Leeks are slow-growing compared with many greenhouse vegetables.
But their slow growth does not mean they require a special sequence of:
low PPFD → medium PPFD → high PPFD → lower preharvest PPFD.
Direct leek research points to a different set of priorities.
Light matters.
But so do:
water supply
temperature
plant spacing
nitrogen
cultivar
planting depth and blanching
and:
bolting history.
Most importantly, the harvested white “shaft” is not a conventional stem.
It is a:
pseudostem formed mainly from elongated overlapping leaf sheaths.
So greenhouse leek production should be managed around:
whole-plant growth and pseudostem development
rather than an invented “stem-density VPD” recipe.
Quick Answer
Leeks are:
cool-season
and generally prefer:
substantial light, consistent water and fertile conditions.
Utah State Extension recommends full sun and emphasizes that leek roots are relatively shallow and require regular watering; water stress reduces both plant size and yield.
Direct controlled research compared young leeks at approximately:
550 µmol/m²/s PPFD
versus only:
20% of that intensity — about 110 µmol/m²/s.
Growth was substantially restricted under the low-light treatment.
But current research does not establish one universal leek DLI target such as:
10–14 mol/m²/day
or a stage-specific:
800–1000 ppm CO₂
requirement.
The evidence supports a better strategy:
avoid severe light limitation, maintain adequate water, manage temperature and bolting risk, and recognize that spacing and nitrogen can have major effects on pseudostem diameter and marketable yield.
The Edible Leek “Stem” Is Actually a Pseudostem
This matters because many leek articles describe the crop as if it thickens a conventional stem.
The edible cylinder consists largely of:
elongated overlapping leaf bases and sheaths.
The true stem is a highly compressed structure at the base of the plant.
So throughout this guide, the more accurate term is:
pseudostem.
This also means old AquaHorti statements about:
“hollow stems”
or:
“stem density being controlled by VPD”
should be removed unless directly supported by research.
Light Clearly Matters for Leek Growth
Leeks are not shade plants in the sense that low light has little consequence.
A Wageningen growth-chamber study grew young vegetative leeks at approximately:
550 µmol/m²/s
and:
20% of that intensity — roughly 110 µmol/m²/s.
The lower-light environment substantially altered plant growth.
This is direct evidence that:
very low PPFD can strongly restrict leek biomass production.
But 550 PPFD Is Not a Universal Optimum
The experiment compared two very different light environments.
It did not test:
100,
200,
300,
400,
500,
600 PPFD
and identify one optimum.
Therefore the correct conclusion is:
110 PPFD represented a strongly light-limited treatment relative to 550 PPFD in that experiment.
Not:
Leeks require 550 PPFD.
Another Greenhouse Study Compared About 153 and 313 PPFD
A more recent greenhouse study grew hydroponic leeks under:
40% shade
versus:
no shade.
Approximate light intensities were:
153 µmol/m²/s
and:
313 µmol/m²/s.
Under lower light, plants became:
taller
but produced less:
leaf number
fresh mass
and:
dry mass.
That directly contradicts the old AquaHorti claim that stronger light simply produces height without thickness.
Low Light Can Produce Taller but Smaller Leeks
This is an important morphological distinction.
A plant can become:
taller
without becoming:
heavier
or:
more marketable.
The shaded leek experiment found exactly that pattern.
Low-light plants increased in height while several biomass traits declined.
So:
height alone is a poor indicator of successful leek growth.
For production, measure:
pseudostem diameter,
fresh weight,
dry matter,
leaf number,
and:
marketable yield.
Higher Light Does Not Optimize Every Quality Trait
The 153-versus-313 PPFD study also found something interesting.
The more shaded plants had:
higher polyphenol content
and differences in volatile compounds.
So once again:
maximum vegetative biomass and maximum concentration of particular secondary metabolites are not necessarily the same objective.
This is another reason there is no one “best PPFD” for every production goal.
