Zucchini (Cucurbita pepo L.) is a fast-growing greenhouse crop capable of producing a large canopy and repeated harvests in a relatively short period.
That rapid growth can make environmental problems difficult to diagnose.
A zucchini plant may continue producing large leaves even while fruit production is limited by low solar radiation, excessive temperature, poor pollination or inadequate water supply.
For this reason, greenhouse zucchini should not be managed using one isolated measurement.
Light, temperature, humidity, CO₂, irrigation and pollination interact.
There is also an important limitation:
Current research does not support a universal stage-by-stage recipe assigning one exact PAR, DLI, CO₂ and VPD value to every zucchini crop.
Published studies instead give us experimentally documented conditions and responses that can be used as reference points.
PAR and DLI: Why Light Matters for Zucchini
PPFD describes the amount of photosynthetically active light reaching the crop at a particular moment.
It is expressed as:
µmol/m²/s
DLI — Daily Light Integral — represents the total photosynthetic light received over the entire day:
mol/m²/day
For greenhouse zucchini, the distinction is especially important because solar radiation can change substantially over a long production cycle.
A high PPFD reading at noon does not necessarily mean that the crop has received enough total daily light.
A 2025 Long-Term Greenhouse Zucchini Study
A particularly useful recent study followed greenhouse zucchini for:
168 days
from October through March.
Researchers grew the cultivar ‘Black Tosca’ in a hydroponic greenhouse and examined the relationship between female flowers, intercepted light, dry-matter production and fruit yield.
One of the most important findings was:
Producing more female flowers did not automatically increase yield when solar radiation was low.
Plants eventually produced a very high proportion of female flowers, but during periods of lower winter radiation, additional female flowers did not translate directly into additional dry matter or fruit yield.
This is highly relevant to greenhouse management.
If zucchini fruit production declines during winter, the problem may not simply be “not enough female flowers.”
Light availability can limit the crop’s ability to support additional fruit.
A Useful Seedling Research Condition
In the same study, zucchini seedlings were initially raised under:
350 µmol/m²/s PPFD
for:
14 hours per day
with:
1,000 ppm CO₂
and temperatures of:
25°C during the day
15°C at night.
The light treatment corresponds to approximately:
17.6 mol/m²/day
because:
350 × 14 × 0.0036 ≈ 17.6 mol/m²/day.
This is a documented nursery condition under which the researchers successfully produced transplants.
It should not be interpreted as proof that every zucchini seedling requires 350 µmol/m²/s or 17.6 mol/m²/day.
That distinction is essential.
A research condition is not automatically an optimum recommendation.
What Happened After Transplanting?
After transplanting, the plants were grown under natural greenhouse light.
Daytime ventilation began when temperature exceeded approximately:
25°C
while heating setpoints were approximately:
22°C during the day
and:
15°C at night.
During the 168-day experiment, average daytime greenhouse temperature ranged approximately:
15.3–23.0°C
and daytime CO₂ concentration ranged approximately:
347–562 ppm.
The researchers supplied CO₂ when concentrations fell below approximately 380 ppm.
Again, these values describe a successful research greenhouse — they are not universal optimum setpoints.
Why Measuring the Canopy Matters
Zucchini produces large leaves very quickly.
That creates significant differences between:
- light above the canopy,
- light at the upper leaves,
- and light deeper inside the canopy.
The 2025 study specifically measured PPFD through several canopy layers and calculated a light-extinction coefficient of approximately:
k = 1.05.
This means light declined substantially as it passed through the zucchini canopy.
For growers, the practical lesson is simple:
A PAR sensor mounted permanently above the crop does not necessarily describe the light environment experienced by all active leaves.
Measure at representative canopy positions and adjust measurement height as plants develop.
Light Can Limit Yield Even When Plants Keep Flowering
The same long-term research provides another useful lesson.
Female flowers became increasingly common at higher nodes, eventually exceeding 70% and later reaching very high proportions.
But under low solar radiation, simply allowing more female flowers to develop did not increase final fruit production proportionally.
This demonstrates an important source–sink principle.
Flowers and young fruits are sinks — they require carbohydrates.
Leaves receiving light are the primary source supplying those carbohydrates.
