Sweet pepper (Capsicum annuum) is a long-cycle greenhouse crop.
A healthy plant can have:
large leaves
thick stems
many flowers
and still produce:
poor fruit set
strong week-to-week yield fluctuations
or:
too many aborted young fruits.
That is because sweet pepper yield is not controlled by one environmental number.
Fruit production depends on the balance between:
photosynthetic carbon supply
and:
the competing demand from leaves, stems, flowers and developing fruits.
Light, CO₂, temperature and water conditions all influence that balance.
For that reason, there is no strong scientific basis for a rigid recipe such as:
400–650 PPFD + 900–1200 ppm CO₂ + 0.6–1.0 kPa VPD = the flowering stage.
Direct sweet-pepper research supports a much more useful model:
Manage light and CO₂ to maintain plant source strength, avoid temperatures that damage reproductive development, monitor water and root-zone conditions, and interpret fruit set in the context of existing fruit load.
Quick Answer
Sweet pepper is a relatively high-light greenhouse crop.
Low light can reduce:
dry-matter production
and:
assimilate availability for flowers and young fruits.
Direct experiments show that reducing source strength through:
shading,
greater plant density,
or leaf removal
increases flower and young-fruit abortion.
But that does not mean:
maximum PPFD always gives maximum marketable yield.
Modern greenhouse experiments show that:
light distribution
spectrum
plant density
existing fruit load
and:
seasonal natural DLI
can all change the response to supplemental lighting.
So PPFD should be managed as part of:
a whole-canopy carbon budget.
PPFD, PAR and DLI Are Different
PAR describes the conventional photosynthetically active radiation waveband:
400–700 nm.
When a quantum sensor reads:
500 µmol/m²/s
the quantity is more precisely:
PPFD — Photosynthetic Photon Flux Density.
DLI integrates PPFD over the whole day:
mol/m²/day.
So:
PPFD = photon flux right now
while:
DLI = total daily photosynthetic photon exposure.
For constant artificial lighting:
DLI = PPFD × hours × 0.0036
For example:
| PPFD | 12 h | 16 h | 18 h |
|---|---|---|---|
| 200 µmol/m²/s | 8.64 | 11.52 | 12.96 |
| 300 | 12.96 | 17.28 | 19.44 |
| 400 | 17.28 | 23.04 | 25.92 |
| 500 | 21.60 | 28.80 | 32.40 |
| 600 | 25.92 | 34.56 | 38.88 |
These are mathematical conversions.
They are not universal sweet-pepper targets.
Sweet Pepper Seedlings Do Not Have a Proven 6–10 DLI Requirement
One of the clearest reasons to remove the old AquaHorti stage table comes from new bell-pepper seedling research.
A 2025 controlled study grew bell pepper seedlings in an indoor vertical farm at approximately:
23.7
31.7
and:
39.6 mol/m²/day DLI.
The respective PPFD treatments were approximately:
275
367
and:
458 µmol/m²/s.
Bell pepper seedlings accumulated the greatest tested biomass at:
39.6 DLI.
That is completely different from the old AquaHorti claim:
Seedling DLI = 6–10.
High-DLI Seedlings Grew Much Faster
Compared with greenhouse-grown seedlings in that experiment, bell pepper seedlings receiving:
23.7 DLI
already accumulated approximately:
264% more total dry biomass.
At:
39.6 DLI
the increase was approximately:
333%.
But this does not mean:
every pepper nursery should use 39.6 DLI.
The study used:
dense 200-cell trays,
artificial LED lighting,
controlled temperature,
specific nutrient-solution EC,
and an indoor vertical-farm system.
The proper conclusion is:
Pepper seedlings can use much more light than the old 6–10 DLI recommendation suggests.
Maximum Seedling Biomass Is Not Necessarily the Commercial Optimum
Lighting seedlings at nearly:
40 DLI
requires substantial electricity.
A nursery may decide that:
23,
30,
or another DLI
produces sufficiently strong transplants at a better:
energy cost per plant.
The scientific experiment identifies:
biological response.
The grower still has to optimize:
economics.
That distinction should remain clear.