There Is No Strong Research-Based Universal Leek DLI
The old AquaHorti article claims:
3–5 DLI
then:
5–8
then:
10–14
then:
8–12 mol/m²/day.
There is not enough direct leek DLI research to justify those numbers as stage requirements.
Current leek studies more often report:
light intensity
natural greenhouse radiation
or:
shade percentage.
Therefore AquaHorti should not simply replace the old DLI chart with another unsupported one.
DLI Is Still Useful
Not having a universal optimum does not make DLI useless.
In a greenhouse, natural PPFD changes continuously with:
cloud cover,
season,
solar angle,
greenhouse glazing,
structural shadows,
and shading screens.
A noon reading of:
400 PPFD
does not tell you whether the crop accumulated:
8,
15,
or:
25 mol/m²/day.
DLI is useful for comparing:
one day with another
and:
one greenhouse zone with another.
Do Not Estimate DLI From a Few Spot Readings
If sunlight is variable:
several manual readings do not reliably reconstruct the daily light curve.
For greenhouse leek:
log PPFD through time
if actual DLI is needed.
With a constant grow light, DLI can be calculated:
DLI = PPFD × hours × 0.0036
For example:
| PPFD | 12 h | 16 h |
|---|---|---|
| 100 µmol/m²/s | 4.32 | 5.76 |
| 150 | 6.48 | 8.64 |
| 200 | 8.64 | 11.52 |
| 300 | 12.96 | 17.28 |
| 400 | 17.28 | 23.04 |
These are mathematical conversions.
They are not leek recommendations.
Water Has Much Stronger Direct Evidence Than a Leek-Specific VPD Target
One of the strongest direct leek studies compared irrigation regimes.
As water supply decreased:
plant height
pseudostem diameter
leaf fresh weight
pseudostem fresh weight
dry matter
and:
leaf area
all declined.
This is very important because the old AquaHorti page attributes thin leeks primarily to:
PPFD,
CO₂
and:
VPD.
Direct evidence says:
root-zone water supply itself can strongly control pseudostem diameter.
Drought Directly Reduces Leek Photosynthesis
A separate experiment on Gigante Suizo leek compared adequate irrigation with drought stress.
Under drought:
photosynthetic rate fell from roughly:
5.0–5.4
to:
3.3–3.4 µmol CO₂/m²/s.
Transpiration also fell substantially.
Total fresh weight dropped dramatically as well.
Depending on treatment, well-watered plants were roughly:
355–453 g
while drought-stressed plants were approximately:
152–255 g.
Drought Also Reduced Pseudostem Diameter
The same experiment reported reductions in:
leaf area,
plant fresh weight,
pseudostem length,
and:
pseudostem diameter
under water stress.
That gives us a much stronger explanation for thin marketable shafts than:
“VPD quietly determines stem density.”
The crop needs:
adequate root-zone water
to maintain growth.
VPD Is Still Useful—but It Is Not a Leek-Diameter Meter
VPD describes:
atmospheric evaporative demand.
It can help growers understand how strongly the air is driving:
transpiration
and:
water use.
But I could not find strong leek-specific trials establishing:
0.3–0.6 kPa for germination
0.5–0.9 for seedlings
0.8–1.2 for vegetative growth
and:
1.0–1.3 before harvest.
Those old values should be deleted.
The stronger practical question is:
Can the root system supply enough water under the current atmospheric demand?
Root-Zone Water and VPD Must Be Separated
Two leek crops can experience the same:
1.0 kPa VPD
but completely different root conditions.
One may have:
adequate moisture.
The other may have:
dry substrate.
Their physiological response will not necessarily be the same.
So VPD should be interpreted together with:
root-zone moisture
temperature
and:
irrigation frequency.
Leek Roots Are Relatively Shallow
Utah State Extension specifically emphasizes that leeks have:
relatively shallow roots
and require regular irrigation.
Water stress reduces:
plant size
and:
yield.
This makes water management particularly important during long greenhouse production cycles.
A crop can look reasonably healthy above ground while gradually losing:
marketable pseudostem diameter.
