• 1.0–1.3

for successive growth stages.

Current Allium fistulosum literature does not establish these as universal optimal ranges.

Those values should be removed.

47. Experimental RH Is Not an Optimum VPD

The 2009 PPFD × CO₂ experiment successfully grew scallions at approximately:

25°C and 50% RH.

The 2020 LED spectrum experiment used approximately:

25/18°C

and:

65% RH.

Both produced healthy crops.

But neither experiment was designed to optimize humidity or VPD.

Therefore:

successful experimental RH ≠ proven optimum VPD.

48. What VPD Is Actually Useful For

VPD helps describe:

atmospheric evaporative demand.

When:

• temperature increases
• RH decreases

VPD generally rises.

That can increase:

• transpiration
• irrigation requirement
• root-zone water loss

This is useful greenhouse information.

But VPD should remain:

a diagnostic measurement.

49. VPD Does Not Directly Measure Stem Thickness

A VPD value does not tell you:

• pseudostem diameter
• wall thickness
• hollowing
• mechanical strength

Those plant traits must be measured independently.

The old statement:

“VPD quietly controls stem thickness”

should therefore be removed.

50. VPD Does Not Directly Measure Regrowth Speed Either

Regrowth speed can be recorded as:

• mm/day
• days to marketable height
• fresh biomass per harvest

Then compare it with:

• harvest interval
• DLI
• CO₂
• water status
• temperature

That creates real production evidence.

51. Temperature Has Direct Crop-Specific Relevance

A recent review of Welsh-onion physiology summarizes effective vegetative growth across approximately:

7–30°C

with a favorable average range around:

13–25°C.

It also reports more specific physiological references such as:

• germination around 13–20°C
• whole-plant dry-matter accumulation around 19–25°C
• leaf-sheath dry-matter accumulation around 13–19°C

These are literature synthesis values, not universal greenhouse setpoints.

52. Pseudostem Quality May Favor Cooler Conditions Than Maximum Whole-Plant Growth

The review notes that higher temperature during leaf-sheath development can reduce:

pseudostem and leaf quality.

This is more defensible than saying one VPD number determines pseudostem texture.

Temperature and developmental stage deserve separate attention.

53. Severe Heat Produces Direct Physiological Stress

A 2023 Allium fistulosum study compared:

• heat-tolerant AF60
• heat-sensitive AF35

under high-temperature stress.

A severe treatment around:

38/35°C day/night

changed:

• water content
• proteins
• antioxidant systems
• metabolic pathways

within approximately 24 hours.

54. Cultivar Strongly Changed Heat Response

The heat-tolerant and heat-sensitive cultivars did not respond identically.

Different molecular pathways were activated.

This means greenhouse heat risk depends partly on:

genetics.

So one temperature rule cannot describe every green-onion cultivar equally.

55. High Temperature Can Also Affect Reproductive Development

Japanese bunching-onion experiments have shown temperature can modify:

• flower-bud formation
• bolting
• devernalization

after cold exposure.

This is particularly relevant for longer-cycle crops or overwintered production.

Again:

temperature is a developmental variable, not merely part of the VPD calculation.

56. Flavor Also Has Direct Light Evidence

Green-onion flavor is strongly influenced by sulfur-containing compounds.

Direct LED research shows light spectrum can change those compounds.

The white + blue treatment produced higher concentrations of several sulfur compounds than comparison spectra.

Therefore, if flavor is commercially important:

record spectrum.

Do not assume PPFD alone predicts flavor.

57. Elevated CO₂ Can Also Alter Allium Flavor Chemistry

Controlled Allium research has detected changes in volatile emissions and sensory characteristics under elevated CO₂.

But the response differs:

• among species
• among cultivars
• with crop age

Therefore:

higher CO₂ does not guarantee stronger or better green-onion flavor.

58. Texture Should Be Measured Directly

If green-onion texture matters, record traits such as:

• pseudostem diameter
• wall thickness
• bending resistance
• crispness
• tissue water content
• sensory texture

Then compare those data with:

• PAR
• DLI
• CO₂
• temperature
• root-zone water

Do not infer texture from VPD alone.

59. Shelf Life Has Excellent Direct 2024 Evidence

A recent study directly examined:

Allium fistulosum

after harvest.

Green onions were stored at:

20°C for 8 days

and:

1°C for 6 weeks.

Researchers compared:

• string-tied bunches
• film packaging

and:

• roots left intact
• roots trimmed to about 5 mm.