Mature Sweet Pepper Also Responds Strongly to Daily Light
A controlled sweet-pepper study compared theoretical DLIs of:
5
10
15
and:
20 mol/m²/day.
The highest fruit production occurred at:
20 DLI
among those treatments.
At approximately:
5 DLI
fruit production was severely restricted.
This is useful evidence that fruiting sweet peppers require substantial daily photon supply.
But because this was one controlled study:
20 DLI should be treated as a research reference, not a universal optimum.
Commercial Pepper DLI Can Be Much Higher
Real greenhouse pepper crops can experience considerably higher DLI.
One greenhouse experiment with sweet pepper Kori experienced average seasonal DLIs of approximately:
16.6 mol/m²/day in winter
and:
32.2 mol/m²/day in summer.
Additional far-red interlighting increased shoot dry weight and fruit yield during the:
lower-light winter period
but produced little additional growth or yield response in summer when sunlight was already abundant.
That tells us:
supplemental light has the greatest value when natural light is limiting.
The Same Extra Light Is Not Equally Valuable in Every Season
If winter greenhouse DLI is low:
additional photons can meaningfully increase crop carbon supply.
If summer DLI is already high:
the same supplemental-light treatment may produce little extra yield.
So a fixed rule such as:
Flowering peppers require 400–650 PPFD
misses the real question.
The useful question is:
How much total light is already reaching the canopy today?
That is why DLI logging matters.
DLI Alone Still Does Not Explain Yield
A 2025 greenhouse experiment provides an especially useful example.
Sweet pepper Mavera was grown under:
a normal polyethylene greenhouse covering
or:
a film that converted part of green sunlight toward red wavelengths.
The light-conversion treatment actually produced approximately:
9.4% lower DLI
with an average around:
21.1 mol/m²/day.
Despite receiving fewer total PAR photons, those plants produced approximately:
29% greater cumulative fruit yield.
This is an important lesson:
DLI measures photon quantity, but does not fully describe spectrum or how effectively the canopy uses those photons.
Spectrum Matters for Sweet Pepper
Sweet pepper has unusually good direct spectrum research.
Researchers have changed:
red,
blue,
green,
and:
far-red
fractions while maintaining substantial photosynthetic lighting.
The results show differences in:
plant architecture,
light penetration,
fruit set,
fruit mass,
yield,
carotenoids,
and:
fruit cracking.
Therefore:
PPFD alone cannot describe the entire lighting treatment.
Green Light Can Improve Light Penetration Into a Tall Pepper Canopy
Sweet pepper creates a:
deep vertical canopy.
Top leaves receive far more light than lower leaves.
A direct lighting experiment found that adding green photons improved light penetration deep within the canopy by approximately:
43–158%
depending on treatment.
Individual fruit weight increased approximately:
2–15%
depending on cultivar and spectrum.
So canopy lighting is not simply about:
the PPFD measured at the top leaf.
Measure More Than the Top of the Canopy
A tall greenhouse pepper plant can be:
well lit at the top
and:
strongly light limited in the middle and lower canopy.
That affects the photosynthetic contribution of:
older leaves
and:
the carbon supply available to nearby developing fruits.
Intra-canopy lighting research has increased spring pepper yield by approximately:
30%
by improving photosynthesis within shaded canopy regions.
For measurement, consider checking:
top canopy
middle canopy
and:
lower productive canopy.
Fruit Set Is a Source–Sink Problem
This is probably the most important part of greenhouse pepper physiology.
A flower or young fruit requires:
assimilates
to survive and continue growing.
But it is competing against:
leaves,
stems,
roots,
and especially:
older rapidly growing fruits.
A classic Wageningen experiment manipulated source strength by:
shading,
plant density,
and leaf pruning.
Every method that reduced source strength increased:
flower and young-fruit abortion.
The First Week After Flowering Is Particularly Sensitive
The same research showed that pepper flowers and young fruits were especially susceptible to abortion:
around the first week after anthesis.
This gives growers a much better explanation for inconsistent fruit set than:
VPD was 1.2 instead of 0.9.
At that moment the plant must have enough carbon supply relative to:
all competing sinks.