Spacing Can Change Pseudostem Diameter
This is one of the biggest missing factors in the old article.
A study in Hokkaido tested leek spacing of:
5 cm
10 cm
and:
15 cm
within the row.
At:
5 cm
the pseudostems were thinner.
A 10-cm spacing performed better for the study’s target of producing pseudostems larger than approximately:
3 cm diameter.
So when leeks are:
long and thin
the answer is not automatically:
“adjust PPFD and VPD.”
Check:
plant density.
High Density Can Produce Long, Thin Leeks
Another density experiment produced the same general lesson.
Higher plant density reduced:
individual pseudostem weight
and:
diameter,
while increasing pseudostem length under some spacing arrangements.
That means:
tall + thin can be a spacing response.
It is not a unique signature of:
high PPFD,
high VPD,
or low CO₂.
Nitrogen Can Strongly Influence Leek Yield and Diameter
Leek also responds strongly to nitrogen availability.
A field experiment comparing:
0,
50,
100
and:
200 kg N/ha
found increasing nitrogen associated with greater:
leaf number,
leaf weight,
pseudostem diameter,
pseudostem weight,
and total yield under that specific soil and cultivar.
At the highest tested nitrogen treatment, pseudostem diameter reached approximately:
36.3 mm
in that experiment.
Again:
pseudostem thickness is not controlled by light alone.
Do Not Turn One Nitrogen Experiment Into a Fertilizer Recipe
The experiment does not mean:
every greenhouse leek crop should receive 200 kg N/ha.
Greenhouse hydroponics,
soil culture,
container systems
and different cultivars all have different nutrient-management requirements.
The useful lesson is:
nitrogen supply can materially influence leek growth and pseudostem development.
So any diagnosis of thin leeks should include nutrition.
Cultivar Matters Too
Greenhouse trials with nine leek cultivars found substantial differences in:
yield,
pseudostem dry matter,
sugars,
antioxidants,
and:
mineral composition.
Some cultivars naturally produce:
thicker
or:
heavier pseudostems
than others.
So avoid comparing:
two cultivars
and attributing every difference to:
PPFD or VPD.
CO₂ Does Not Have the Response the Old Article Claims
The old page says moderate or elevated CO₂ helped biomass and recommends:
600–800 ppm
then:
800–1000 ppm
before later dropping to:
700–900 ppm.
But there is direct leek-specific evidence that makes this claim questionable.
A 1994 experiment increased atmospheric CO₂ from approximately:
355 ppm
to:
800–900 ppm
across eight vegetable crops.
Several vegetables showed significant yield responses.
Leek did not.
Elevated CO₂ Did Not Significantly Increase Leek Yield in That Experiment
The tested leek cultivars did not show a statistically significant yield increase from:
355 → 800–900 ppm CO₂.
That is a very useful correction.
It means AquaHorti should definitely remove statements such as:
“800–1000 ppm is the ideal vegetative leek CO₂ range.”
There is no direct basis for that.
This Does Not Mean CO₂ Never Matters
CO₂ remains an essential substrate for photosynthesis.
In a tightly closed greenhouse, concentration can potentially fall during active photosynthesis.
Monitoring can therefore still be useful.
But:
avoiding severe CO₂ depletion
and:
deliberately enriching to 900 ppm
are two different management decisions.
Current leek-specific evidence does not justify a universal enrichment target.
Leek Temperature Matters
A controlled greenhouse study compared leaf gas exchange at:
20°C
and:
27°C
in two leek cultivars.
Higher temperature reduced:
photosynthesis
and:
transpiration,
with especially large reductions in one cultivar.
The researchers noted greenhouse conditions approaching:
25°C
were near temperatures associated with inhibited leek growth in their production context.
So leeks deserve their reputation as a:
cool-season crop.
But There Is No Universal 18–24°C “Vegetative Target”
The old page assigns narrow temperature bands to every stage.
Again, those values may be reasonable operational conditions.