60. Room-Temperature Marketability Was Only About Three Days

At:

20°C

leaf yellowing and wilting progressed rapidly.

Overall marketable storage life was approximately:

3 days.

This is a much stronger direct shelf-life result than any preharvest VPD claim.

61. At 1°C, Marketability Reached About Five Weeks

Under:

1°C cold storage

the green onions remained commercially acceptable for approximately:

5 weeks

based on the study’s overall visual and physicochemical criteria.

That is an enormous difference from room-temperature storage.

62. Film Packaging Helped Under Cold Storage

At:

1°C

film-packaged green onions maintained better marketability than string-tied bunches.

Packaging helped limit important quality losses.

63. Keeping the Roots Intact Also Helped

Green onions stored with their roots left attached showed:

• slower leaf-color deterioration
• better stem color
• better firmness

than root-trimmed onions.

This is direct evidence that:

postharvest trimming practice affects shelf life.

64. Yellowing Was a Major Shelf-Life Limitation

The 2024 study found that changes in:

• leaf color
• wilting from moisture loss

were among the primary factors determining the marketability limit.

So if postharvest yellowing is the problem, focus first on:

• cold storage
• water loss
• packaging
• trimming

rather than a speculative preharvest VPD setpoint.

65. Allicin and Quercetin Also Changed During Storage

Interestingly, allicin and quercetin in the pseudostem initially increased as quality deterioration began.

They later declined as deterioration became more severe.

This demonstrates that:

postharvest chemistry is dynamic.

A crop cannot be assumed to have one fixed nutritional composition after cutting.

66. Shelf Life Is Therefore Primarily a Postharvest Management Problem

Preharvest conditions matter.

But after cutting, major variables include:

• storage temperature
• moisture loss
• packaging
• root trimming
• handling duration

Current direct green-onion evidence does not support:

“VPD 1.0–1.3 kPa before harvest prevents yellowing.”

67. A Practical Greenhouse Measurement Workflow

Step 1 — Identify the Crop Type

Record whether the crop is:

• Japanese bunching onion
• Welsh onion
• scallion cultivar
• repeated-harvest crop
• single-harvest bunching crop

Step 2 — Measure PPFD at Leaf Height

Green-onion leaves are vertical.

Measure where the actual canopy intercepts light.

Step 3 — Record DLI

The direct crop-specific experiment shows major responses across:

8.6 / 17.3 / 25.9 DLI.

Measure the complete day rather than relying on noon PPFD.

Step 4 — Record Spectrum

White, blue and mixed blue-white LEDs can produce different:

• growth
• gas exchange
• flavor chemistry

even at similar photon quantities.

Step 5 — Monitor CO₂ With PAR

Track CO₂ during:

• lights-on
• peak sunlight
• greenhouse closure
• ventilation events

Step 6 — Track Temperature

Record:

• daytime peaks
• nighttime temperature
• prolonged heat events

Step 7 — Track RH and VPD

Use VPD to understand atmospheric demand.

Do not treat it as a texture or regrowth setpoint.

Step 8 — Measure Root-Zone Conditions

Track:

• nutrient-solution EC
• substrate moisture
• irrigation-water salinity
• drainage / circulation

Step 9 — Record Every Harvest

For cut-and-regrow production, record:

• cutting height
• harvest date
• days to next harvest
• fresh biomass

Step 10 — Measure Commercial Quality

Track:

• leaf length
• pseudostem diameter
• fresh biomass
• color
• yellowing
• marketable percentage

If flavor or texture matters, measure those separately.

68. Practical Research-Based PPFD Reference

The strongest factorial scallion experiment directly tested:

150

300

and:

450 µmol/m²/s

under a 16-hour photoperiod.

Increasing:

150 → 300 PPFD

approximately doubled biomass across all tested CO₂ concentrations.

Increasing:

300 → 450

provided smaller gains under ambient CO₂ but a larger gain near 1200 ppm.

Therefore:

150–450 PPFD is a strongly studied controlled-environment range.

It is not a universal optimum band.

69. Practical Research-Based DLI Reference

The same treatments corresponded to:

8.6

17.3

and:

25.9 mol/m²/day.

The strongest relative improvement occurred:

8.6 → 17.3 DLI.

The value of increasing to 25.9 DLI depended substantially on CO₂.

This gives AquaHorti something much better than the old invented stage table.

70. Practical CO₂ Reference

Direct crop-specific research tested:

400 / 1200 / 4000 ppm.

Around:

1200 ppm

strongly increased biomass under adequate light.