Existing Fruits Can Cause New Flowers to Abort
Sweet pepper crops often exhibit:
waves of fruit set.
A heavy flush of growing fruit creates powerful sinks for assimilates.
New flowers appearing while those fruits are rapidly expanding may receive less carbon and become more likely to abort.
A review of reproductive abortion in sweet pepper describes exactly this cyclical pattern.
So a plant can have:
good light,
good CO₂,
reasonable temperature
and still temporarily set fewer fruits because:
existing fruit load is consuming the available assimilates.
Fruit Load Is Therefore Part of “Light Management”
This may sound counterintuitive.
Fruit load is not a light measurement.
But if the same canopy must support:
five rapidly growing fruits
versus:
fifteen,
the source–sink balance changes dramatically.
A 2022 greenhouse study found that fruit-set ratio increased with:
dry-matter production
and:
source-to-sink ratio
while decreasing as competing fruit sink strength increased.
That is why PPFD should be interpreted together with:
crop load.
More Light Can Reduce Flower and Fruit Abortion
Because more usable light can increase source strength, supplemental lighting can improve fruit set under light-limited conditions.
Winter intra-canopy lighting studies have shown better:
fruit set,
fruit survival,
and:
fruit yield
under supplemental illumination.
But again:
this does not establish one flowering PPFD threshold.
It demonstrates:
a carbon-balance response.
Far-Red Can Alter Source Strength and Fruit Set
A 2024 experiment added:
0, 50 or 100 µmol/m²/s far-red
to approximately:
190 µmol/m²/s white supplemental light
during generative growth.
Additional far-red increased:
plant dry matter,
fruit set,
and:
fruit yield.
It also substantially reduced moderate/severe fruit cracking in the tested cultivars.
But far-red photons were not used as efficiently as PAR photons for biomass production.
So this is not evidence for:
more far-red is always better.
It shows that:
spectrum changes plant architecture, source strength and reproductive behavior.
Too Much Blue Relative to Red Can Reduce Fruit Set
Spectrum can also work in the opposite direction.
A 2024 sweet-pepper experiment found that a very high:
blue:red ratio of 9:1
reduced fruit set compared with lower blue:red ratios.
The reduced fruit set was associated with:
lower flower/fruit starch
and:
changes in several plant hormones linked with abortion.
This is another reason not to treat:
“500 PPFD”
as a complete lighting description.
Temperature Can Directly Damage Pepper Reproduction
Light is not always the problem when flowers drop.
Bell pepper reproductive development is particularly sensitive to:
high temperature.
A controlled experiment found that exposure to approximately:
33°C
during specific early flower-development stages reduced:
pollen viability,
fruit set,
seed number,
and:
fruit quality.
Flowers reaching anthesis during high-temperature treatment were also vulnerable.
So when greenhouse flowers abort:
check temperature history before blaming PPFD or VPD.
A Brief Heat Event Can Matter Before the Flower Even Opens
One particularly important detail is that the sensitive flower may still be:
a small bud.
Damage to pollen development can occur before anthesis.
That means:
today’s flower drop may reflect a temperature event several days earlier.
A single current temperature reading cannot reconstruct that history.
Environmental logging is therefore much more useful than:
spot checks.
33°C Is Not a Universal Pepper Death Threshold
The controlled research demonstrates reproductive sensitivity around:
33°C
under its experimental conditions.
It does not mean:
32.9°C is safe and 33.0°C is failure.
Cultivars differ in:
heat tolerance,
plant water status,
exposure duration,
night temperature,
and stage of floral development.
Use 33°C as:
direct research evidence of heat-sensitive reproductive development
rather than a universal switch.
Old Flowering VPD Targets Are Too Simplistic
The old AquaHorti article says flowering sweet peppers should receive:
0.6–1.0 kPa VPD
and that they require lower VPD than tomatoes.
Direct sweet-pepper humidity research does not support such a simple rule.
In one greenhouse experiment:
daytime VPD ranged about:
0.33–0.66 kPa
and nighttime VPD about:
0.27–0.86 kPa.
Lower daytime VPD increased:
fruit set
and:
seed number.