But the direct literature shows:
cultivar differences
and interactions with:
light,
development,
and reproductive induction.
Use temperature as a monitored variable rather than another rigid stage table.
Cold Can Trigger Bolting
This is especially important for a long-cycle leek crop.
Leek has a:
vernalization requirement
for flowering.
Controlled experiments found the strongest vernalization response around:
5°C
with inductive temperatures extending roughly from:
0–18°C.
Temperatures above approximately:
18°C
could reverse or reduce the vernalization response in the experiment.
Plant Size Changes Bolting Sensitivity
The same study identified a juvenile phase ending around:
2 g plant weight
or approximately:
five visible leaves.
Young plants below that developmental stage are less reproductively competent.
As leeks become larger:
exposure to prolonged cold can become much more relevant to:
bolting risk.
So greenhouse temperature management is not simply:
cooler is always better.
Daylength Interacts With Vernalization
The research showed:
short days during vernalization
and:
long days afterward
could promote bolting.
This is a much more biologically meaningful photoperiod story than giving leeks one:
PPFD + DLI growth-stage table.
For long-cycle production:
monitor the plant’s:
cold history
and:
developmental stage.
Strong Transplants Can Influence Final Yield
Research on early leek production found final harvest weight strongly related to:
transplant weight.
Across multiple experiments, transplant weight accounted for a large portion of final weight variation.
That gives propagation another important objective:
produce adequately developed transplants
rather than:
maintain seedlings at exactly 3–5 DLI.
But Bigger Transplants Can Carry More Bolting Risk
The same research found larger transplants could be more responsive to:
cold-induced flower initiation.
Plants raised around:
9°C
had substantially greater bolting than comparable plants raised at warmer temperatures.
So there is a real tradeoff:
large transplants can accelerate production
but:
developmental size + cold exposure can increase reproductive risk.
That is much more useful than the old:
“low VPD makes better seedlings”
claim.
The White Leek Shaft Is Produced by Blanching
This is another major omission in the old article.
Consumers often want a:
long white pseudostem.
That white tissue develops because light is excluded from the lower leaf sheaths.
Common techniques include:
deep planting
and:
hilling soil around the plant.
Utah State specifically recommends deep dibble planting and hilling during growth to produce a longer blanched portion.
Do Not Reduce Whole-Canopy PPFD to Make a Whiter Leek
This distinction is crucial.
If the production goal is:
a longer white shaft,
the solution is not:
reduce greenhouse PPFD before harvest.
Instead:
exclude light from the lower pseudostem while keeping foliage adequately illuminated.
Hilling itself primarily affects:
blanching
rather than magically increasing pseudostem weight or diameter. Direct density/hilling research similarly found hilling important for whitening but not a major driver of diameter or weight.
This Is Why the Old “Preharvest Light Reduction” Stage Should Go
The old AquaHorti article recommends reducing:
250–400 PPFD
during vegetative growth
to:
200–350 PPFD
near harvest
and claims this produced:
firmer shafts,
less internal hollowing,
better uniformity,
and better postharvest handling.
There is no adequate evidence for that treatment.
If the goal is white pseudostem development:
use:
blanching practices.
If the goal is postharvest quality:
use:
proper harvest maturity and cooling.
Leeks Continue Growing After Harvest
Leek has an unusual postharvest quality problem:
inner leaf extension
or:
telescoping.
After trimming, inner leaves can continue growing and extend above the cut pseudostem during storage.
This reduces:
appearance
and:
marketability.
So postharvest quality is not simply determined by the preharvest VPD.
Cold Storage Is Important
Utah State recommends harvested leeks be held in:
cold, humid conditions
to minimize moisture loss.
Postharvest research likewise shows storage temperature strongly influences:
inner leaf extension
and quality deterioration.
Therefore:
shelf life should not be attributed to a preharvest VPD of 1.0–1.3 kPa.
