Growing Romaine Lettuce in a Greenhouse

Romaine lettuce (Lactuca sativa var. longifolia) is recognizable by its upright growth habit, elongated leaves and prominent midribs.

Compared with loose-leaf lettuce, a mature romaine plant can look structurally robust.

But that appearance does not mean romaine simply benefits from:

more light

more CO₂

and:

drier air.

Direct romaine research shows clear tradeoffs.

Increasing Daily Light Integral can increase:

• fresh biomass
• dry biomass
• leaf number
• head circumference

while simultaneously increasing:

tipburn.

CO₂ enrichment can increase:

• fresh weight
• leaf area

while also changing:

• leaf nitrogen
• chlorophyll-related measurements
• tipburn risk

Humidity can alter:

• growth
• calcium distribution
• tipburn timing

but its effects differ between the:

light period

and:

dark period.

There is therefore no scientifically established stage-by-stage PAR, CO₂ and VPD recipe that guarantees:

• dense heads
• crisp leaves
• fast growth
• no tipburn
• mild flavor
• long shelf life

The better greenhouse strategy is to measure the environment and connect those measurements with marketable crop quality.

Quick Reference

Variable	What It Tells You	What Romaine Research Supports
PPFD / PAR	Photosynthetic light reaching the canopy now	Direct romaine studies commonly include roughly 160–320 µmol/m²/s; response depends on photoperiod, cultivar and other conditions
DLI	Total photosynthetic light accumulated through the day	Direct romaine research tested 9.2–17.3 mol/m²/day and found higher DLI increased biomass but also tipburn
Photoperiod	Hours of light per day	Extending the day sometimes increased biomass more efficiently than simply raising PPFD
CO₂	Carbon available for photosynthesis	Direct open-heart romaine greenhouse research at ~400 vs ~800 ppm increased fresh weight about 24.7%
VPD / RH	Atmospheric water-demand environment	Direct romaine research shows humidity affects calcium and tipburn, but differently during day and night
Airflow	Boundary-layer and transpiration environment	Tipburn can respond strongly to local airflow around young leaves
Root-zone temperature	Temperature experienced by roots	Direct romaine research shows root-zone temperature can substantially alter biomass and photosynthesis
Cultivar	Genetic background	Romaine cultivars differ strongly in yield, tipburn, sensory quality and shelf life
Storage temperature	Postharvest environment	Near 0°C provides roughly 21 days expected storage life for romaine under appropriate conditions

These are research references, not universal greenhouse specifications.

1. Romaine Is Not Just “A Tougher Lettuce”

Romaine cultivars can differ substantially in:

• plant size
• head compactness
• leaf shape
• pigment concentration
• nitrate accumulation
• sensory characteristics
• tipburn susceptibility
• shelf life

For example, controlled-environment studies comparing midi and mini romaine cultivars have found large genetic differences in:

• fresh mass
• nutrients
• bioactive compounds
• sensory quality

Therefore, a response observed in one romaine cultivar should not automatically become a universal romaine requirement.

2. PAR and DLI Answer Different Questions

PPFD measures photosynthetic photon flux at one moment.

It is expressed as:

µmol/m²/s.

It answers:

How much photosynthetic light is reaching the romaine canopy right now?

DLI integrates those photons across the complete day.

It is expressed as:

mol/m²/day.

It answers:

How much photosynthetic light did the crop receive today?

For constant artificial lighting:

DLI = PPFD × hours × 0.0036

For example:

160 PPFD × 16 h
≈ 9.2 DLI

240 PPFD × 16 h
≈ 13.8 DLI

160 PPFD × 20 h
≈ 11.5 DLI

240 PPFD × 20 h
≈ 17.3 DLI

These exact combinations have been tested directly in romaine lettuce.

3. Romaine Has Excellent Direct DLI Evidence

A 2022 controlled-environment study examined two romaine cultivars:

‘Casual’

a midi romaine,

and:

‘Elizium’

a mini / Little Gem type.

Researchers compared:

160 vs 240 µmol/m²/s PPFD

and:

16 vs 20 h photoperiods.

That created four DLIs:

9.2

11.5

13.8

and:

17.3 mol/m²/day.

Plants were grown for:

30 days.

4. Higher DLI Strongly Increased ‘Casual’ Biomass

For ‘Casual’ romaine:

the lowest-light treatment:

160 PPFD × 16 h = 9.2 DLI

produced approximately:

215 g fresh weight.

The highest-light treatment:

240 PPFD × 20 h = 17.3 DLI

produced approximately:

353 g.

That is about:

64% greater fresh biomass.

Higher DLI also increased:

• dry mass
• leaf number
• head circumference.

5. But Tipburn Increased at the Same Time

This is the key tradeoff.

For ‘Casual’ at:

9.2 DLI

approximately:

• 4% of outer leaves
• 2% of inner leaves

showed tipburn.

At:

17.3 DLI

approximately:

• 14% of outer leaves
• 18% of inner leaves

showed tipburn.

So:

the treatment producing the greatest biomass did not produce the greatest marketable quality.

6. Mini Romaine Showed an Even Stronger Tipburn Response

For ‘Elizium’:

at:

9.2 DLI

approximately:

• 4% outer leaves
• 0% inner leaves

showed tipburn.

At:

17.3 DLI

approximately:

• 50% outer leaves
• 8% inner leaves

showed symptoms.

This demonstrates how strongly:

cultivar × light environment

can affect commercial quality.

7. Therefore 17.3 DLI Is Not Simply “The Romaine Optimum”

At 17.3 DLI:

‘Casual’ reached approximately:

350 g

within 30 days.

That is an excellent biological yield.

But tipburn increased.

An environmental setting cannot be called optimal based only on:

fresh weight.

For commercial lettuce, the better target is:

marketable fresh weight.

8. Mini Romaine Needed Less DLI for High Biomass

For ‘Elizium’, high fresh weights around:

239 g

were obtained across approximately:

11.5–17.3 DLI.

Increasing DLI beyond the lower end of this region did not produce the same large response seen in ‘Casual’.

Again:

cultivar matters.

9. The Lowest DLI Produced the Best Visual Quality

The researchers reported that the lowest:

9.2 DLI

treatment generally had the best visual quality because tipburn was rare.

But it also produced lower biomass.

This creates a real production tradeoff:

Lower DLI

Less biomass
Less tipburn

Higher DLI

More biomass
More tipburn

Neither condition is automatically “best.”

10. Nitrate Moved in the Opposite Direction From Tipburn

Increasing DLI reduced:

nitrate accumulation.

Even at the lowest DLI, nitrate remained below the applicable European regulatory limit in that experiment.

So higher daily light produced:

lower nitrate

but:

greater tipburn risk.

This is another reason crop quality cannot be reduced to a single metric.

11. Sensory Quality Did Not Simply Follow Maximum DLI

For ‘Elizium’, the highest overall sensory quality occurred at:

240 PPFD × 16 h

which equals approximately:

13.8 DLI.

That was not the maximum 17.3-DLI treatment.

For ‘Casual’, sensory responses differed again.

Therefore:

maximum yield ≠ maximum sensory quality.

12. Photoperiod and PPFD Were Not Interchangeable

For ‘Casual’, extending photoperiod from:

16 → 20 hours

increased biomass by approximately:

30%.

Increasing PPFD:

160 → 240

increased biomass by approximately:

26%.

So delivering more photons over a longer period was slightly more effective than increasing instantaneous light in that experiment.

13. This Matters for Supplemental-Lighting Economics

Suppose you need to increase DLI.

There are multiple ways to do it:

• higher PPFD
• longer photoperiod
• both

Plants may not respond identically.

Electricity cost may also differ.

Therefore:

DLI should be recorded together with PPFD and photoperiod.

14. Another Romaine Study Confirms Strong Cultivar Effects

A 2023 hydroponic experiment compared seven romaine cultivars at:

160

and:

200 µmol/m²/s

under:

16-hour photoperiods.

This corresponds approximately to:

9.2

and:

11.5 DLI.

15. Higher Light Did Not Unambiguously Improve Quality

The researchers found:

genotype had a major effect

on:

• fresh mass
• nutrients
• bioactive compounds
• nitrate
• sensory quality.

Increasing PPFD from:

160 → 200

did not consistently improve all measured quality traits.

That independently supports the same conclusion:

more PPFD is not automatically better romaine.

16. Romaine Type Matters Too

In the seven-cultivar study:

midi romaine

generally produced greater fresh yield and higher concentrations of several bioactive compounds than:

mini romaine.

Mini cultivars, however, showed other nutritional advantages.