If photosynthetic carbon supply becomes limiting, increasing the number of potential fruits does not automatically increase harvestable yield.
That is why greenhouse zucchini should be evaluated by both:
reproductive development
and
available daily light.
What Other Greenhouse Research Tells Us About Radiation
A separate hydroponic greenhouse study compared zucchini grown during spring–summer and summer–fall.
Marketable fresh yield differed between the seasons:
6.2 kg/m² in spring–summer
versus:
4.7 kg/m² in summer–fall.
The researchers found that radiation-use efficiency and water-use efficiency changed with solar radiation, air temperature and VPD.
This reinforces an important point:
Light cannot be interpreted independently of water demand and greenhouse climate.
Does Zucchini Always Need Maximum Light?
No.
Zucchini is a relatively high-light crop, but this does not mean that every reduction in incoming radiation causes the same response.
A greenhouse experiment using insect-proof screens found that the greenhouse covering itself reduced PPFD by approximately 30% relative to outdoors.
The tested insect screens caused a much smaller additional reduction — roughly 5% relative to the unscreened greenhouse treatment.
One screen treatment eventually produced approximately:
23% lower yield
and:
18% fewer fruits
than the unscreened control.
However, this should not be interpreted as proof that a 5% light reduction caused a 23% yield reduction.
Screens also modify:
- ventilation,
- temperature,
- humidity,
- pest access,
- and greenhouse microclimate.
That is exactly why greenhouse environmental measurements should be considered together.
Temperature Has a Direct Effect on Female Flower Formation
Zucchini produces separate male and female flowers on the same plant.
Temperature can influence that sex expression.
In a controlled study, researchers compared several day/night temperature combinations.
At:
30°C day / 30°C night
the percentage of female flowers was significantly lower than under:
30°C / 20°C
or:
30°C / 10°C.
Female flowering and fruit-setting were also markedly inhibited under continuous 30°C day and night conditions.
This gives growers a much stronger scientific explanation than claiming that one particular VPD range “controls female flowers.”
Persistently high night temperature can directly alter zucchini reproductive development.
High Temperature Can Affect Fruit Quality Too
Research conducted during spring–summer greenhouse production found that high greenhouse temperatures were strongly associated with zucchini fruits retaining attached flowers.
The effect also varied greatly among cultivars, showing that genetics and temperature interact.
So when unusual fruit development appears during hot weather, temperature should be investigated before assuming that the cause is PAR or VPD alone.
Pollination Is a Major Part of Fruit Set
For many zucchini cultivars, pollination strongly influences normal fruit development.
In the 2025 long-term greenhouse experiment, researchers released bumblebees inside the greenhouse specifically to promote fruit set.
A separate 2024 greenhouse experiment studied honey-bee visits to zucchini flowers.
Fruit set increased with the number of visits to female flowers.
With approximately:
8 bee visits per female flower
fruit set exceeded:
90%.
Fruit from flowers receiving four or more bee visits was also heavier than fruit receiving only two visits.
This is much stronger evidence than attributing fruit abortion primarily to a narrow humidity or VPD range.
Pollination Requirement Depends on Cultivar
Not all zucchini cultivars behave identically.
Research into parthenocarpy — fruit development without fertilization — shows substantial genetic differences among zucchini cultivars.
In one greenhouse study, researchers compared pollinated and unpollinated flowers from cultivars differing in parthenocarpic capacity under approximately:
24°C / 15°C day/night temperatures
and:
75% relative humidity.
This means that when fruit set is poor, growers should know whether their cultivar:
- normally requires effective pollination,
- has partial parthenocarpic ability,
- or was bred specifically for protected cultivation.
A universal greenhouse pollination rule does not apply to every cultivar.
CO₂: Avoid Turning Experimental Conditions Into a Recipe
The original article gives stage-specific CO₂ values as high as:
1,100 ppm
without providing zucchini-specific evidence that those concentrations are optimal.
The long-term 2025 zucchini research gives us a better example of how CO₂ should be interpreted.
Seedlings were raised at approximately:
1,000 ppm CO₂,
but after transplanting the production greenhouse generally operated around:
347–562 ppm during daytime
with CO₂ supplied when concentration dropped below approximately:
380 ppm.