A Better Leek Production Framework
| Factor | What direct evidence tells us |
|---|---|
| Low PPFD | ~110 PPFD substantially restricts growth compared with ~550 PPFD in one direct study |
| Moderate shade | ~153 vs 313 PPFD altered height, biomass and phytochemicals |
| DLI | Useful for tracking greenhouse photon supply, but no validated universal leek DLI target exists |
| Water | Deficit irrigation directly reduces pseudostem diameter and yield |
| VPD | Useful for atmospheric water demand; no validated leek stage-specific kPa recipe |
| CO₂ | 355 → 800–900 ppm did not significantly increase leek yield in one multi-crop experiment |
| Temperature | High temperature can depress gas exchange; cold exposure can induce vernalization |
| Plant spacing | Strong influence on individual pseudostem diameter and marketable size |
| Nitrogen | Can materially alter pseudostem growth and yield |
| Blanching | Deep planting/hilling increases white pseudostem length |
| Cultivar | Significant differences in yield and quality |
| Postharvest | Cold, humid storage and control of inner-leaf extension matter |
These are:
research-based relationships
not:
one greenhouse recipe.
Frequently Asked Questions
What PPFD should greenhouse leeks receive?
There is no single validated universal optimum.
Direct research compared approximately:
110 vs 550 µmol/m²/s
and found substantial growth restriction at the lower intensity.
Another greenhouse experiment compared approximately:
153 vs 313 PPFD
and found lower-light plants became taller but accumulated less biomass.
Is 250–400 PPFD the ideal leek vegetative range?
It may be a workable greenhouse environment.
But there is not enough direct research to call it:
the universal optimum.
The old AquaHorti range should therefore not be presented as a biological requirement.
What DLI do leeks need?
No strong crop-specific universal DLI optimum has been established.
Use DLI mainly to compare:
daily light availability
and:
seasonal greenhouse conditions.
Are 10–14 DLI required for mature leeks?
No reliable evidence establishes:
10–14 mol/m²/day
as the universal vegetative leek requirement.
Does low light make leeks taller?
It can.
A direct greenhouse study found lower-light leeks became taller while producing less fresh and dry biomass.
So taller does not automatically mean:
better growth.
What makes leek pseudostems thicker?
Direct evidence points to several factors, including:
light
water availability
plant spacing
nitrogen
cultivar
and:
overall growth duration.
There is no evidence that one VPD number controls pseudostem thickness.
Does plant spacing matter?
Yes.
Higher density can produce:
thinner individual pseudostems,
while wider spacing can increase individual plant size.
Is the white part of a leek a stem?
Botanically, it is primarily a:
pseudostem formed by overlapping elongated leaf sheaths.
How do I get a longer white leek shaft?
Use:
deep planting
and/or:
hilling
to exclude light from the lower pseudostem.
Do not reduce light to the entire canopy just to make the shaft white.
Should I reduce PPFD before leek harvest?
There is no strong evidence supporting a universal preharvest PPFD reduction for improved:
density,
texture
or:
shelf life.
What CO₂ level do leeks need?
No universal leek enrichment optimum has been established.
In one direct experiment, increasing CO₂ from approximately:
355 to 800–900 ppm
did not significantly increase yield in two leek cultivars.
Does that mean CO₂ never matters?
No.
CO₂ is still required for photosynthesis, and depletion can matter in tightly enclosed greenhouses.
But the available evidence does not justify:
800–1000 ppm as a universal leek target.
What VPD should leek use?
There is no validated leek-specific stage table such as:
0.3–0.6 → 0.5–0.9 → 0.8–1.2 kPa.
Use VPD to understand:
atmospheric water demand
and interpret it together with root-zone moisture.
Are leeks sensitive to drought?
Yes.
Direct experiments show drought reduces:
photosynthesis,
transpiration,
fresh weight,
leaf area
and:
pseudostem diameter.
Why are my leeks tall but thin?
Possible causes include:
low light,
high planting density,
water stress,
cultivar,
nutrition,
or insufficient time.
Do not diagnose this from PPFD alone.
What temperature do leeks prefer?