Therefore, even the commercial category:

“romaine”

contains physiologically different crop types.

17. A Practical Romaine Light Reference

Direct romaine evidence strongly supports approximately:

160–240 µmol/m²/s

and:

9–17 mol/m²/day

as a useful controlled-environment research region.

But this does not mean every greenhouse should maintain exactly that range.

Higher light has also been studied.

The important point is that:

marketable yield begins to be constrained by physiological disorders such as tipburn before photosynthetic growth necessarily stops responding.

18. Tipburn Is One of the Most Important Limits on Intensive Romaine Production

Tipburn appears as:

• browning
• necrosis
• damaged margins

especially on rapidly expanding young leaves.

It is strongly associated with insufficient:

calcium delivery to those tissues.

The nutrient solution can contain adequate calcium while the inner leaves still develop a local deficiency.

19. Tipburn Is Not Simply “Too Much PAR”

Higher DLI can promote rapid growth.

Rapid growth increases:

calcium demand.

If calcium transport cannot keep pace, tipburn risk rises.

Therefore light is part of the mechanism.

But it is not the only variable.

20. Romaine Has Direct Humidity × Tipburn Research

A classic experiment used the tipburn-sensitive romaine cultivar:

‘Lobjoits Green Cos’.

Plants received approximately:

320 µmol/m²/s

for:

16 hours/day.

That corresponds to approximately:

18.4 DLI.

Researchers changed relative humidity during:

• light periods
• dark periods

separately.

21. Lower Daytime RH Delayed Tipburn — but Reduced Growth

When daytime RH decreased from approximately:

74% → 51%,

plant growth slowed.

Leaf calcium concentration increased.

The onset of tipburn was delayed.

This demonstrates that increasing daytime evaporative demand can improve calcium transport while simultaneously reducing growth.

22. That Does Not Mean “Dryer Air Is Better”

Growth was reduced.

So once again:

lower tipburn

did not mean:

maximum productivity.

If the greenhouse is made excessively dry simply to increase transpiration, production can suffer.

23. Nighttime Humidity Produced the Opposite Pattern

The same experiment found that reducing nighttime RH from approximately:

95% → 90%

resulted in:

• lower calcium concentration
• earlier tipburn
• reduced growth.

Further decreases did not create a simple linear improvement.

This is extremely important.

24. Day and Night Humidity Cannot Be Replaced by One VPD Number

The crop reacted differently to humidity depending on:

time of day.

Therefore a statement such as:

“Maintain VPD at 1.0 kPa”

does not capture the biological response.

You need to know:

• when that VPD occurred
• what temperature produced it
• whether stomata were active
• how the root system was behaving.

25. The Daily Water-Transport Cycle Matters

The 1984 researchers concluded that calcium accumulation and tipburn were influenced by humidity patterns that encouraged:

• daytime transpiration
• nighttime root-pressure flow

The complete diurnal cycle mattered.

That is much more informative than the old statement:

“VPD quietly controls romaine structure.”

26. VPD Is Useful — but It Is Not a Texture Setpoint

VPD describes:

atmospheric evaporative demand.

As:

• temperature rises
• RH falls

VPD generally increases.

This can influence:

• transpiration
• calcium transport
• irrigation demand
• plant water status.

That makes VPD useful.

But it does not directly measure:

• crispness
• head density
• bitterness
• shelf life.

27. Local Airflow Can Matter More Than Room-Average VPD

The most vulnerable leaves in romaine are often:

young inner leaves.

These can be enclosed within the canopy and surrounded by humid boundary-layer air.

Room-level RH or VPD may therefore fail to describe the actual environment around:

the shoot tip.

28. This Is Why Airflow Can Reduce Lettuce Tipburn

Targeted airflow reduces the humid boundary layer around young tissue.

That can increase:

• local transpiration
• calcium delivery.

Recent greenhouse lettuce research continues to show that vertical airflow can dramatically reduce tipburn even when overall room:

• temperature
• RH
• VPD

change very little.

The local leaf environment matters.

29. Romaine Spectrum Can Also Change Tipburn

A 2026 study directly examined romaine lettuce under:

200 µmol/m²/s

for:

16 h/day

with total photon intensity held constant.

The researchers changed the proportion of:

green light.

CO₂ was approximately:

1000 ppm

and air temperature around:

20°C.

30. More Green Light Reduced Tipburn

Tipburn incidence decreased approximately from:

49%

in the lower-green treatment

to:

25%

under 100% green light.

That is a substantial reduction.

But there was a tradeoff.

31. Maximum Green Light Also Reduced Fresh Biomass

Compared with the lower-green treatment:

80% green

reduced shoot fresh mass by approximately:

7%.

100% green

reduced fresh mass by approximately:

25%.

So:

a spectrum that reduced tipburn also reduced yield.

Again:

maximum marketable quality and maximum biological biomass are different objectives.

32. PAR Quantity Alone Cannot Explain This Result

Every treatment received approximately:

200 PPFD.

The difference was:

spectrum.

Therefore equal PAR can still produce different:

• growth
• morphology
• tipburn.

A PAR meter tells you photon quantity.

It does not fully characterize spectrum.

33. CO₂ Has Direct Romaine-Specific Greenhouse Evidence

A 2020 Oklahoma State University experiment grew:

‘Auvona’ open-heart romaine

in greenhouse NFT hydroponics.

Researchers compared approximately:

400 ppm CO₂

with an enriched environment averaging:

800 ppm.

34. 800 ppm Increased Romaine Fresh Weight by About 24.7%

Fresh weight increased approximately from:

203.8 g

under ambient CO₂

to:

254.2 g

under enrichment.

That is approximately:

+24.7%.

Total leaf area increased from roughly:

4885 → 5989 cm².

So there is strong direct evidence that:

romaine can respond positively to moderate CO₂ enrichment.

35. But Leaf Nutritional Composition Also Changed

Under elevated CO₂:

• tissue nitrogen decreased
• phosphorus decreased
• SPAD chlorophyll-related readings decreased

relative to ambient-grown lettuce.

This is another classic dilution / compositional tradeoff.

More biomass does not automatically mean:

higher nutrient concentration per gram.

36. Tipburn Also Appeared Under Elevated CO₂

The researchers reported:

inner-leaf tipburn

at the later growth stage in CO₂-enriched ‘Auvona’.

Plants under ambient conditions did not show the same disorder.

This is extremely important.

CO₂ enrichment increased:

growth

but rapid growth also increased a:

marketability problem.

37. Therefore 800 ppm Is Not Simply “The Best CO₂”

It is a strong:

romaine-specific research reference.

The experiment demonstrated:

• greater fresh biomass
• greater leaf area

but also:

• altered tissue composition
• tipburn.

So the correct question is not:

Does 800 ppm increase growth?

It did in that experiment.

The better question is:

Does it increase marketable yield without creating quality problems in this production system?

38. Do Not Automatically Push CO₂ to 1000–1500 ppm

Other controlled-environment romaine studies sometimes use:

1000–1500 ppm

as production conditions.

That demonstrates that romaine can be grown under those concentrations.

It does not establish them as:

universal optima.

A full crop-specific CO₂-response curve is needed before making that claim.

39. High CO₂ Should Be Interpreted With DLI

If light is limiting:

extra CO₂ may provide a different return than under high DLI.

If both DLI and CO₂ promote rapid biomass accumulation:

calcium demand can also rise.

This is why:

PAR / DLI + CO₂ + tipburn

should be considered together.

40. Temperature Adds Another Major Layer

Lettuce is a cool-season crop.

Temperature affects:

• development
• respiration
• water demand
• bolting
• photosynthesis
• head quality.

Romaine-specific field research across multiple planting seasons has shown strong relationships between temperature and:

• days to harvest
• marketable heads
• bolting.

41. Cooler Conditions Can Slow Production Dramatically

In multi-season romaine research, cooler planting periods required substantially more time to reach harvest.

Across the tested seasonal environments:

approximately every:

1°C decrease

in growing-season minimum or maximum temperature was associated with about:

5 additional days to harvest

in the study’s regression.

This demonstrates the production cost of overly cool conditions.

42. But Excessive Heat Creates a Different Problem

Warm conditions can accelerate:

• development
• bolting
• quality deterioration.

Therefore, the correct objective is not:

minimum possible temperature

or:

maximum possible growth temperature.

Temperature must be balanced against crop maturity and marketability.

43. Tropical Greenhouse Research Shows Cooling Strategy Matters

Direct green-romaine greenhouse research in hot conditions compared:

• no climate treatment
• fogging + ventilation
• fogging + shading
• fogging + ventilation + shading.