But numbers of flowers and fruits were positively correlated with:
higher nighttime VPD.
That is already enough to show:
day and night responses cannot be summarized by one flowering VPD.
A Separate Humidity Experiment Found Little Yield Effect
Another glasshouse sweet-pepper experiment used day/night VPD environments with approximately:
0.30–0.78 kPa 24-hour mean VPD.
Neither vegetative growth nor early or final yield was significantly correlated with:
day,
night,
or:
24-hour humidity treatment.
Higher nighttime humidity did increase:
mean fruit weight.
So publishing:
VPD 0.6–1.0 = reliable fruit set
would overstate the evidence.
VPD Is Still Useful
VPD remains a valuable measurement because it describes:
atmospheric evaporative demand.
It helps you understand potential:
transpiration pressure
and:
water demand.
But in sweet pepper it should be interpreted alongside:
temperature,
root-zone water,
crop load,
light,
and:
time of day.
It is not a direct:
pollen-viability meter
or:
fruit-set meter.
CO₂ Can Increase Sweet Pepper Yield
Sweet pepper has direct greenhouse CO₂ enrichment research.
One experiment compared approximately:
natural concentration,
500 ppm
and:
800 ppm CO₂
in three Italian-type sweet-pepper cultivars.
CO₂ enrichment increased:
plant growth
and:
yield.
The greatest yields generally occurred under the:
800 ppm treatment,
and average fruit weight also increased.
That makes 500–800 ppm a useful direct research region.
But it does not establish:
800 ppm as the universal optimum.
Another CO₂ Experiment Shows Why Depletion Matters
A classic greenhouse study compared sweet pepper around:
200
340
and:
500 ppm CO₂.
The crop at:
500 ppm
produced approximately:
60% more harvested fruits per square meter
than the crop maintained near:
200 ppm.
This demonstrates one particularly important point:
preventing severe daytime CO₂ depletion can be extremely important.
CO₂ Should Be Monitored During the Light Period
A tightly closed greenhouse can experience:
active photosynthesis
while ventilation is limited.
CO₂ may then fall below outside concentration.
So before asking:
Should I enrich to 900 or 1200 ppm?
first ask:
What CO₂ is actually present around the canopy during peak photosynthesis?
A grower who prevents:
200–300 ppm depletion
may gain more than one trying to fine-tune:
950 versus 1,050 ppm.
The Old 900–1200 ppm Flowering Target Is Not Proven
The current AquaHorti article specifically assigns:
900–1200 ppm CO₂
to sweet-pepper flowering.
There is no evidence that flowers have a unique biological requirement for this band.
CO₂ helps through:
photosynthetic carbon supply.
Its value depends strongly on:
light,
temperature,
ventilation,
crop load,
and economics.
So delete the stage-specific CO₂ ladder.
Elevated CO₂ Can Also Change Other Crop Outcomes
The 500/800-ppm greenhouse experiment found improved:
water-use efficiency
under enrichment.
But blossom-end rot was actually more common in some enriched treatments than in controls.
This is another useful warning:
higher yield does not guarantee fewer physiological disorders.
CO₂, Temperature and Water Interact
A pepper crop cannot use additional CO₂ optimally if:
temperature damages reproduction,
stomata close because of water stress,
or:
root-zone conditions are poor.
A review of elevated CO₂ effects in greenhouse fruit crops finds that pepper yield responses vary strongly with:
irrigation,
salinity,
nitrogen,
pruning,
and other environmental factors.
So CO₂ should never be interpreted in isolation.
Blossom-End Rot Is Not a “Late-Stage VPD Problem”
The old AquaHorti article claims that changing late-stage VPD reduced:
blossom-end issues.
That is too simplistic.
Blossom-end rot in pepper is associated with a complex interaction involving:
calcium distribution,
water movement,
root health,
salinity,
rapid tissue development,
and environmental stress.
Root-zone management is especially important.
Adequate Calcium in the Nutrient Solution Does Not Guarantee Healthy Fruit
Pepper fruits compete poorly with leaves for transpiration-driven calcium transport.
Leaves usually transpire much more strongly.