The experiment was not designed to determine optimum CO₂.
Therefore neither 380 ppm nor 1,000 ppm should be presented as the universal zucchini target.
They are documented experimental conditions.
Why CO₂ Depletion Still Matters
During strong photosynthesis, a dense greenhouse crop can consume CO₂ faster than fresh air replaces it.
If the greenhouse is closed or only lightly ventilated, canopy CO₂ can fall below outdoor concentration.
The practical question should therefore be:
Does CO₂ around the active canopy become depleted during high-light periods?
rather than:
What is the one perfect zucchini CO₂ number?
If CO₂ is already adequate and light is limiting, additional enrichment may provide little benefit.
If light is abundant and CO₂ is strongly depleted, CO₂ may become a more important limitation.
VPD: Important for Water Demand, but Not a Pollination Setpoint
VPD is useful because it describes the drying demand of the air.
As VPD rises, potential transpiration generally increases.
For a crop such as zucchini with large leaves, this can substantially change irrigation demand.
Long-term greenhouse zucchini research has shown that radiation-use efficiency and water-use efficiency vary with:
- solar radiation,
- air temperature,
- and VPD.
But current scientific evidence does not justify statements such as:
0.6–1.0 kPa is the correct flowering VPD
or:
high VPD directly causes hollow zucchini fruit.
Those claims are too specific for the available evidence.
Root-Zone Water Supply Is Critical
A greenhouse irrigation experiment tested zucchini under different soil matric-potential thresholds:
−10 kPa
−25 kPa
and:
−40 kPa.
The researchers concluded that approximately −25 kPa provided the best balance between commercial production and efficient water and nutrient use under their specific sand-mulched greenhouse system.
Plants at the driest treatment produced fewer and smaller fruits.
This gives us a useful warning:
When atmospheric water demand increases, root-zone water availability becomes increasingly important.
A VPD measurement by itself cannot tell you whether the plant is actually experiencing water stress.
Uniform Irrigation Matters Too
Another greenhouse zucchini experiment investigated both fertigation amount and distribution uniformity.
When irrigation distribution uniformity dropped to approximately 50%, yield fell by around:
45%
regardless of the irrigation-volume treatment.
This is particularly relevant to greenhouse diagnosis.
If one part of a crop produces small or aborted fruit while another performs normally, the first assumption should not always be light or humidity.
Check:
- emitter performance,
- root-zone moisture,
- nutrient distribution,
- and irrigation uniformity.
A Better Way to Use VPD
Rather than treating VPD as a rigid zucchini growth-stage target, use it as an indicator of atmospheric demand.
Ask:
Is VPD rising because greenhouse temperature is climbing?
Can the root system supply enough water at that time?
Is irrigation responding to increasing solar radiation?
Does leaf temperature differ significantly from air temperature?
Are plants closing stomata despite adequate soil moisture?
This makes VPD useful as a diagnostic measurement instead of turning it into a magic number.
Research-Based Reference Conditions
Published zucchini research provides several useful reference environments:
| Research situation | Conditions | What it tells us |
|---|---|---|
| 2025 seedling production | 350 µmol/m²/s, 14 h, ~17.6 mol/m²/day; 1,000 ppm CO₂; 25/15°C | A documented nursery condition, not a universal optimum |
| 168-day greenhouse crop | Natural greenhouse light; daytime 15.3–23.0°C; CO₂ 347–562 ppm | Low solar radiation could limit yield even with many female flowers |
| Temperature experiment | 30/10, 30/20 and 30/30°C among treatments | High night temperature reduced female-flower proportion and fruit set |
| Bee-pollination experiment | Multiple visits to female flowers | About 8 visits produced >90% fruit set |
| Greenhouse irrigation trial | −10, −25 and −40 kPa soil matric potential | Root-zone water status affected fruit production and efficiency |
| Seasonal greenhouse trial | Spring–summer vs summer–fall | Radiation, temperature and VPD influenced water and radiation-use efficiency |
These data come from different experiments and different production systems.
They should not be combined into one supposedly exact zucchini environmental recipe.