Fogging plus ventilation performed particularly well for:

• growth
• yield
• sensory evaluation.

44. Heavy Shading Can Solve Heat While Creating a Light Problem

In the same tropical greenhouse work:

shading reduced photosynthesis

and could reduce marketable fresh weight.

This is a classic greenhouse tradeoff.

If shading is used to reduce temperature:

measure the resulting PAR and DLI.

Do not assume temperature control is free.

45. Root-Zone Temperature Is Another Independent Variable

A hydroponic crop can have acceptable:

air temperature

while the nutrient solution becomes:

• too warm
• too cold.

That can affect:

• roots
• water uptake
• nutrient uptake
• photosynthesis.

46. Romaine Has Direct Root-Zone Temperature Evidence

A 2021 NFT experiment tested:

17 lettuce cultivars

including several romaine cultivars.

Nutrient-solution temperatures included:

18.3°C

21.1°C

and an ambient treatment averaging approximately:

20–26.5°C.

47. Around 21.1°C Increased Shoot Fresh Weight

Across the lettuce cultivars:

plants grown around:

21.1°C root-zone temperature

produced approximately:

15% greater shoot fresh weight

than plants under ambient nutrient-solution temperatures.

But °Brix was approximately:

26% lower.

Again:

maximum biomass and maximum soluble-solids concentration were not the same outcome.

48. Romaine Cultivars Responded Differently

‘Parris Island’ and other romaine cultivars showed different:

• growth
• CO₂ assimilation
• °Brix

responses.

Therefore root-zone management also needs to be interpreted by:

cultivar.

49. New 2026 Mini-Romaine Research Confirms Root Temperature Matters

A 2026 experiment grew mini romaine in:

• NFT
• aeroponics

with nutrient-solution minimum temperatures around:

11–12°C

14°C

18°C

and:

22°C.

50. Warming a Cold Root Zone Dramatically Increased Leaf Yield

Compared with the cold ambient root zone:

18–22°C

increased leaf mass per production area by approximately:

127–232% in NFT

and:

54–75% in aeroponics.

This is a major response.

51. But Photosynthetic Indicators Did Not Simply Follow Biomass

Some photosynthetic-performance indicators were greater at lower root-zone temperatures even while crop biomass was restricted.

The researchers concluded that low temperature limited:

growth / sink performance

more strongly than photosynthetic capacity alone.

This is another reminder:

one physiological measurement cannot describe total crop performance.

52. “Head Density” Cannot Be Predicted From VPD Alone

Romaine architecture depends on:

• cultivar
• leaf number
• leaf expansion
• DLI
• temperature
• water
• calcium-related disorders
• harvest maturity.

There is no validated relationship showing:

VPD X = dense head

and:

VPD Y = loose head.

Measure head characteristics directly.

53. Crispness Also Requires Direct Measurement

The old article claimed the preharvest:

1.0–1.3 kPa VPD

range produced crisper leaves and stronger midribs.

Current direct evidence does not establish that relationship.

Crispness can depend on:

• tissue water content
• cell structure
• cultivar
• maturity
• storage
• temperature
• dehydration.

If crispness matters commercially:

measure sensory or mechanical texture directly.

54. Bitterness Is Also Not a VPD Reading

Lettuce bitterness can change with:

• cultivar
• maturity
• bolting
• heat stress
• chemistry.

The old statement:

“too much dryness increased bitterness”

may sound biologically plausible.

But a VPD reading does not directly measure bitter compounds.

Without a controlled sensory or chemical experiment, that claim should not be presented as established romaine science.

55. Shelf Life Has Much Stronger Direct Evidence

UC Davis currently recommends storing romaine near:

0°C

with relative humidity above:

95%.

At approximately:

0°C

expected storage life is around:

21 days.

At:

5°C

approximately:

14 days

may be expected if ethylene is absent.

This is direct practical postharvest guidance.

56. Cooling Romaine Quickly Is Important

Romaine has a substantial respiration rate.

As storage temperature increases:

respiration increases rapidly.

That accelerates deterioration.

Commercial cooling options include:

• vacuum cooling
• hydro-vacuum cooling
• hydrocooling
• forced-air cooling.

Therefore, shelf life is strongly controlled after harvest.

57. Fresh-Cut Romaine Has Direct Packaging Evidence

A controlled study compared fresh-cut romaine at:

0°C

and:

5°C

under several packaging films.

Packaging reduced:

weight loss

at both temperatures.

The best low-temperature packaging combinations helped preserve:

• color
• ascorbic acid
• sensory quality.

58. Storage Temperature Is Much More Directly Linked to Shelf Life Than Preharvest VPD

The old article claimed:

a preharvest VPD around 1.0–1.3 kPa improves shelf life.

Current romaine evidence is much stronger for:

• rapid cooling
• near-0°C storage
• high RH
• suitable packaging
• ethylene avoidance.

Therefore, shelf-life management should not be reduced to one preharvest atmospheric number.

59. Genetics Can Also Strongly Affect Romaine Shelf Life

A recent study evaluated many romaine genotypes and found large differences in:

postharvest longevity.

For example:

• breeding line 60184 deteriorated relatively quickly
• ‘Manatee’ was intermediate
• ‘Okeechobee’ had longer shelf life.

This demonstrates:

genotype itself is a major postharvest variable.

60. The Long-Shelf-Life Cultivar Was Not Simply the Thickest Leaf

Interestingly, the short-shelf-life genotype had:

• thicker leaf lamina
• higher stomatal index

than the long-shelf-life cultivar.

So even seemingly intuitive traits such as:

thicker leaf = longer shelf life

do not automatically hold.

Again:

measure the actual outcome.

61. Preharvest Light Can Affect Specific Postharvest Disorders

Recent 2025 romaine research examined:

midrib pinking

after harvest.

Two cultivars were grown under substantially different supplemental-light intensities.

The study found both:

• cultivar
• preharvest light environment

could influence postharvest pinking-related responses.

This demonstrates that preharvest light can matter.

But it does not prove:

one VPD prevents pinking.

62. Production CO₂ and Postharvest CO₂ Are Completely Different

During cultivation:

CO₂ is used for:

photosynthesis.

After harvest:

controlled-atmosphere CO₂ can affect:

• respiration
• browning
• physiological injury.

UC Davis notes that intact romaine heads generally do not benefit from high postharvest CO₂ and may be injured above roughly:

5% CO₂.

Fresh-cut romaine, however, may use higher CO₂ in specialized modified-atmosphere packaging.

Do not confuse these applications.

63. A Practical Romaine Measurement Workflow

Step 1 — Record the Cultivar

Specify:

• midi romaine
• mini / Little Gem
• open-heart type
• cultivar name.

Step 2 — Measure PPFD at Canopy Height

Do not rely on fixture specifications.

Step 3 — Map the Crop

Measure:

• center
• edges
• structural shadows
• dense canopy positions.

Step 4 — Record DLI

Compare:

• sunny vs cloudy days
• summer vs winter
• supplemental-light schedules.

Step 5 — Record Photoperiod

The same DLI can be produced by different:

PPFD × time

combinations.

Step 6 — Monitor CO₂ With PAR

Observe CO₂ during:

• lights-on
• strong sunlight
• greenhouse closure
• ventilation.

Step 7 — Track Temperature

Record:

• daytime average
• midday peaks
• nighttime values.

Step 8 — Monitor Root-Zone Temperature

Especially in:

• NFT
• DWC
• other recirculating hydroponics.

Step 9 — Record RH and VPD

Use VPD to understand:

atmospheric water demand.

Do not use it as a crispness target.

Step 10 — Measure Air Movement

Room-average VPD does not describe the boundary layer around:

inner young leaves.

Step 11 — Inspect for Tipburn

Record:

• first appearance
• number of affected leaves
• inner vs outer tipburn
• severity.

Step 12 — Measure Marketable Yield

Separate:

total fresh weight

from:

marketable fresh weight.

64. Practical Research-Based PPFD Reference

Direct romaine experiments provide particularly strong evidence around:

160–240 µmol/m²/s

for indoor production.

A classic tipburn study also used approximately:

320 µmol/m²/s.

These are useful research references.

They do not define a universal 160–320 PPFD optimum.

65. Practical Research-Based DLI Reference

The strongest direct controlled-environment study tested:

9.2–17.3 mol/m²/day.

Increasing DLI generally increased biomass.

For ‘Casual’:

17.3 DLI

produced the greatest yield.

For ‘Elizium’:

high biomass occurred across:

11.5–17.3 DLI.

But higher DLI also increased tipburn.

Therefore:

roughly 10–17 DLI is a strong romaine research region, not a rigid optimum band.