Developing fruits transpire comparatively little.
As a result, calcium can be abundant in the:
root zone
and:
vegetative tissues
while fruit tissues still receive inadequate local supply.
So simply adding calcium is not always enough.
Root-Zone EC and Salinity Matter
Direct greenhouse sweet-pepper research shows that increasing salinity can:
reduce water status,
reduce marketable yield,
and:
increase blossom-end rot.
That is why a fruit-quality investigation should include:
root-zone EC
and:
irrigation uniformity
rather than only:
VPD.
Irregular Water Supply Can Trigger Fruit Problems
Environmental stress that alters water movement can contribute to:
blossom-end rot
and:
fruit cracking.
For greenhouse pepper, maintain consistent:
root-zone moisture
rather than alternating strongly between:
dry
and:
wet.
Canadian greenhouse guidance also identifies fluctuating moisture, temperature and humidity as factors associated with calcium-distribution problems in developing pepper fruits.
Fruit Cracking Is Also Not Controlled by One VPD
A fruit can crack because of interactions among:
water availability,
root pressure,
fruit growth,
cultivar,
temperature,
and light environment.
Recent far-red research reduced moderate/severe cracking from:
approximately 17–25%
to around:
8%
in the two tested cultivars.
That result alone shows how:
crop morphology and spectrum
can influence fruit disorders independently of one room VPD.
Do Not Reduce Light Automatically During Ripening
The old AquaHorti article recommends lowering PPFD from fruit development to ripening.
There is no strong evidence that sweet pepper has a biological:
“lower-light ripening stage.”
Colored pepper fruit remain attached to a photosynthetically active crop that still requires carbon for:
fruit filling,
new flowers,
new fruits,
roots,
and:
vegetative growth.
Reducing supplemental light may make economic sense if:
natural DLI increases
or the crop’s remaining harvest value is low.
But it should not be published as:
a universal physiological requirement.
Do Not Automatically Reduce CO₂ During Ripening Either
The same problem applies to the old progression:
900–1200 ppm at flowering
→ 800–1000 during fruit development
→ 700–900 during ripening.
There is no established sweet-pepper rule requiring those staged reductions.
CO₂ enrichment should instead be adjusted according to:
light availability
ventilation
remaining crop load
crop photosynthetic demand
and:
economics.
Fruit Color Is Influenced by Spectrum
Supplemental interlighting has also changed:
ascorbic acid,
soluble sugars,
and:
carotenoid composition
in red and yellow sweet peppers.
For example, red + blue interlighting and additional far-red increased yield in the tested greenhouse system, but spectral treatments also changed fruit carotenoid profiles.
So:
fruit color development ≠ simply a PPFD problem.
High Yield and Maximum Carotenoid Content May Not Be the Same Lighting Goal
This is similar to what we have already seen in other crops.
A lighting treatment can improve:
fruit number
or:
total yield
without maximizing:
pigment concentration
or another quality trait.
Therefore define what you are optimizing:
kg/m²
individual fruit size
wall thickness
color
nutritional composition
energy efficiency
or:
marketability.
There may not be one lighting treatment that maximizes all of them simultaneously.
Pepper Canopies Need Spatial Light Measurements
Sweet pepper is a particularly good crop for demonstrating why:
one canopy PPFD reading is not enough.
A tall trained crop has strong vertical light gradients.
Measure representative positions such as:
top leaves
middle fruiting zone
and:
lower canopy.
If using interlighting, measure positions:
near
and:
between fixtures.
The goal is to understand:
the light environment of the productive canopy, not just the brightest leaf.
DLI Should Be Logged Across the Season
Winter pepper DLI may be:
less than half
the summer value.
The same artificial-light program can therefore be highly valuable in:
winter
and nearly redundant in:
summer.
Logging DLI lets you answer:
How much additional light does the crop actually need today?
That is much more useful than permanently maintaining:
500 PPFD
because a stage chart says so.