Increasing further to:

4000 ppm

provided little or no additional benefit and sometimes produced less biomass than 1200 ppm.

Therefore:

~1200 ppm is a direct research treatment, not a universal optimum.

71. Practical Spectrum Reference

At approximately:

300 PPFD

white and blue light produced stronger green-onion growth and photosynthetic performance than:

• red
• green
• yellow

monochromatic treatments.

A separate white + blue study also produced favorable:

• growth
• nutrient accumulation
• sulfur-compound profiles.

Therefore:

spectrum deserves independent consideration.

72. Practical Regrowth Reference

Direct repeated-harvest research successfully compared:

• 7-day
• 14-day
• single-harvest

strategies.

The response to CO₂ changed with harvest schedule.

Therefore:

regrowth should be optimized through actual harvest data

rather than assigned one VPD value.

73. Practical Root-Zone Reference

Hydroponic green onions have been directly tested across nutrient-solution salinities from:

1.5 to 9.0 dS/m.

Increasing salinity reduced:

• dry matter
• nutrient accumulation

and increased Na⁺ / Cl⁻ stress.

Do not turn 1.5 dS/m into a universal nutrient EC optimum.

The experiment was designed to study salinity.

74. Practical Temperature Reference

Current literature synthesis supports approximately:

13–25°C

as a useful broad vegetative-growth reference for Welsh onion.

More specifically:

• germination: around 13–20°C
• whole-plant dry matter: around 19–25°C
• leaf-sheath dry matter: around 13–19°C

These are useful biological references, not exact greenhouse requirements.

75. Practical VPD Reference

At present:

no validated stage-specific green-onion VPD optimum exists.

Therefore remove the old:

0.4–1.3 kPa

stage table.

Use VPD to understand:

• atmospheric water demand
• effects of heat / RH changes
• irrigation demand

and interpret it together with:

• root-zone water
• EC
• crop response.

76. A Better Way to Think About Green-Onion Measurements

Instead of asking:

What PPFD makes green onions grow fastest?

ask:

Is the crop still light-limited at the current CO₂ concentration?

Instead of:

What DLI is ideal?

ask:

How much daily light is accumulating, and are additional photons still increasing marketable biomass?

Instead of:

What CO₂ level should I maintain?

ask:

Does additional CO₂ still increase biomass under the actual light level, or has the response begun to saturate?

Instead of:

What VPD makes thicker stems?

ask:

What are the actual pseudostem dimensions, root-zone water status, EC and temperature?

Instead of:

What VPD gives fastest regrowth?

ask:

How do harvest interval, cutting height, DLI and root reserves affect the next harvest?

Those questions are much closer to the direct Allium fistulosum evidence.

Final Takeaway

Green onions do not have one scientifically established PAR, CO₂ and VPD recipe for rapid growth, strong pseudostems, tender texture and fast regrowth.

But Allium fistulosum has unusually strong direct evidence showing how light and CO₂ interact.

A controlled hydroponic experiment tested:

150 / 300 / 450 PPFD

combined with:

400 / 1200 / 4000 ppm CO₂

under a:

16-hour photoperiod.

That produced DLIs of approximately:

8.6 / 17.3 / 25.9 mol/m²/day.

At ambient CO₂, fresh biomass increased approximately:

1.4 → 3.4 → 4.1 g/plant

as PPFD increased.

At 1200 ppm CO₂:

3.4 → 6.8 → 9.0 g/plant.

At 4000 ppm:

4.1 → 7.2 → 8.4 g/plant.

This tells us several important things.

Moving from 150 to 300 PPFD approximately doubled biomass.

But:

moving from 300 to 450 PPFD produced smaller returns under ambient CO₂.

The additional light became much more useful around:

1200 ppm CO₂.

And:

4000 ppm did not outperform 1200 ppm.

That is direct evidence that:

PAR, DLI and CO₂ should be interpreted together.

It also demonstrates:

more CO₂ is not automatically better.

Quality followed a different pattern.

Elevated CO₂ strongly stimulated biomass but reduced antioxidant activity per unit dry matter, whereas increasing light supported biomass and antioxidant production more proportionally.

Therefore:

the environment maximizing biomass is not automatically the environment maximizing antioxidant density.

Spectrum matters too.

At approximately:

300 PPFD

with the same photon quantity, white and blue light produced much stronger green-onion growth and photosynthetic performance than several other monochromatic spectra.

Separate research found a:

white + blue

combination particularly effective for growth, nutrient accumulation and sulfur-containing compounds.