66. Practical CO₂ Reference

Direct open-heart romaine research compared:

~400 vs ~800 ppm.

At approximately 800 ppm:

fresh weight increased about:

24.7%.

Leaf area increased about:

22.6%.

But:

• tissue N decreased
• tipburn appeared at the later stage.

Therefore:

~800 ppm is a strong research reference, not a universal optimum.

67. Practical VPD Reference

There is no scientifically justified stage table such as:

0.4–0.8 → 0.6–1.0 → 0.8–1.2 → 1.0–1.3 kPa.

Direct romaine research instead shows that:

• daytime humidity
• nighttime humidity
• local airflow
• calcium delivery
• growth rate

interact.

Use VPD diagnostically.

68. What Should You Watch When DLI Is Increased?

As DLI rises, record:

• fresh growth
• leaf number
• tipburn
• nitrate
• marketable percentage.

If biomass increases while tipburn rises faster:

the crop has passed the point where:

more biological growth = more saleable crop.

69. What Should You Watch When CO₂ Is Increased?

Record:

• fresh weight
• leaf area
• tipburn
• DLI
• temperature
• nutrient status.

Do not assume a larger plant automatically has:

• better nutritional concentration
• better storage quality
• less physiological disorder.

70. What Should You Watch When VPD Changes?

Ask:

• Did temperature change?
• Did RH change?
• Was it daytime or nighttime?
• Did airflow change?
• Is the root zone hydrated?
• Is tipburn increasing?
• Are inner leaves actually receiving air movement?

These questions are much more useful than asking whether:

1.0 kPa is “good” and 1.4 kPa is “bad.”

71. A Better Way to Think About Romaine Measurements

Instead of asking:

What PPFD makes dense romaine heads?

ask:

How much daily light increases marketable biomass before tipburn becomes unacceptable?

Instead of:

What DLI is optimal?

ask:

Where is the yield–tipburn tradeoff for this cultivar?

Instead of:

What CO₂ concentration produces the fastest crop?

ask:

Does CO₂ enrichment increase marketable yield under the available DLI without creating excessive tipburn?

Instead of:

What VPD creates crisp leaves?

ask:

How are atmospheric demand, root water supply and calcium transport affecting the young leaves?

Instead of:

How do I improve shelf life before harvest?

ask:

How quickly can the crop be cooled and maintained near its correct postharvest temperature and humidity?

Those are much closer to the direct romaine evidence.

Final Takeaway

Greenhouse romaine lettuce does not have one scientifically established PAR, CO₂ and VPD recipe for dense heads, crisp leaves, rapid growth and long shelf life.

But romaine has unusually strong crop-specific environmental evidence.

DLI strongly affects biomass — and tipburn.

A direct 2022 experiment compared:

160 / 240 PPFD

with:

16 / 20 h photoperiods

to produce:

9.2 / 11.5 / 13.8 / 17.3 DLI.

For ‘Casual’ romaine:

fresh weight increased from approximately:

215 g at 9.2 DLI

to:

353 g at 17.3 DLI.

But inner-leaf tipburn increased from approximately:

2%

to:

18%.

For ‘Elizium’, outer-leaf tipburn rose as high as approximately:

50%

under the highest DLI treatment.

That means:

maximum biomass was not maximum marketable quality.

Nitrate moved in another direction:

higher DLI reduced nitrate accumulation.

And sensory quality had yet another optimum:

‘Elizium’ performed particularly well at approximately:

240 PPFD × 16 h = 13.8 DLI

rather than at the maximum DLI.

This is exactly why a single “best light” number is misleading.

CO₂ has strong direct romaine evidence too.

Greenhouse NFT ‘Auvona’ grown around:

800 ppm CO₂

produced approximately:

24.7% more fresh weight

than lettuce near:

400 ppm.

Leaf area increased by about:

22.6%.

But leaf nitrogen declined and inner-leaf tipburn appeared under enrichment.

Therefore:

higher CO₂ can increase growth while simultaneously increasing another production problem.

Humidity and VPD require much more careful interpretation than the old article suggested.

In direct ‘Lobjoits Green Cos’ research, lowering daytime RH from approximately:

74% to 51%

delayed tipburn and increased leaf calcium but reduced growth.

Yet reducing nighttime RH from:

95% to 90%

caused earlier tipburn and lower calcium.

So:

daytime and nighttime humidity can produce different physiological effects.

There is no justification for replacing this complexity with one universal:

“ideal VPD.”

Local airflow matters as well.

Young inner leaves can remain surrounded by humid air even if room-average VPD appears acceptable.

And new romaine research shows spectrum itself can change tipburn:

at the same:

200 PPFD

increasing green-light proportion reduced tipburn from approximately:

49% to 25%

but the highest-green treatment also reduced fresh mass by approximately:

25%.

Again:

yield and quality trade off.

Temperature must also be separated into:

• air temperature
• root-zone temperature.

New 2026 mini-romaine research found that warming cold nutrient solution from approximately:

11–12°C

toward:

18–22°C

increased leaf production dramatically in NFT and aeroponic systems.

Finally, shelf life should be treated primarily as a postharvest problem.

Current UC Davis guidance expects approximately:

21 days near 0°C

and:

14 days around 5°C

under appropriate conditions.

Rapid cooling and RH above roughly:

95%

are far better-supported shelf-life tools than the old article’s claim that a particular preharvest VPD creates longer storage life.

The stronger greenhouse strategy is therefore:

Measure PAR at the actual canopy.

Record DLI through the full day.

Record photoperiod and spectrum separately.

Monitor CO₂ together with PAR.

Track air and root-zone temperature.

Use RH and VPD to understand atmospheric demand.

Evaluate airflow around the inner leaves.

Monitor tipburn and calcium-related disorders.

Then compare those measurements with:

marketable fresh weight, tipburn incidence, nitrate, sensory quality and storage performance.

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

References

Matysiak, B., Ropelewska, E., Wrzodak, A., Kowalski, A. & Kaniszewski, S. Yield and Quality of Romaine Lettuce at Different Daily Light Integral in an Indoor Controlled Environment. Agronomy, 2022.

Assessment of Romaine Lettuce Cultivars Grown in a Vertical Hydroponic System at Two Levels of LED Light Intensity. Scientia Horticulturae, 2023.

Singh, H., Poudel, M.R., Dunn, B.L., Fontanier, C. & Kakani, G. Effect of Greenhouse CO₂ Supplementation on Yield and Mineral Element Concentrations of Leafy Greens Grown Using Nutrient Film Technique. Agronomy, 2020.

Collier, G.F. & Tibbitts, T.W. Effects of Relative Humidity and Root Temperature on Calcium Concentration and Tipburn Development in Lettuce. Journal of the American Society for Horticultural Science, 1984.

High Green Light Substitution Reduces Tipburn Incidence in Romaine Lettuce Grown in a Plant Factory with Artificial Lighting. Plants, 2026.

Moccio, M. et al. Nutrient Solution Temperature Affects Growth and °Brix Parameters of Seventeen Lettuce Cultivars Grown in an NFT Hydroponic System. Horticulturae, 2021.

Bantis, F. et al. Root-Zone Heating Boosts the Production of Mini Romaine Lettuce Grown in Nutrient Film Technique and Aeroponics Systems. Plants, 2026.

Dufault, R.J. et al. Dynamic Relationships Between Field Temperatures and Romaine Lettuce Yield and Head Quality. Scientia Horticulturae, 2009.

Kumsong, P. et al. Comparison of Different Temperature Control Systems in Tropical-Adapted Greenhouses for Green Romaine Lettuce Production. Horticulturae, 2023.

Examining Preharvest Genetic and Morphological Factors Contributing to Lettuce (Lactuca sativa L.) Shelf-Life. Scientific Reports, 2024.

Yahya, M.H., Chadwick, M.J. & Wagstaff, C. The Effect of Supplementary LED Illumination of Romaine Lettuce on Midribs Pinking After Harvest. Journal of Horticultural Science & Biotechnology, 2025.

Manolopoulou, H., Lambrinos, G., Chatzis, E., Xanthopoulos, G. & Aravantinos, E. Effect of Temperature and Modified Atmosphere Packaging on Storage Quality of Fresh-Cut Romaine Lettuce. Journal of Food Quality, 2010.

UC Davis Postharvest Research and Extension Center. Lettuce (Romaine and Loose-Leaf): Produce Facts.

Related AquaHorti Tools

For instantaneous PAR / PPFD measurements and mapping romaine-canopy light distribution, see AquaHorti AH-Quantuv.