A Better Sweet Pepper Measurement Framework
| Variable | What it actually helps you understand |
|---|---|
| PPFD | Instantaneous photon supply at a specific canopy position |
| DLI | Total photosynthetic photon supply through the day |
| Spectrum | How photons are distributed by wavelength |
| CO₂ | Carbon available for photosynthesis |
| Temperature | Photosynthesis, development and reproductive heat stress |
| VPD | Atmospheric water demand |
| Root-zone moisture | Actual water available to the plant |
| Root-zone EC | Salinity / nutrient concentration stress |
| Fruit load | Competing sink demand |
| Plant density | Light interception and assimilate availability per plant |
| Airflow | Canopy microclimate, transpiration and disease environment |
None of these variables should be interpreted alone.
Frequently Asked Questions
What PPFD should greenhouse sweet peppers receive?
There is no one universal PPFD optimum.
Commercial and research systems expose peppers to widely different PPFDs depending on:
natural sunlight,
supplemental lighting,
photoperiod,
season,
and canopy position.
Use PPFD together with:
DLI and crop response.
Is 400–650 PPFD required during flowering?
No.
There is no scientifically established flowering threshold of:
400–650 µmol/m²/s.
Fruit set depends strongly on:
source–sink balance,
temperature,
existing fruit load,
cultivar,
and overall carbon supply.
What DLI is useful for fruiting sweet peppers?
Direct research supports productive pepper growth around:
20 mol/m²/day
and commercial greenhouse experiments frequently operate across environments from roughly:
16 to more than 30 DLI.
That is better treated as research context than as one universal target.
Is 5 DLI enough for fruiting pepper?
In one controlled DLI experiment:
5 mol/m²/day severely restricted fruit production.
That indicates very low daily light can become strongly limiting.
Does more light always increase pepper yield?
No.
More usable light can increase plant source strength when light is limiting.
But response becomes dependent on:
season,
crop load,
canopy structure,
spectrum,
plant density,
and economics.
Why are my sweet pepper flowers dropping?
Possible causes include:
insufficient assimilate supply,
strong competition from existing fruits,
high temperature,
water stress,
nutrient stress,
and cultivar characteristics.
Direct research shows reduced source strength increases flower and young-fruit abortion.
Can existing peppers cause new flowers to abort?
Yes.
Rapidly growing fruits are strong carbon sinks and can compete with new flowers and young fruits for assimilates.
When are pepper flowers most sensitive to abortion?
Research indicates that flowers and young fruits are particularly sensitive during approximately:
the first week after anthesis.
Does high temperature reduce pepper fruit set?
Yes.
Direct bell-pepper research found that exposure to approximately:
33°C
during sensitive flower-development stages reduced pollen viability and fruit set.
What VPD should sweet pepper use during flowering?
There is no validated universal flowering VPD.
Direct greenhouse research showed different responses to:
daytime
versus:
nighttime
humidity/VPD.
Therefore one number such as:
0.8 kPa
cannot guarantee fruit set.
Do sweet peppers need lower VPD than tomatoes during flowering?
Current evidence is not strong enough to make that universal statement.
Delete it from the old AquaHorti article.
Is 900–1200 ppm CO₂ required for flowering?
No.
Direct sweet-pepper experiments show responses to enrichment around:
500–800 ppm,
but no universal flowering requirement of 900–1200 ppm has been established.
Does CO₂ enrichment increase pepper yield?
It can.
A direct greenhouse experiment found increased growth and yield at:
500 and 800 ppm
compared with natural concentration, with the strongest yield response generally at 800 ppm.
Is 800 ppm therefore the ideal CO₂ concentration?
Not universally.
The response depends on:
light,
temperature,
ventilation,
irrigation,
cultivar,
and economics.
Why does my greenhouse CO₂ fall during the day?
Active photosynthesis consumes CO₂.
In a tightly closed greenhouse, crop demand can exceed air exchange.
This is why CO₂ should be logged during:
high-light periods.
What causes blossom-end rot in pepper?
Blossom-end rot is associated with problems involving:
fruit calcium relations,
water movement,
root-zone salinity,
water stress,
rapid tissue development,
and environmental stress.
It is not simply caused by:
VPD being too high.
Will adding more calcium always fix blossom-end rot?
No.
The nutrient solution may already contain adequate calcium while transport into developing fruits remains inadequate.
