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

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

When I first started growing spinach in a greenhouse, I treated it much like I did other leafy greens: give it bright light, steady water, and a healthy soil mix. For the first few weeks, seedlings grew quickly and leaves unfurled as expected. But as plants progressed into larger vegetative growth, I began noticing differences in leaf size, texture, and overall vigor between seemingly identical spots. That inconsistency pushed me to start monitoring not only usable light (PAR), but also carbon dioxide (CO₂) levels and vapor pressure deficit (VPD) throughout the crop cycle.

Over several growing seasons, tracking these three variables together revealed clear patterns in how spinach responds to its environment at each stage of development. Below is what I learned from real greenhouse measurements and plant responses.

Why PAR, CO₂, and VPD Matter for Spinach

Spinach is a cool-season, leafy crop that thrives when its photosynthesis demands are balanced with environmental conditions. Three factors proved especially important:

  • PAR (Photosynthetically Active Radiation) measures usable light (in µmol/m²/s), which provides the energy spinach uses to grow leaves and build biomass.
  • CO₂ (Carbon Dioxide) is the carbon source plants fix through photosynthesis to produce sugars and structural carbohydrates.
  • VPD (Vapor Pressure Deficit) represents the driving force for transpiration and gas exchange; it influences how open stomata can remain to allow CO₂ uptake without excessive water loss.

Tracking all three together gave me a comprehensive view of what the plants were experiencing, rather than guessing from light alone or relying only on temperature and humidity.

Seedling and Early Growth Stage

In the early phase, spinach plants are establishing roots and developing their first functional leaves. At this stage they are sensitive to environmental conditions that influence how quickly they expand foliage.

From my greenhouse measurements:

  • PAR: Usable light in the range of 150–300 µmol/m²/s around midday supported steady leaf development without elongated stems. Where PAR remained consistently below 150 µmol/m²/s, plants tended to grow longer petioles and smaller leaves, indicating they were stretching for usable light.
  • CO₂: During active photosynthesis periods, maintaining CO₂ near ambient outdoor levels (about 400–450 ppm) supported steady early growth. In stagnant zones where CO₂ dipped below 350 ppm midday, seedlings expanded more slowly and looked less robust.
  • VPD: I monitored VPD to gauge stomatal behavior. A VPD range of 0.8–1.3 kPa seemed to support balanced transpiration and gas exchange. When VPD climbed above 1.5 kPa due to dry, warm air, I saw slight leaf curl and slower expansion even when PAR and CO₂ were adequate.

Recording these variables helped explain why seedlings in some areas of my greenhouse developed more slowly despite similar watering and soil conditions.

Vegetative Growth: Expanding Leaf Canopy

Once spinach moves past the seedling stage and transitions into vigorous vegetative growth, its demand for usable light and carbon increases, and atmospheric conditions become even more critical.

During this middle phase of development:

  • PAR: Midday usable light near 300–500 µmol/m²/s tended to produce lush, broad leaves and thicker canopies. In spots where midday PAR rarely exceeded 250 µmol/m²/s, leaves were smaller and less dense.
  • CO₂: As leaf area increased and photosynthesis intensified, midday CO₂ sometimes dropped in poorly ventilated zones. By improving airflow and keeping CO₂ closer to 450–600 ppm during peak light, I observed more consistent leaf growth across plants. Low CO₂ during strong light periods correlated with slower expansion and more variation in plant size.
  • VPD: Moderate VPD — generally 1.0–1.8 kPa — helped stomata remain open for efficient gas exchange without excessive water loss. When VPD spiked above 2.0 kPa on hot dry afternoons, plants showed slight stress responses such as tighter leaf posture and slower assimilate flow into new leaves.

Balancing ventilation and humidity control helped maintain VPD in a range where spinach leaves could expand effectively without water stress.

Mature Growth and Pre-Harvest Stage

In later stages, when spinach approaches full size and leaf production slows before harvest, the balance of usable light, carbon, and atmospheric demand influences leaf thickness, color, and uniformity.

From greenhouse records:

  • PAR: Usable light near 450–650 µmol/m²/s around midday supported full leaf expansion and richer green coloration. When the daily integral of usable light (DLI) stayed below about 18–22 mol/m²/day, I observed less uniform leaf development.
  • CO₂: Maintaining midday CO₂ near 500–650 ppm during active photosynthetic hours seemed to support steady carbohydrate production and fuller leaf texture. In areas where CO₂ dipped below 400 ppm, leaves were sometimes thinner and plants showed slower growth despite adequate light.
  • VPD: Moderate VPD — roughly 1.2–1.8 kPa — seemed to support robust stomatal conductance and efficient gas exchange. When VPD remained very low (often due to very high humidity), stomatal response slowed and growth lagged even when PAR and CO₂ were favorable.

By paying attention to how VPD changed through the day — especially during warm midday conditions — I was able to make adjustments that improved growth consistency.

How I Monitor and Adjust Conditions

I take measurements at multiple times each day — morning, midday, and late afternoon — to see how PAR, CO₂, and VPD vary with sunlight and ventilation. Logging these values over several days helps identify trends rather than isolated snapshots.

  • Ventilation: Ensuring fresh air exchange prevents midday CO₂ from dropping too low and helps stabilize VPD. On days with strong sunlight and minimal breeze, I increase circulation to maintain these conditions.
  • Humidity and Shading: On very hot, dry afternoons, I use shade cloth or partial shading to prevent excessive PAR spikes that drive VPD out of balance. On humid days, adding gentle airflow improves stomatal conductance without drying leaves excessively.
  • Light Distribution: Measuring usable light at canopy height helped me spot places where shading from structure or neighboring plants was reducing effective PAR, even in spots that appeared bright by eye.

Practical Tips for Everyday Growers

Here are some lessons from my experience that helped align PAR, CO₂, and VPD for stronger spinach growth:

  • Take repeated measurements throughout the day rather than relying on a single reading. Patterns over time matter for plant response.
  • Balance fresh air exchange with environmental control to maintain CO₂ and VPD in ranges that support active gas exchange.
  • Moderate midday extremes — very high usable light without supportive atmospheric conditions can stress plants rather than help them grow.
  • Watch plant responses as feedback — changes in leaf size, texture, and posture often reflect how conditions are aligning with physiological processes.

Final Reflection

Growing greenhouse spinach taught me that plants do not respond to single factors in isolation. Lettuce integrates usable light, carbon availability, and atmospheric demand over the course of the day to determine growth rates, leaf quality, and uniformity. Usable light supplies the energy; CO₂ provides the carbon required to build biomass; and VPD influences how freely stomata can open for gas exchange. When these three factors are aligned, spinach grows predictably and produces fuller, healthier foliage.

Instead of guessing from how bright a space looks or how often you water, monitoring PAR, CO₂, and VPD together gave me a much clearer view of what the plants were actually experiencing — and how to adjust the greenhouse environment for consistent results. For everyday greenhouse growers who want spinach that grows vigorously and matures uniformly, considering these interacting variables provides a practical framework for better decisions.

View on Amazon

Amazon is a trademark of Amazon.com, Inc. or its affiliates.