For recording greenhouse PAR throughout the complete 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.

Romaine lettuce (Lactuca sativa var. longifolia) is recognizable by its upright growth habit, elongated leaves and prominent midribs.

Compared with loose-leaf lettuce, a mature romaine plant can look structurally robust.

But that appearance does not mean romaine simply benefits from:

more light

more CO₂

and:

drier air.

Direct romaine research shows clear tradeoffs.

Increasing Daily Light Integral can increase:

• fresh biomass
• dry biomass
• leaf number
• head circumference

while simultaneously increasing:

tipburn.

CO₂ enrichment can increase:

• fresh weight
• leaf area

while also changing:

• leaf nitrogen
• chlorophyll-related measurements
• tipburn risk

Humidity can alter:

• growth
• calcium distribution
• tipburn timing

but its effects differ between the:

light period

and:

dark period.

There is therefore no scientifically established stage-by-stage PAR, CO₂ and VPD recipe that guarantees:

• dense heads
• crisp leaves
• fast growth
• no tipburn
• mild flavor
• long shelf life

The better greenhouse strategy is to measure the environment and connect those measurements with marketable crop quality.

Quick Reference

Variable	What It Tells You	What Romaine Research Supports
PPFD / PAR	Photosynthetic light reaching the canopy now	Direct romaine studies commonly include roughly 160–320 µmol/m²/s; response depends on photoperiod, cultivar and other conditions
DLI	Total photosynthetic light accumulated through the day	Direct romaine research tested 9.2–17.3 mol/m²/day and found higher DLI increased biomass but also tipburn
Photoperiod	Hours of light per day	Extending the day sometimes increased biomass more efficiently than simply raising PPFD
CO₂	Carbon available for photosynthesis	Direct open-heart romaine greenhouse research at ~400 vs ~800 ppm increased fresh weight about 24.7%
VPD / RH	Atmospheric water-demand environment	Direct romaine research shows humidity affects calcium and tipburn, but differently during day and night
Airflow	Boundary-layer and transpiration environment	Tipburn can respond strongly to local airflow around young leaves
Root-zone temperature	Temperature experienced by roots	Direct romaine research shows root-zone temperature can substantially alter biomass and photosynthesis
Cultivar	Genetic background	Romaine cultivars differ strongly in yield, tipburn, sensory quality and shelf life
Storage temperature	Postharvest environment	Near 0°C provides roughly 21 days expected storage life for romaine under appropriate conditions

These are research references, not universal greenhouse specifications.

1. Romaine Is Not Just “A Tougher Lettuce”

Romaine cultivars can differ substantially in:

• plant size
• head compactness
• leaf shape
• pigment concentration
• nitrate accumulation
• sensory characteristics
• tipburn susceptibility
• shelf life

For example, controlled-environment studies comparing midi and mini romaine cultivars have found large genetic differences in:

• fresh mass
• nutrients
• bioactive compounds
• sensory quality

Therefore, a response observed in one romaine cultivar should not automatically become a universal romaine requirement.

2. PAR and DLI Answer Different Questions

PPFD measures photosynthetic photon flux at one moment.

It is expressed as:

µmol/m²/s.

It answers:

How much photosynthetic light is reaching the romaine canopy right now?

DLI integrates those photons across the complete day.

It is expressed as:

mol/m²/day.

It answers:

How much photosynthetic light did the crop receive today?

For constant artificial lighting:

DLI = PPFD × hours × 0.0036

For example:

160 PPFD × 16 h
≈ 9.2 DLI

240 PPFD × 16 h
≈ 13.8 DLI

160 PPFD × 20 h
≈ 11.5 DLI

240 PPFD × 20 h
≈ 17.3 DLI

These exact combinations have been tested directly in romaine lettuce.

3. Romaine Has Excellent Direct DLI Evidence

A 2022 controlled-environment study examined two romaine cultivars:

‘Casual’

a midi romaine,

and:

‘Elizium’

a mini / Little Gem type.

Researchers compared:

160 vs 240 µmol/m²/s PPFD

and:

16 vs 20 h photoperiods.

That created four DLIs:

9.2

11.5

13.8

and:

17.3 mol/m²/day.

Plants were grown for:

30 days.

4. Higher DLI Strongly Increased ‘Casual’ Biomass

For ‘Casual’ romaine:

the lowest-light treatment:

160 PPFD × 16 h = 9.2 DLI

produced approximately:

215 g fresh weight.

The highest-light treatment:

240 PPFD × 20 h = 17.3 DLI

produced approximately:

353 g.

That is about:

64% greater fresh biomass.

Higher DLI also increased:

• dry mass
• leaf number
• head circumference.

5. But Tipburn Increased at the Same Time

This is the key tradeoff.

For ‘Casual’ at:

9.2 DLI

approximately:

• 4% of outer leaves
• 2% of inner leaves

showed tipburn.

At:

17.3 DLI

approximately:

• 14% of outer leaves
• 18% of inner leaves

showed tipburn.

So:

the treatment producing the greatest biomass did not produce the greatest marketable quality.

6. Mini Romaine Showed an Even Stronger Tipburn Response

For ‘Elizium’:

at:

9.2 DLI

approximately:

• 4% outer leaves
• 0% inner leaves

showed tipburn.

At:

17.3 DLI

approximately:

• 50% outer leaves
• 8% inner leaves

showed symptoms.

This demonstrates how strongly:

cultivar × light environment

can affect commercial quality.

7. Therefore 17.3 DLI Is Not Simply “The Romaine Optimum”

At 17.3 DLI:

‘Casual’ reached approximately:

350 g

within 30 days.

That is an excellent biological yield.

But tipburn increased.

An environmental setting cannot be called optimal based only on:

fresh weight.

For commercial lettuce, the better target is:

marketable fresh weight.

8. Mini Romaine Needed Less DLI for High Biomass

For ‘Elizium’, high fresh weights around:

239 g

were obtained across approximately:

11.5–17.3 DLI.

Increasing DLI beyond the lower end of this region did not produce the same large response seen in ‘Casual’.

Again:

cultivar matters.

9. The Lowest DLI Produced the Best Visual Quality

The researchers reported that the lowest:

9.2 DLI

treatment generally had the best visual quality because tipburn was rare.

But it also produced lower biomass.

This creates a real production tradeoff:

Lower DLI

Less biomass
Less tipburn

Higher DLI

More biomass
More tipburn

Neither condition is automatically “best.”

10. Nitrate Moved in the Opposite Direction From Tipburn

Increasing DLI reduced:

nitrate accumulation.

Even at the lowest DLI, nitrate remained below the applicable European regulatory limit in that experiment.

So higher daily light produced:

lower nitrate

but:

greater tipburn risk.

This is another reason crop quality cannot be reduced to a single metric.

11. Sensory Quality Did Not Simply Follow Maximum DLI

For ‘Elizium’, the highest overall sensory quality occurred at:

240 PPFD × 16 h

which equals approximately:

13.8 DLI.

That was not the maximum 17.3-DLI treatment.

For ‘Casual’, sensory responses differed again.

Therefore:

maximum yield ≠ maximum sensory quality.

12. Photoperiod and PPFD Were Not Interchangeable

For ‘Casual’, extending photoperiod from:

16 → 20 hours

increased biomass by approximately:

30%.

Increasing PPFD:

160 → 240

increased biomass by approximately:

26%.

So delivering more photons over a longer period was slightly more effective than increasing instantaneous light in that experiment.

13. This Matters for Supplemental-Lighting Economics

Suppose you need to increase DLI.

There are multiple ways to do it:

• higher PPFD
• longer photoperiod
• both

Plants may not respond identically.

Electricity cost may also differ.

Therefore:

DLI should be recorded together with PPFD and photoperiod.

14. Another Romaine Study Confirms Strong Cultivar Effects

A 2023 hydroponic experiment compared seven romaine cultivars at:

160

and:

200 µmol/m²/s

under:

16-hour photoperiods.

This corresponds approximately to:

9.2

and:

11.5 DLI.

15. Higher Light Did Not Unambiguously Improve Quality

The researchers found:

genotype had a major effect

on:

• fresh mass
• nutrients
• bioactive compounds
• nitrate
• sensory quality.

Increasing PPFD from:

160 → 200

did not consistently improve all measured quality traits.

That independently supports the same conclusion:

more PPFD is not automatically better romaine.

16. Romaine Type Matters Too

In the seven-cultivar study:

midi romaine

generally produced greater fresh yield and higher concentrations of several bioactive compounds than:

mini romaine.

Mini cultivars, however, showed other nutritional advantages.

Therefore, even the commercial category:

“romaine”

contains physiologically different crop types.