Therefore:

PPFD alone cannot predict green-onion flavor or quality.

Regrowth also requires a different framework.

Direct Japanese bunching-onion research compared:

• weekly harvest
• roughly 14-day harvest
• single harvest

and found that harvest schedule strongly altered cumulative biomass and the crop’s response to elevated CO₂.

That means:

regrowth is not controlled by VPD alone.

Root-zone conditions matter as well.

Hydroponic green-onion research across:

1.5–9.0 dS/m

nutrient-solution salinity showed increasing salinity reduced:

• dry matter
• nutrient accumulation

and increased Na⁺ / Cl⁻ stress.

So a crop with apparently acceptable PAR and room climate may still suffer from a hidden:

root-zone problem.

For VPD, current green-onion-specific evidence does not justify the old:

0.4–1.3 kPa

stage table.

VPD should instead be used to understand:

atmospheric water demand

and interpreted together with:

• temperature
• irrigation
• root-zone moisture
• EC
• actual crop measurements.

Finally, shelf life has exceptionally strong direct evidence.

A 2024 Allium fistulosum study found marketable storage life of only approximately:

3 days at 20°C

but approximately:

5 weeks at 1°C.

Film packaging improved cold-storage marketability, and keeping the roots attached slowed:

• color deterioration
• loss of firmness

compared with root trimming.

Therefore, postharvest yellowing is much more directly controlled by:

temperature, moisture loss, packaging and trimming practices

than by one unsupported preharvest VPD value.

The stronger greenhouse strategy is:

Measure PAR at the actual canopy.

Record DLI across the full day.

Treat spectrum separately from PPFD.

Monitor CO₂ together with light.

Track temperature, RH and VPD together.

Measure root-zone water and EC.

Record cutting height and harvest interval.

Then compare those measurements with the commercial trait that actually matters:

fresh biomass, pseudostem diameter, regrowth, sulfur compounds, texture, color or shelf life.

That provides a much stronger technical basis for greenhouse green onions than unsupported stage-by-stage PAR / CO₂ / VPD targets.

References

Levine, L.H. & Paré, P.W. Antioxidant Capacity Reduced in Scallions Grown Under Elevated CO₂ Independent of Assayed Light Intensity. Advances in Space Research, 2009.

Gao, S. et al. Photosynthetic Characteristics and Chloroplast Ultrastructure of Welsh Onion (Allium fistulosum L.) Grown Under Different LED Wavelengths. BMC Plant Biology, 2020.

Gao, S. et al. Comparison of the Effects of LED Light Quality Combination on Growth and Nutrient Accumulation in Green Onion (Allium fistulosum L.). Protoplasma, 2021.

Gao, S. et al. Effect of Different LED Light Quality Combination on the Content of Vitamin C, Soluble Sugar, Organic Acids, Amino Acids, Antioxidant Capacity and Mineral Elements in Green Onion (Allium fistulosum L.). Food Research International, 2022.

Broome, A.L. & Peffley, E.B. Effect of Planting Density, CO₂, and Harvest Intervals on Biomass of Three Allium Species. HortScience, 2005.

Broome, A.L. et al. Effect of Elevated CO₂ and Harvest Schedule on Allium Biomass and Sensory Quality of A. fistulosum. Texas Tech University / NASA controlled-environment research.

Thompson, L. et al. Biomass, Flavonol Levels and Sensory Characteristics of Allium Cultivars Grown Hydroponically at Ambient and Elevated CO₂. SAE Technical Paper, 2004.

Souza, C.D.S. et al. Nutrient and Inorganic Solute (Na⁺ and Cl⁻) Content in Green Onion Plants Under Hydroponic Cultivation Using Brackish Water. Ciência e Agrotecnologia, 2020.

Guo, Y. et al. Integrated Multi-Omic Data and Analyses Reveal the Response Pathways of Green Onion (Allium fistulosum L.) to High-Temperature Stress. Physiologia Plantarum, 2023.

Liu, C. et al. Welsh Onion (Allium fistulosum L.) Seed Physiology, Breeding, Production and Trade. Plants, 2022.

Choi, J.W. et al. Effects of Packaging Method and Root Trimming on Quality of Green Onion (Allium fistulosum L.) During Storage. Food Science and Preservation, 2024.

Related AquaHorti Tools

For instantaneous PAR / PPFD measurements and comparing green-onion light distribution, see AquaHorti AH-Quantuv.

For recording greenhouse PAR throughout the full 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.