PAR, CO₂, and VPD Requirements for Greenhouse Cucumbers at Different Growth Stages

PAR, CO₂, and VPD Requirements for Greenhouse Cucumbers at Different Growth Stages

When I first began growing cucumbers in my greenhouse, I assumed that light and water were the main variables to manage. I gave them strong sunlight, balanced irrigation, and typical fertilizer, expecting vigorous vines and plentiful fruit. Early on, that approach produced decent growth, but as plants matured I saw inconsistent flowering and fruit development that did not make sense based on light levels or watering alone. This inconsistency pushed me to record and analyze not just usable light (PAR), but also carbon dioxide (CO₂) levels and vapor pressure deficit (VPD) throughout cucumber growth stages.

Over multiple seasons, tracking these three environmental factors — PAR, CO₂, and VPD — gave me far better insight into plant behavior and why some stages of growth proceeded faster or slower than expected. Combining these measurements revealed environmental conditions that help cucumbers thrive from seedling to harvest.

Why PAR, CO₂, and VPD Matter Together

Cucumbers are productive, fast-growing vines that rely on:

  • Usable light (PAR) to fuel photosynthesis
  • Carbon dioxide (CO₂) as the source of carbon for building sugars and biomass
  • Vapor pressure deficit (VPD) to regulate stomatal function and water/carbon exchange

Each of these factors interacts. Strong PAR provides energy, but without available CO₂ and balanced atmospheric demand (indicated by VPD), that light energy can’t be used effectively. I found that treating them as separate variables was not enough; they must be managed as an integrated environment.

Seedling Stage: Establishing Strong Beginnings

During the early seedling stage cucumbers are forming root systems and their first true leaves. At this point their usable light and carbon needs are relatively modest compared with later stages, but conditions still influence foundational growth.

From my measurements:

  • PAR: Seedlings did best with mid-morning and midday usable light around 150–250 µmol/m²/s. Lower than this and leaf expansion slowed; higher than this without sufficient CO₂ or balanced VPD increased stress.
  • CO₂: At this stage, keeping CO₂ near ambient outdoor levels (around 400–450 ppm) helped seedlings develop without slowing. In areas where CO₂ dipped below about 350 ppm during active light periods, leaves appeared smaller and internodes stretched.
  • VPD: I aimed for a moderate VPD range near 0.8–1.3 kPa. This supported stomatal opening for CO₂ uptake without excessive transpiration. When VPD moved above 1.5 kPa in warm dry air, I saw slowed leaf formation even with good PAR.

Recording these values helped me group seedlings where micro­climate conditions were most favorable rather than simply where it looked bright.

Vegetative Growth: Rapid Leaf and Vine Expansion

Once cucumbers pass the seedling phase and enter vigorous vegetative growth, their demand for usable light and carbon increases significantly. At this stage, balanced conditions helped produce larger leaves and vigorous vine growth.

In my greenhouse:

  • PAR: Midday usable light near 300–500 µmol/m²/s consistently produced broad, healthy leaves and rapid vine expansion. Spots where midday PAR rarely exceeded 250 µmol/m²/s often lagged in leaf area development.
  • CO₂: As vines expanded and photosynthesis intensified, midday CO₂ levels sometimes dropped in less ventilated zones. By ensuring airflow or periodic fresh air intake, I kept CO₂ closer to 400–550 ppm during active light periods, which correlated with faster vegetative growth.
  • VPD: During this phase I watched VPD closely. When mid-day VPD rose above 1.8–2.0 kPa — common on hot dry days — stomata closed slightly, limiting CO₂ uptake and slowing photosynthesis even when light was abundant. Keeping VPD in the 1.1–1.8 kPa range supported steady expansion.

Balancing airflow and shade cloth on the hottest days made a noticeable difference in preventing excessive VPD spikes, which in turn allowed vines to stay actively transpiring and photosynthesizing.

Flowering and Fruit Set

Flowering and initial fruit set are among the most sensitive phases in cucumber production. Carbon demand rises sharply and environmental imbalances become more visible in plant responses.

From my measurements and observations:

  • PAR: Midday usable light near 450–650 µmol/m²/s encouraged flower development and early fruit set. In spots where the daily integral of usable light stayed consistently below 15–18 mol/m²/day, flower development was uneven and fruit set was patchy.
  • CO₂: In flowering blocks with strong light, CO₂ sometimes dipped mid-day below 350 ppm if ventilation was poor. Improving air exchange kept CO₂ closer to 450–600 ppm during peak light, and I saw more consistent flower retention and early fruit development.
  • VPD: Moderate VPD — roughly 1.2–1.8 kPa — seemed to support flower longevity and fruit set. High VPD (>2.0 kPa) coincided with wilted flowers and reduced set, likely due to stomatal closure reducing CO₂ uptake despite good light.

In practice, this meant adjusting vent openings and circulation fans based on real-time VPD data rather than humidity alone.

Fruit Development and Maturation

As cucumbers transition into rapid fruit growth, their balance of usable light, CO₂, and VPD continues to influence size, shape, and overall quality.

In my greenhouse records:

  • PAR: Usable light peaks near 600–800 µmol/m²/s at midday were beneficial for fruit bulking, provided that extreme heat and VPD spikes were avoided. In direct midday sun on very hot days, usable light was high but VPD also spiked, stressing the plants.
  • CO₂: Midday CO₂ levels sustained closer to 500–650 ppm during active photosynthesis correlated with increased fruit size and consistent development. When CO₂ dipped below 400 ppm during sunny periods, fruit development slowed, even if PAR was high.
  • VPD: Balanced VPD — generally 1.3–1.8 kPa — helped maintain open stomata for continued carbon uptake. On days where VPD exceeded 2.0 kPa, fruit backs became lighter and vines appeared slightly stressed.

Using shade cloth or evaporative cooling to prevent midday heat spikes helped manage VPD and allowed plants to use usable light and CO₂ effectively for fruit development.

How I Manage PAR, CO₂, and VPD Together

In practice, managing these factors is an ongoing process:

Monitoring and Logging

I take multiple measurements daily — early morning, midday, and late afternoon — to see how PAR, CO₂, and VPD change through the day. Logging these over time helps identify trends and adjust ventilation, shading, or supplemental lighting.

Ventilation and Fresh Air Exchange

Because cucumbers can draw CO₂ down quickly under strong light, I keep side vents and circulation on schedules that bring in fresh air during peak photosynthesis periods, which helps maintain usable carbon and moderate VPD.

Shading and Temperature Control

On very bright, hot days, I use shade cloth to reduce extreme midday PAR peaks that can elevate VPD beyond optimal ranges. Reducing heat influx can keep VPD in a range where stomata remain open and gas exchange continues efficiently.

Final Reflection

Growing greenhouse cucumbers taught me that focusing on light alone is only part of the picture. Usable light needs to be paired with sufficient carbon and balanced atmospheric demand for plants to use that light effectively.

PAR provides the energy for photosynthesis. CO₂ supplies the building blocks for sugars and biomass. VPD governs how freely stomata can open to take in carbon and release water vapor. When these three factors are aligned, cucumbers develop robust foliage, consistent flowers, and abundant, high-quality fruit.

Instead of reacting only to how bright a spot looks or how often you water, integrating data from PAR, CO₂, and VPD helped me understand what cucumbers experience in their environment and how to manage it more effectively at every stage of growth.

If you want predictable, high-quality cucumber production in a greenhouse, paying attention to usable light, carbon availability, and atmospheric conditions together is far more effective than managing any one factor in isolation.

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