17. A Practical Romaine Light Reference

Direct romaine evidence strongly supports approximately:

160–240 µmol/m²/s

and:

9–17 mol/m²/day

as a useful controlled-environment research region.

But this does not mean every greenhouse should maintain exactly that range.

Higher light has also been studied.

The important point is that:

marketable yield begins to be constrained by physiological disorders such as tipburn before photosynthetic growth necessarily stops responding.

18. Tipburn Is One of the Most Important Limits on Intensive Romaine Production

Tipburn appears as:

• browning
• necrosis
• damaged margins

especially on rapidly expanding young leaves.

It is strongly associated with insufficient:

calcium delivery to those tissues.

The nutrient solution can contain adequate calcium while the inner leaves still develop a local deficiency.

19. Tipburn Is Not Simply “Too Much PAR”

Higher DLI can promote rapid growth.

Rapid growth increases:

calcium demand.

If calcium transport cannot keep pace, tipburn risk rises.

Therefore light is part of the mechanism.

But it is not the only variable.

20. Romaine Has Direct Humidity × Tipburn Research

A classic experiment used the tipburn-sensitive romaine cultivar:

‘Lobjoits Green Cos’.

Plants received approximately:

320 µmol/m²/s

for:

16 hours/day.

That corresponds to approximately:

18.4 DLI.

Researchers changed relative humidity during:

• light periods
• dark periods

separately.

21. Lower Daytime RH Delayed Tipburn — but Reduced Growth

When daytime RH decreased from approximately:

74% → 51%,

plant growth slowed.

Leaf calcium concentration increased.

The onset of tipburn was delayed.

This demonstrates that increasing daytime evaporative demand can improve calcium transport while simultaneously reducing growth.

22. That Does Not Mean “Dryer Air Is Better”

Growth was reduced.

So once again:

lower tipburn

did not mean:

maximum productivity.

If the greenhouse is made excessively dry simply to increase transpiration, production can suffer.

23. Nighttime Humidity Produced the Opposite Pattern

The same experiment found that reducing nighttime RH from approximately:

95% → 90%

resulted in:

• lower calcium concentration
• earlier tipburn
• reduced growth.

Further decreases did not create a simple linear improvement.

This is extremely important.

24. Day and Night Humidity Cannot Be Replaced by One VPD Number

The crop reacted differently to humidity depending on:

time of day.

Therefore a statement such as:

“Maintain VPD at 1.0 kPa”

does not capture the biological response.

You need to know:

• when that VPD occurred
• what temperature produced it
• whether stomata were active
• how the root system was behaving.

25. The Daily Water-Transport Cycle Matters

The 1984 researchers concluded that calcium accumulation and tipburn were influenced by humidity patterns that encouraged:

• daytime transpiration
• nighttime root-pressure flow

The complete diurnal cycle mattered.

That is much more informative than the old statement:

“VPD quietly controls romaine structure.”

26. VPD Is Useful — but It Is Not a Texture Setpoint

VPD describes:

atmospheric evaporative demand.

As:

• temperature rises
• RH falls

VPD generally increases.

This can influence:

• transpiration
• calcium transport
• irrigation demand
• plant water status.

That makes VPD useful.

But it does not directly measure:

• crispness
• head density
• bitterness
• shelf life.

27. Local Airflow Can Matter More Than Room-Average VPD

The most vulnerable leaves in romaine are often:

young inner leaves.

These can be enclosed within the canopy and surrounded by humid boundary-layer air.

Room-level RH or VPD may therefore fail to describe the actual environment around:

the shoot tip.

28. This Is Why Airflow Can Reduce Lettuce Tipburn

Targeted airflow reduces the humid boundary layer around young tissue.

That can increase:

• local transpiration
• calcium delivery.

Recent greenhouse lettuce research continues to show that vertical airflow can dramatically reduce tipburn even when overall room:

• temperature
• RH
• VPD

change very little.

The local leaf environment matters.

29. Romaine Spectrum Can Also Change Tipburn

A 2026 study directly examined romaine lettuce under:

200 µmol/m²/s

for:

16 h/day

with total photon intensity held constant.

The researchers changed the proportion of:

green light.

CO₂ was approximately:

1000 ppm

and air temperature around:

20°C.

30. More Green Light Reduced Tipburn

Tipburn incidence decreased approximately from:

49%

in the lower-green treatment

to:

25%

under 100% green light.

That is a substantial reduction.

But there was a tradeoff.

31. Maximum Green Light Also Reduced Fresh Biomass

Compared with the lower-green treatment:

80% green

reduced shoot fresh mass by approximately:

7%.

100% green

reduced fresh mass by approximately:

25%.

So:

a spectrum that reduced tipburn also reduced yield.

Again:

maximum marketable quality and maximum biological biomass are different objectives.

32. PAR Quantity Alone Cannot Explain This Result

Every treatment received approximately:

200 PPFD.

The difference was:

spectrum.

Therefore equal PAR can still produce different:

• growth
• morphology
• tipburn.

A PAR meter tells you photon quantity.

It does not fully characterize spectrum.

33. CO₂ Has Direct Romaine-Specific Greenhouse Evidence

A 2020 Oklahoma State University experiment grew:

‘Auvona’ open-heart romaine

in greenhouse NFT hydroponics.

Researchers compared approximately:

400 ppm CO₂

with an enriched environment averaging:

800 ppm.

34. 800 ppm Increased Romaine Fresh Weight by About 24.7%

Fresh weight increased approximately from:

203.8 g

under ambient CO₂

to:

254.2 g

under enrichment.

That is approximately:

+24.7%.

Total leaf area increased from roughly:

4885 → 5989 cm².

So there is strong direct evidence that:

romaine can respond positively to moderate CO₂ enrichment.

35. But Leaf Nutritional Composition Also Changed

Under elevated CO₂:

• tissue nitrogen decreased
• phosphorus decreased
• SPAD chlorophyll-related readings decreased

relative to ambient-grown lettuce.

This is another classic dilution / compositional tradeoff.

More biomass does not automatically mean:

higher nutrient concentration per gram.

36. Tipburn Also Appeared Under Elevated CO₂

The researchers reported:

inner-leaf tipburn

at the later growth stage in CO₂-enriched ‘Auvona’.

Plants under ambient conditions did not show the same disorder.

This is extremely important.

CO₂ enrichment increased:

growth

but rapid growth also increased a:

marketability problem.

37. Therefore 800 ppm Is Not Simply “The Best CO₂”

It is a strong:

romaine-specific research reference.

The experiment demonstrated:

• greater fresh biomass
• greater leaf area

but also:

• altered tissue composition
• tipburn.

So the correct question is not:

Does 800 ppm increase growth?

It did in that experiment.

The better question is:

Does it increase marketable yield without creating quality problems in this production system?

38. Do Not Automatically Push CO₂ to 1000–1500 ppm

Other controlled-environment romaine studies sometimes use:

1000–1500 ppm

as production conditions.

That demonstrates that romaine can be grown under those concentrations.

It does not establish them as:

universal optima.

A full crop-specific CO₂-response curve is needed before making that claim.

39. High CO₂ Should Be Interpreted With DLI

If light is limiting:

extra CO₂ may provide a different return than under high DLI.

If both DLI and CO₂ promote rapid biomass accumulation:

calcium demand can also rise.

This is why:

PAR / DLI + CO₂ + tipburn

should be considered together.

40. Temperature Adds Another Major Layer

Lettuce is a cool-season crop.

Temperature affects:

• development
• respiration
• water demand
• bolting
• photosynthesis
• head quality.

Romaine-specific field research across multiple planting seasons has shown strong relationships between temperature and:

• days to harvest
• marketable heads
• bolting.

41. Cooler Conditions Can Slow Production Dramatically

In multi-season romaine research, cooler planting periods required substantially more time to reach harvest.

Across the tested seasonal environments:

approximately every:

1°C decrease

in growing-season minimum or maximum temperature was associated with about:

5 additional days to harvest

in the study’s regression.

This demonstrates the production cost of overly cool conditions.

42. But Excessive Heat Creates a Different Problem

Warm conditions can accelerate:

• development
• bolting
• quality deterioration.

Therefore, the correct objective is not:

minimum possible temperature

or:

maximum possible growth temperature.

Temperature must be balanced against crop maturity and marketability.

43. Tropical Greenhouse Research Shows Cooling Strategy Matters

Direct green-romaine greenhouse research in hot conditions compared:

• no climate treatment
• fogging + ventilation
• fogging + shading
• fogging + ventilation + shading.