A Practical Greenhouse Zucchini Monitoring Workflow
1. Measure PAR at canopy height
Do not rely only on outdoor solar radiation or lamp specifications.
Measure what the leaves actually receive.
As plants grow, reposition the sensor.
2. Track DLI
A midday reading cannot tell you whether winter solar radiation has been sufficient for the entire day.
DLI makes different days and seasons easier to compare.
3. Monitor day and night temperature
Night temperature deserves special attention because experimental evidence shows that persistently high night temperature can alter female-flower formation and fruit set.
4. Monitor CO₂ during bright periods
Look for depletion when the canopy is actively photosynthesizing.
Interpret CO₂ together with available light.
5. Use VPD together with irrigation data
Higher atmospheric demand means little without knowing whether roots can keep up.
Monitor root-zone moisture, irrigation frequency and distribution uniformity.
6. Evaluate pollination separately
If female flowers appear but fruits fail to develop normally, investigate pollinator activity or hand pollination before blaming VPD.
7. Consider cultivar characteristics
Different zucchini cultivars vary in:
- sex expression,
- parthenocarpy,
- heat response,
- and fruit-development characteristics.
Cultivar choice can therefore change how the greenhouse should be managed.
What Should Growers Actually Optimize?
For greenhouse zucchini, current research supports several practical priorities.
Provide enough daily light to support both vegetative growth and repeated fruit production.
Avoid prolonged excessive night temperatures during reproductive development.
Maintain reliable pollination when the cultivar requires it.
Keep root-zone water supply matched to radiation and atmospheric demand.
Prevent severe CO₂ depletion during active photosynthesis.
Measure the actual canopy environment instead of relying only on greenhouse setpoints.
This is more defensible than assigning five exact PAR / CO₂ / VPD recipes to five arbitrary crop stages.
Key Takeaway
Greenhouse zucchini is not simply a crop that needs “high light, high CO₂ and the correct VPD.”
Recent research shows a more complex picture.
A 168-day greenhouse study found that additional female flowers could not increase yield when solar radiation was limiting.
Controlled temperature research found that 30°C nights reduced female-flower formation and strongly inhibited fruit set compared with cooler nights.
Greenhouse pollination research found that fruit set exceeded 90% when female flowers received about eight bee visits.
And irrigation studies show that root-zone water availability and irrigation uniformity can substantially affect yield.
For growers, the stronger strategy is therefore to:
measure canopy PAR, track DLI, monitor day and night temperature, watch CO₂ and VPD, verify irrigation performance, and evaluate pollination separately.
That gives a much more realistic picture of what is limiting fruit production.
References
Oda, A., Nomura-Ando, K., Isozaki, M., & Ahn, D.-H. (2025). Relationship among the Rate of Female Flowers, Dry Matter Production, and Fruit Yield in Long-term Zucchini (Cucurbita pepo L.) Production. The Horticulture Journal, 94(4), 464–471.
Ikeura, H., Tokuda, T., & Hayata, Y. (2012). Effects of Different Day and Night Temperatures on Female Flower Formation in Zucchini. Horticultural Research (Japan), 11(3), 357–361.
Rouphael, Y., & Colla, G. (2005). Radiation and water use efficiencies of greenhouse zucchini squash in relation to different climate parameters. European Journal of Agronomy, 23(2), 183–194.
Contreras, J. I., Alonso, F., Cánovas, G., & Baeza, R. (2017). Irrigation management of greenhouse zucchini with different soil matric potential level: Agronomic and environmental effects. Agricultural Water Management, 183, 26–34.
Effect of Distribution Uniformity and Fertigation Volume on the Bio-Productivity of the Greenhouse Zucchini Crop. (2020). Water, 12(8), 2183.
Improved Porosity of Insect Proof Screens Enhances Quality Aspects of Zucchini Squash without Compromising the Yield. (2020). Plants, 9(10), 1264.
A Hive Entrance System That Directs Honey Bees Inside or Outside a Greenhouse Reduced Colony Decline While Effectively Pollinating Zucchini Squash. (2024). Agriculture, 14(6), 805.
First RNA-seq approach to study fruit set and parthenocarpy in zucchini (Cucurbita pepo L.). (2019). BMC Plant Biology.