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

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

When I first started growing arugula in my greenhouse, I approached it much like I did other leafy greens: give it bright light, consistent moisture, and balanced soil nutrients, and it should grow quickly. In the early weeks that approach seemed to work — seedlings developed leaves rapidly — but as plants matured I began noticing differences in leaf size, texture, and overall vigor that I could not explain from light or watering alone. Some plants grew lush and tender, while others remained sparse or developed slower despite similar visible light exposure. That inconsistency led me to start measuring three environmental variables together: usable light (PAR), carbon dioxide (CO₂) concentration, and vapor pressure deficit (VPD). Over multiple growing cycles, tracking these factors gave me a much clearer understanding of how arugula responds to its environment at different stages of growth.

Below is a practical, experience-based explanation of how PAR, CO₂, and VPD interact and how managing them helped improve arugula growth in a greenhouse.

Why PAR, CO₂, and VPD Matter for Arugula

Arugula is a leafy green that depends on efficient photosynthesis to produce abundant, tender leaves. Photosynthesis depends not only on light, but also on a steady supply of carbon and favorable atmospheric conditions that allow stomata to remain open for gas exchange.

  • PAR (Photosynthetically Active Radiation) is the usable range of light (400–700 nm) that plants convert into energy, measured in micromoles per square meter per second (µmol/m²/s).
  • CO₂ (Carbon Dioxide) is the carbon source that plants fix into sugars and leaf biomass during photosynthesis.
  • VPD (Vapor Pressure Deficit) reflects the atmospheric demand around leaf surfaces, influencing how freely stomata can open for gas exchange without causing excessive water loss.

Tracking these variables together helped me interpret why plants performed differently under conditions that looked similar to the eye.

Seedling and Early Leaf Development Stage

In the earliest stage, arugula is establishing roots and producing its first functional leaves. How it responds to environmental conditions at this stage influences its growth pattern later.

From my greenhouse measurements:

  • PAR: Usable light around 150–300 µmol/m²/s around midday supported compact seedlings with balanced early leaves. In areas where midday PAR stayed below 150 µmol/m²/s, seedlings developed narrower leaves and elongated petioles, suggesting they were stretching to capture more usable light.
  • CO₂: During active photosynthesis, maintaining CO₂ near ambient outdoor levels (about 400–450 ppm) supported steady early growth. In stagnant air zones where midday CO₂ dipped below 350 ppm, seedlings expanded more slowly and looked less vigorous.
  • VPD: Moderate VPD — around 0.8–1.3 kPa — helped stomata remain open for gas exchange without triggering excessive transpiration. On particularly warm, dry afternoons when VPD spiked above 1.5 kPa, leaf edges showed slight stress and expansion slowed even when PAR and CO₂ were adequate.

Logging these values helped me recognize early why some seedlings appeared stronger than others despite similar care.

Vegetative Growth: Leaf Expansion and Canopy Development

Once arugula passed the seedling phase and entered vigorous vegetative growth, its demand for usable light and carbon increased, and atmospheric conditions played a larger role in how quickly it added biomass.

In this phase:

  • PAR: Midday usable light near 300–500 µmol/m²/s supported broad leaf expansion and lush canopies. In spots where midday PAR rarely exceeded 250 µmol/m²/s, leaves were smaller and overall growth was slower.
  • CO₂: As leaf area expanded and photosynthesis intensified, midday CO₂ sometimes dropped in zones with limited airflow. Encouraging circulation and maintaining CO₂ closer to 450–600 ppm during peak light hours correlated with more uniform leaf production and faster growth. In areas where CO₂ frequently dipped below 400 ppm, growth slowed despite good PAR.
  • VPD: Moderate VPD values — typically between 1.0–1.8 kPa — supported active stomatal conductance. On especially hot, dry afternoons when VPD exceeded 2.0 kPa, leaves became slightly stressed and growth rate slowed even though usable light and CO₂ were sufficient.

Balancing ventilation and humidity control helped keep VPD in a range where stomata could remain open and gas exchange could proceed efficiently.

Mature Growth and Pre-Harvest Stage

In later stages, when arugula approaches full leaf density and tender texture is the goal, the combination of usable light, carbon availability, and atmospheric demand influences leaf quality and overall vigor.

From greenhouse tracking:

  • PAR: Usable midday light near 450–650 µmol/m²/s supported full leaf expansion and deeper green color. In areas where the daily light integral (daily light total) stayed below 18–22 mol/m²/day, foliage tended to be lighter and less dense.
  • CO₂: Maintaining midday CO₂ near 500–650 ppm during active photosynthesis supported steady carbohydrate production and fuller leaf sets. In zones where CO₂ dipped below 400 ppm during bright periods, leaves matured more slowly and the crop looked less vigorous at harvest.
  • VPD: Moderate VPD — around 1.2–1.8 kPa — supported efficient gas exchange. Very low VPD due to high humidity suppressed stomatal motion and slowed growth even when PAR and CO₂ were favorable. On hot, dry days when VPD spiked above 2.0 kPa, leaves showed slight stress and growth became uneven.

Managing shading, airflow, and humidity during peak heat helped keep VPD within a range where stomatal function remained effective without compromising usable light or carbon availability.

How I Monitor and Adjust Conditions

To manage these variables effectively, I take measurements at multiple times — early morning, midday, and late afternoon — to capture how PAR, CO₂, and VPD change with light intensity, temperature, and ventilation. Logging these values over several days reveals patterns rather than isolated snapshots.

Ventilation and Airflow

Introducing fresh air during periods of peak photosynthesis prevents midday CO₂ drawdown and helps stabilize VPD. On calm or hot days, I use circulation fans and strategically open vents to maintain stable airflow.

Humidity and Temperature Management

Temperature and humidity together determine VPD. On hot, dry afternoons, I apply shade cloth and increase airflow to prevent midday VPD spikes. On humid days, increasing circulation prevents moisture stagnation and keeps stomatal responsiveness.

Usable Light Distribution

Measuring usable light at canopy height — rather than assuming brightness from appearance — showed me where shading from greenhouse structures or neighboring plants reduced effective PAR. I adjusted plant placement and supplemental lighting accordingly.

Practical Tips for Everyday Growers

Here are practical lessons from my experience that helped align PAR, CO₂, and VPD for healthier arugula growth:

  • Take repeated measurements throughout the day rather than relying on a single reading. Conditions change and trends matter.
  • Balance fresh air exchange with humidity control to keep CO₂ and VPD in ranges that support gas exchange.
  • Moderate midday extremes — very high usable light without supportive atmospheric conditions can stress plants and slow growth.
  • Use plant responses as feedback — leaf size, posture, color intensity, and growth rate often reflect how conditions are affecting physiology.

Final Reflection

Growing greenhouse arugula taught me that plants do not respond to single environmental factors in isolation. Instead, arugula integrates usable light energy, carbon availability, and atmospheric demand across the day to determine growth rates, leaf quality, and uniformity. Usable light supplies the energy for photosynthesis, CO₂ provides the carbon building blocks, and VPD influences how freely stomata can open for gas exchange without undue water loss.

By tracking PAR, CO₂, and VPD together rather than in isolation, I gained a much clearer understanding of what my arugula plants were actually experiencing — and how to adjust greenhouse conditions for more predictable, vigorous, and high-quality growth at every stage. For everyday greenhouse growers who want arugula with tender foliage and uniform growth, thinking in terms of these interacting variables offers a practical, evidence-based framework for better outcomes.

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