Fogging plus ventilation performed particularly well for:

• growth
• yield
• sensory evaluation.

44. Heavy Shading Can Solve Heat While Creating a Light Problem

In the same tropical greenhouse work:

shading reduced photosynthesis

and could reduce marketable fresh weight.

This is a classic greenhouse tradeoff.

If shading is used to reduce temperature:

measure the resulting PAR and DLI.

Do not assume temperature control is free.

45. Root-Zone Temperature Is Another Independent Variable

A hydroponic crop can have acceptable:

air temperature

while the nutrient solution becomes:

• too warm
• too cold.

That can affect:

• roots
• water uptake
• nutrient uptake
• photosynthesis.

46. Romaine Has Direct Root-Zone Temperature Evidence

A 2021 NFT experiment tested:

17 lettuce cultivars

including several romaine cultivars.

Nutrient-solution temperatures included:

18.3°C

21.1°C

and an ambient treatment averaging approximately:

20–26.5°C.

47. Around 21.1°C Increased Shoot Fresh Weight

Across the lettuce cultivars:

plants grown around:

21.1°C root-zone temperature

produced approximately:

15% greater shoot fresh weight

than plants under ambient nutrient-solution temperatures.

But °Brix was approximately:

26% lower.

Again:

maximum biomass and maximum soluble-solids concentration were not the same outcome.

48. Romaine Cultivars Responded Differently

‘Parris Island’ and other romaine cultivars showed different:

• growth
• CO₂ assimilation
• °Brix

responses.

Therefore root-zone management also needs to be interpreted by:

cultivar.

49. New 2026 Mini-Romaine Research Confirms Root Temperature Matters

A 2026 experiment grew mini romaine in:

• NFT
• aeroponics

with nutrient-solution minimum temperatures around:

11–12°C

14°C

18°C

and:

22°C.

50. Warming a Cold Root Zone Dramatically Increased Leaf Yield

Compared with the cold ambient root zone:

18–22°C

increased leaf mass per production area by approximately:

127–232% in NFT

and:

54–75% in aeroponics.

This is a major response.

51. But Photosynthetic Indicators Did Not Simply Follow Biomass

Some photosynthetic-performance indicators were greater at lower root-zone temperatures even while crop biomass was restricted.

The researchers concluded that low temperature limited:

growth / sink performance

more strongly than photosynthetic capacity alone.

This is another reminder:

one physiological measurement cannot describe total crop performance.

52. “Head Density” Cannot Be Predicted From VPD Alone

Romaine architecture depends on:

• cultivar
• leaf number
• leaf expansion
• DLI
• temperature
• water
• calcium-related disorders
• harvest maturity.

There is no validated relationship showing:

VPD X = dense head

and:

VPD Y = loose head.

Measure head characteristics directly.

53. Crispness Also Requires Direct Measurement

The old article claimed the preharvest:

1.0–1.3 kPa VPD

range produced crisper leaves and stronger midribs.

Current direct evidence does not establish that relationship.

Crispness can depend on:

• tissue water content
• cell structure
• cultivar
• maturity
• storage
• temperature
• dehydration.

If crispness matters commercially:

measure sensory or mechanical texture directly.

54. Bitterness Is Also Not a VPD Reading

Lettuce bitterness can change with:

• cultivar
• maturity
• bolting
• heat stress
• chemistry.

The old statement:

“too much dryness increased bitterness”

may sound biologically plausible.

But a VPD reading does not directly measure bitter compounds.

Without a controlled sensory or chemical experiment, that claim should not be presented as established romaine science.

55. Shelf Life Has Much Stronger Direct Evidence

UC Davis currently recommends storing romaine near:

0°C

with relative humidity above:

95%.

At approximately:

0°C

expected storage life is around:

21 days.

At:

5°C

approximately:

14 days

may be expected if ethylene is absent.

This is direct practical postharvest guidance.

56. Cooling Romaine Quickly Is Important

Romaine has a substantial respiration rate.

As storage temperature increases:

respiration increases rapidly.

That accelerates deterioration.

Commercial cooling options include:

• vacuum cooling
• hydro-vacuum cooling
• hydrocooling
• forced-air cooling.

Therefore, shelf life is strongly controlled after harvest.

57. Fresh-Cut Romaine Has Direct Packaging Evidence

A controlled study compared fresh-cut romaine at:

0°C

and:

5°C

under several packaging films.

Packaging reduced:

weight loss

at both temperatures.

The best low-temperature packaging combinations helped preserve:

• color
• ascorbic acid
• sensory quality.

58. Storage Temperature Is Much More Directly Linked to Shelf Life Than Preharvest VPD

The old article claimed:

a preharvest VPD around 1.0–1.3 kPa improves shelf life.

Current romaine evidence is much stronger for:

• rapid cooling
• near-0°C storage
• high RH
• suitable packaging
• ethylene avoidance.

Therefore, shelf-life management should not be reduced to one preharvest atmospheric number.

59. Genetics Can Also Strongly Affect Romaine Shelf Life

A recent study evaluated many romaine genotypes and found large differences in:

postharvest longevity.

For example:

• breeding line 60184 deteriorated relatively quickly
• ‘Manatee’ was intermediate
• ‘Okeechobee’ had longer shelf life.

This demonstrates:

genotype itself is a major postharvest variable.

60. The Long-Shelf-Life Cultivar Was Not Simply the Thickest Leaf

Interestingly, the short-shelf-life genotype had:

• thicker leaf lamina
• higher stomatal index

than the long-shelf-life cultivar.

So even seemingly intuitive traits such as:

thicker leaf = longer shelf life

do not automatically hold.

Again:

measure the actual outcome.

61. Preharvest Light Can Affect Specific Postharvest Disorders

Recent 2025 romaine research examined:

midrib pinking

after harvest.

Two cultivars were grown under substantially different supplemental-light intensities.

The study found both:

• cultivar
• preharvest light environment

could influence postharvest pinking-related responses.

This demonstrates that preharvest light can matter.

But it does not prove:

one VPD prevents pinking.

62. Production CO₂ and Postharvest CO₂ Are Completely Different

During cultivation:

CO₂ is used for:

photosynthesis.

After harvest:

controlled-atmosphere CO₂ can affect:

• respiration
• browning
• physiological injury.

UC Davis notes that intact romaine heads generally do not benefit from high postharvest CO₂ and may be injured above roughly:

5% CO₂.

Fresh-cut romaine, however, may use higher CO₂ in specialized modified-atmosphere packaging.

Do not confuse these applications.

63. A Practical Romaine Measurement Workflow

Step 1 — Record the Cultivar

Specify:

• midi romaine
• mini / Little Gem
• open-heart type
• cultivar name.

Step 2 — Measure PPFD at Canopy Height

Do not rely on fixture specifications.

Step 3 — Map the Crop

Measure:

• center
• edges
• structural shadows
• dense canopy positions.

Step 4 — Record DLI

Compare:

• sunny vs cloudy days
• summer vs winter
• supplemental-light schedules.

Step 5 — Record Photoperiod

The same DLI can be produced by different:

PPFD × time

combinations.

Step 6 — Monitor CO₂ With PAR

Observe CO₂ during:

• lights-on
• strong sunlight
• greenhouse closure
• ventilation.

Step 7 — Track Temperature

Record:

• daytime average
• midday peaks
• nighttime values.

Step 8 — Monitor Root-Zone Temperature

Especially in:

• NFT
• DWC
• other recirculating hydroponics.

Step 9 — Record RH and VPD

Use VPD to understand:

atmospheric water demand.

Do not use it as a crispness target.

Step 10 — Measure Air Movement

Room-average VPD does not describe the boundary layer around:

inner young leaves.

Step 11 — Inspect for Tipburn

Record:

• first appearance
• number of affected leaves
• inner vs outer tipburn
• severity.

Step 12 — Measure Marketable Yield

Separate:

total fresh weight

from:

marketable fresh weight.

64. Practical Research-Based PPFD Reference

Direct romaine experiments provide particularly strong evidence around:

160–240 µmol/m²/s

for indoor production.

A classic tipburn study also used approximately:

320 µmol/m²/s.

These are useful research references.

They do not define a universal 160–320 PPFD optimum.

65. Practical Research-Based DLI Reference

The strongest direct controlled-environment study tested:

9.2–17.3 mol/m²/day.

Increasing DLI generally increased biomass.

For ‘Casual’:

17.3 DLI

produced the greatest yield.

For ‘Elizium’:

high biomass occurred across:

11.5–17.3 DLI.

But higher DLI also increased tipburn.

Therefore:

roughly 10–17 DLI is a strong romaine research region, not a rigid optimum band.