Water movement and root health matter.
Does root-zone EC matter?
Yes.
Sweet-pepper salinity research shows that increasing salinity can lower plant water status and increase blossom-end rot, reducing marketable yield.
Should I reduce light during fruit ripening?
There is no universal biological requirement to do so.
Adjust supplemental lighting according to:
available natural DLI,
remaining fruit load,
crop value,
and energy economics.
Does spectrum matter for pepper?
Yes.
Direct studies show red, blue, green and far-red treatments can change:
fruit set,
canopy architecture,
yield,
fruit mass,
pigments,
and fruit cracking.
Should PPFD be measured only at the top of the crop?
No.
A tall sweet-pepper canopy has a strong vertical light gradient.
Measure representative locations through the:
productive canopy.
Should I measure PPFD or DLI?
Measure both.
Use PPFD to understand:
where photons are reaching the canopy.
Use DLI to understand:
how many photons the crop receives over the whole day.
The Main Takeaway
The old AquaHorti Sweet Pepper article should no longer publish this staged recipe:
120–220 PPFD → 300–500 → 400–650 → 350–600 → 300–500
combined with changing:
CO₂
and:
VPD
targets.
Direct research gives us a much stronger model:
Sweet pepper fruit set is strongly controlled by the balance between source strength and competing sink demand.
When source strength was reduced through:
shading, high plant density or leaf pruning, flower and young-fruit abortion increased.
Existing fruit load also matters because rapidly growing fruits compete strongly for available assimilates.
Light quantity matters—but:
spectrum and canopy distribution matter too.
Modern sweet-pepper studies demonstrate substantial yield responses from:
interlighting,
green-light penetration,
and:
far-red treatments.
Temperature can independently reduce reproductive success, with direct research demonstrating reduced pollen viability and fruit set around:
33°C
during sensitive flower-development stages.
And humidity research shows that:
daytime and nighttime VPD can influence sweet pepper differently, so one stage-specific kPa number cannot explain flowering success.
So the better greenhouse strategy is:
Measure PPFD throughout the productive canopy → log DLI → monitor CO₂ during active photosynthesis → track temperature around flower development → interpret VPD as atmospheric water demand → monitor root-zone moisture and EC → record fruit load and fruit-set waves.
That is much closer to how commercial greenhouse sweet pepper actually works.
Measuring Greenhouse Sweet Pepper Conditions
For long-cycle greenhouse peppers, environmental measurements become most useful when reviewed together:
PPFD
DLI
CO₂
air temperature
humidity / VPD
plus:
root-zone moisture and EC.
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
Marcelis et al. — Flower and Fruit Abortion in Sweet Pepper in Relation to Source and Sink Strength. Journal of Experimental Botany, 2004.
Wubs, Heuvelink & Marcelis — Abortion of Reproductive Organs in Sweet Pepper: A Review.
Homma et al. — Dry Matter Production and Fruit Sink Strength Affect Fruit Set Ratio of Greenhouse Sweet Pepper, 2022.
Adame-Adame et al. — Daily Light Integral and Nutrient Solution Electrical Conductivity for Tomato and Bell Pepper Seedling Production in an Indoor Vertical Farm, 2025.
Kang, Lee & Ahn — Enhancement of Sweet Pepper Photosynthesis and Yield with Green-to-Red Sunlight Conversion Despite Reduced Daily Light Integral, 2025.
Additional Far-Red Increases Fruit Yield of Greenhouse Sweet Pepper Mainly Through Enhancing Plant Source Strength, 2024.
High Ratio of Blue:Red Light Reduces Fruit Set in Sweet Pepper, 2024.
Erickson & Markhart — Flower Developmental Stage and Organ Sensitivity of Bell Pepper to Elevated Temperature.
Bakker — The Effects of Air Humidity on Flowering, Fruit Set, Seed Set and Fruit Growth of Glasshouse Sweet Pepper.
Baba et al. — Agronomic Response of Sweet Pepper to CO₂ Enrichment in Greenhouses with Static Ventilation.
UF/IFAS — Blossom-End Rot in Bell Pepper: Causes and Prevention.