Leek is a cool-season crop.
A direct greenhouse gas-exchange study found reduced photosynthesis and transpiration at:
27°C compared with 20°C
in the tested cultivars.
Can cold make leeks bolt?
Yes.
Leek has a vernalization requirement.
Cold around:
5°C
was especially effective at flower induction in direct research, with inductive temperatures extending broadly through approximately:
0–18°C.
Does plant size affect bolting?
Yes.
The juvenile phase was estimated to end around:
2 g plant weight
or approximately:
five visible leaves
in the vernalization experiment.
Does daylength affect bolting?
Yes.
Long days following vernalization can promote bolting.
Why are my greenhouse leeks not thickening?
Check:
light availability
water
plant density
nitrogen
cultivar
root health
and:
production time.
PPFD, CO₂ and VPD alone are not enough to diagnose pseudostem size.
The Main Takeaway
The old AquaHorti leek article should no longer publish this staged recipe:
60–120 PPFD / 3–5 DLI
→ 120–220 / 5–8
→ 250–400 / 10–14
→ 200–350 / 8–12
combined with a staged:
400–1000 ppm CO₂
and:
0.3–1.3 kPa VPD
program.
Direct evidence gives us a much stronger story.
Low light can reduce leek biomass, and approximately:
110 PPFD
was strongly limiting relative to:
550 PPFD
in one direct growth experiment.
But pseudostem quality cannot be explained by light alone.
Direct irrigation research shows that water deficit reduces:
pseudostem diameter
and:
yield.
Plant-spacing studies show crowding can create:
longer, thinner individual leeks.
Nitrogen and cultivar also influence:
pseudostem diameter and marketable production.
And contrary to the old CO₂ ladder, one direct experiment found:
no significant leek-yield response when CO₂ increased from about 355 to 800–900 ppm.
Most importantly:
the long white edible shaft is produced mainly through blanching of the pseudostem—not by reducing the entire crop’s PPFD near harvest.
So the better greenhouse strategy is:
Measure PPFD and DLI → avoid severe light limitation → maintain consistent root-zone water → manage plant density and nitrogen → monitor temperature and cold history for bolting → use deep planting or hilling for blanching → evaluate pseudostem diameter and marketable weight directly.
Measuring Greenhouse Leek Conditions
Useful measurements include:
PPFD
DLI
air temperature
humidity / VPD
and, where possible:
root-zone moisture and EC.
CO₂ can also be logged in enclosed production, but it should not automatically be enriched to a fixed leek-specific ppm target.
Related AquaHorti pages:
Horticulture Measurement Guide → /horticulture-measurement
PPFD, CO₂ and VPD Interaction → /the-relationship-between-par-co%e2%82%82-and-vpd-in-plant-growth/
AH-200 → /ah-200
References
van der Werf, Enserink, Smit & Booij — Components of Relative Growth Rate and Nitrogen Productivity of Brussels Sprouts and Leeks Grown at Two Widely Differing Light Intensities. Netherlands Journal of Agricultural Science, 1996.
Ntobela et al. — Evaluating the Effect of 40% and 0% Shading Levels on Allium porrum Cultivated Hydroponically Under Greenhouse Conditions, 2022.
Mortensen — Effects of Elevated CO₂ Concentrations on Growth and Yield of Eight Vegetable Species in a Cool Climate. Scientia Horticulturae, 1994.
Jezdinský et al. — Effect of Drought Stress and Mycorrhizal Inoculation on Growth, Photosynthetic Activity and Water Use Efficiency of Leek, 2012.
Kiremit & Arslan — Response of Leek to Different Irrigation Water Levels Under Rain Shelter, 2018.
Effects of Temperature and Daylength on Bolting of Leek. Scientia Horticulturae, 1994.
Doran et al. — Leaf Photosynthesis and Transpiration of Two Leek Cultivars With Differing Pseudostem Pungency Levels. Acta Horticulturae.
Utah State University Extension — How to Grow Leeks in Your Garden.