66. Practical CO₂ Reference

Direct open-heart romaine research compared:

~400 vs ~800 ppm.

At approximately 800 ppm:

fresh weight increased about:

24.7%.

Leaf area increased about:

22.6%.

But:

• tissue N decreased
• tipburn appeared at the later stage.

Therefore:

~800 ppm is a strong research reference, not a universal optimum.

67. Practical VPD Reference

There is no scientifically justified stage table such as:

0.4–0.8 → 0.6–1.0 → 0.8–1.2 → 1.0–1.3 kPa.

Direct romaine research instead shows that:

• daytime humidity
• nighttime humidity
• local airflow
• calcium delivery
• growth rate

interact.

Use VPD diagnostically.

68. What Should You Watch When DLI Is Increased?

As DLI rises, record:

• fresh growth
• leaf number
• tipburn
• nitrate
• marketable percentage.

If biomass increases while tipburn rises faster:

the crop has passed the point where:

more biological growth = more saleable crop.

69. What Should You Watch When CO₂ Is Increased?

Record:

• fresh weight
• leaf area
• tipburn
• DLI
• temperature
• nutrient status.

Do not assume a larger plant automatically has:

• better nutritional concentration
• better storage quality
• less physiological disorder.

70. What Should You Watch When VPD Changes?

Ask:

• Did temperature change?
• Did RH change?
• Was it daytime or nighttime?
• Did airflow change?
• Is the root zone hydrated?
• Is tipburn increasing?
• Are inner leaves actually receiving air movement?

These questions are much more useful than asking whether:

1.0 kPa is “good” and 1.4 kPa is “bad.”

71. A Better Way to Think About Romaine Measurements

Instead of asking:

What PPFD makes dense romaine heads?

ask:

How much daily light increases marketable biomass before tipburn becomes unacceptable?

Instead of:

What DLI is optimal?

ask:

Where is the yield–tipburn tradeoff for this cultivar?

Instead of:

What CO₂ concentration produces the fastest crop?

ask:

Does CO₂ enrichment increase marketable yield under the available DLI without creating excessive tipburn?

Instead of:

What VPD creates crisp leaves?

ask:

How are atmospheric demand, root water supply and calcium transport affecting the young leaves?

Instead of:

How do I improve shelf life before harvest?

ask:

How quickly can the crop be cooled and maintained near its correct postharvest temperature and humidity?

Those are much closer to the direct romaine evidence.

Final Takeaway

Greenhouse romaine lettuce does not have one scientifically established PAR, CO₂ and VPD recipe for dense heads, crisp leaves, rapid growth and long shelf life.

But romaine has unusually strong crop-specific environmental evidence.

DLI strongly affects biomass — and tipburn.

A direct 2022 experiment compared:

160 / 240 PPFD

with:

16 / 20 h photoperiods

to produce:

9.2 / 11.5 / 13.8 / 17.3 DLI.

For ‘Casual’ romaine:

fresh weight increased from approximately:

215 g at 9.2 DLI

to:

353 g at 17.3 DLI.

But inner-leaf tipburn increased from approximately:

2%

to:

18%.

For ‘Elizium’, outer-leaf tipburn rose as high as approximately:

50%

under the highest DLI treatment.

That means:

maximum biomass was not maximum marketable quality.

Nitrate moved in another direction:

higher DLI reduced nitrate accumulation.

And sensory quality had yet another optimum:

‘Elizium’ performed particularly well at approximately:

240 PPFD × 16 h = 13.8 DLI

rather than at the maximum DLI.

This is exactly why a single “best light” number is misleading.

CO₂ has strong direct romaine evidence too.

Greenhouse NFT ‘Auvona’ grown around:

800 ppm CO₂

produced approximately:

24.7% more fresh weight

than lettuce near:

400 ppm.

Leaf area increased by about:

22.6%.

But leaf nitrogen declined and inner-leaf tipburn appeared under enrichment.

Therefore:

higher CO₂ can increase growth while simultaneously increasing another production problem.

Humidity and VPD require much more careful interpretation than the old article suggested.

In direct ‘Lobjoits Green Cos’ research, lowering daytime RH from approximately:

74% to 51%

delayed tipburn and increased leaf calcium but reduced growth.

Yet reducing nighttime RH from:

95% to 90%

caused earlier tipburn and lower calcium.

So:

daytime and nighttime humidity can produce different physiological effects.

There is no justification for replacing this complexity with one universal:

“ideal VPD.”

Local airflow matters as well.

Young inner leaves can remain surrounded by humid air even if room-average VPD appears acceptable.

And new romaine research shows spectrum itself can change tipburn:

at the same:

200 PPFD

increasing green-light proportion reduced tipburn from approximately:

49% to 25%

but the highest-green treatment also reduced fresh mass by approximately:

25%.

Again:

yield and quality trade off.

Temperature must also be separated into:

• air temperature
• root-zone temperature.

New 2026 mini-romaine research found that warming cold nutrient solution from approximately:

11–12°C

toward:

18–22°C

increased leaf production dramatically in NFT and aeroponic systems.

Finally, shelf life should be treated primarily as a postharvest problem.

Current UC Davis guidance expects approximately:

21 days near 0°C

and:

14 days around 5°C

under appropriate conditions.

Rapid cooling and RH above roughly:

95%

are far better-supported shelf-life tools than the old article’s claim that a particular preharvest VPD creates longer storage life.

The stronger greenhouse strategy is therefore:

Measure PAR at the actual canopy.

Record DLI through the full day.

Record photoperiod and spectrum separately.

Monitor CO₂ together with PAR.

Track air and root-zone temperature.

Use RH and VPD to understand atmospheric demand.

Evaluate airflow around the inner leaves.

Monitor tipburn and calcium-related disorders.

Then compare those measurements with:

marketable fresh weight, tipburn incidence, nitrate, sensory quality and storage performance.

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

References

Matysiak, B., Ropelewska, E., Wrzodak, A., Kowalski, A. & Kaniszewski, S. Yield and Quality of Romaine Lettuce at Different Daily Light Integral in an Indoor Controlled Environment. Agronomy, 2022.

Assessment of Romaine Lettuce Cultivars Grown in a Vertical Hydroponic System at Two Levels of LED Light Intensity. Scientia Horticulturae, 2023.

Singh, H., Poudel, M.R., Dunn, B.L., Fontanier, C. & Kakani, G. Effect of Greenhouse CO₂ Supplementation on Yield and Mineral Element Concentrations of Leafy Greens Grown Using Nutrient Film Technique. Agronomy, 2020.

Collier, G.F. & Tibbitts, T.W. Effects of Relative Humidity and Root Temperature on Calcium Concentration and Tipburn Development in Lettuce. Journal of the American Society for Horticultural Science, 1984.

High Green Light Substitution Reduces Tipburn Incidence in Romaine Lettuce Grown in a Plant Factory with Artificial Lighting. Plants, 2026.

Moccio, M. et al. Nutrient Solution Temperature Affects Growth and °Brix Parameters of Seventeen Lettuce Cultivars Grown in an NFT Hydroponic System. Horticulturae, 2021.

Bantis, F. et al. Root-Zone Heating Boosts the Production of Mini Romaine Lettuce Grown in Nutrient Film Technique and Aeroponics Systems. Plants, 2026.

Dufault, R.J. et al. Dynamic Relationships Between Field Temperatures and Romaine Lettuce Yield and Head Quality. Scientia Horticulturae, 2009.

Kumsong, P. et al. Comparison of Different Temperature Control Systems in Tropical-Adapted Greenhouses for Green Romaine Lettuce Production. Horticulturae, 2023.

Examining Preharvest Genetic and Morphological Factors Contributing to Lettuce (Lactuca sativa L.) Shelf-Life. Scientific Reports, 2024.

Yahya, M.H., Chadwick, M.J. & Wagstaff, C. The Effect of Supplementary LED Illumination of Romaine Lettuce on Midribs Pinking After Harvest. Journal of Horticultural Science & Biotechnology, 2025.

Manolopoulou, H., Lambrinos, G., Chatzis, E., Xanthopoulos, G. & Aravantinos, E. Effect of Temperature and Modified Atmosphere Packaging on Storage Quality of Fresh-Cut Romaine Lettuce. Journal of Food Quality, 2010.

UC Davis Postharvest Research and Extension Center. Lettuce (Romaine and Loose-Leaf): Produce Facts.

Related AquaHorti Tools

For instantaneous PAR / PPFD measurements and mapping romaine-canopy light distribution, see AquaHorti AH-Quantuv.

For recording greenhouse PAR throughout the complete 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.