PAR, CO₂, and VPD Requirements for Greenhouse Coriander (Cilantro) at Different Growth Stages

PAR, CO₂, and VPD Requirements for Greenhouse Coriander (Cilantro) at Different Growth Stages

When I first began growing coriander (also known as cilantro) in my greenhouse, I assumed it would be as straightforward as other leafy herbs: give it enough light, keep the soil moist, and it will thrive. In the earliest weeks that often held true, but as plants grew larger I began noticing differences in leaf size, aroma, and overall vigor that simply couldn’t be explained by light or water alone. This inconsistency led me to start tracking three key environmental factors together: usable light (PAR), carbon dioxide (CO₂) levels, and vapor pressure deficit (VPD). What I found through repeated measurements and observation was that these three variables intersect in meaningful ways at different coriander growth stages.

Below is a practical, experience-based guide that explains how PAR, CO₂, and VPD interact and why tracking them helped me grow healthier, more productive coriander.

Why PAR, CO₂, and VPD Matter for Coriander

Coriander is a leafy annual herb that relies heavily on efficient photosynthesis for rapid leaf production and strong aromatic character. Photosynthesis depends on three interrelated environmental components:

  • PAR (Photosynthetically Active Radiation) — usable light in the 400–700 nm range, measured in micromoles per square meter per second (µmol/m²/s), which drives photosynthesis.
  • CO₂ (Carbon Dioxide) — the carbon source plants fix into sugars and flesh out leaf tissue.
  • VPD (Vapor Pressure Deficit) — the difference between humidity inside the leaf boundary and the surrounding air, affecting water loss and stomatal openness for gas exchange.

Understanding how these three variables interact gave me a clearer picture of what my coriander plants actually experienced — not just what it looked like to the eye.

Seedling and Early Leaf Development Stage

In the early growth stage, coriander plants are establishing roots and their first true leaves. This is a period where environmental balance matters just as much as light intensity.

From my greenhouse measurements:

  • PAR: Usable light around 150–300 µmol/m²/s at midday encouraged compact, healthy seedlings with robust leaf formation. In areas where midday PAR stayed below 150 µmol/m²/s, seedlings tended to develop longer petioles and narrower leaves, a sign they were stretching for usable light.
  • CO₂: During active photosynthesis periods, maintaining CO₂ near ambient outdoor levels (about 400–450 ppm) helped early leaf expansion. In stagnant corners of the greenhouse where CO₂ dipped below 350 ppm during peaks of light, seedlings produced smaller first leaves and grew more slowly.
  • VPD: Balanced VPD — roughly 0.8–1.2 kPa — supported stomatal conductance without causing excessive water loss. On particularly dry, warm afternoons when VPD spiked above 1.5 kPa, I noticed slight leaf curling and slower leaf expansion despite adequate PAR.

Recording these variables helped me understand early performance differences I had previously attributed to random variation.

Vegetative Growth: Leaf Canopy Expansion

Once coriander passed the seedling stage and entered rapid vegetative growth, its needs for usable light and carbon increased substantially, and atmospheric demand became more significant in determining growth rates.

In this phase:

  • PAR: Midday usable light near 300–500 µmol/m²/s supported broad, vibrant leaf growth. In zones where midday PAR rarely exceeded 250 µmol/m²/s, leaves were smaller and fewer in number, slowing canopy expansion.
  • CO₂: With increased leaf area and photosynthesis, CO₂ levels in some areas dropped during peak light hours. By improving air circulation and keeping CO₂ in the 450–600 ppm range during midday, I observed more uniform leaf production and faster vegetative growth. Low CO₂ (below 400 ppm) coincided with slower expansion even when PAR was sufficient.
  • VPD: Moderate VPD values — typically between 1.0–1.8 kPa — helped stomata stay open for gas exchange without causing undue water stress. On especially hot, dry afternoons with VPD above 2.0 kPa, leaf edges showed slight signs of stress and growth slowed despite adequate PAR and CO₂.

Balancing ventilation and humidity helped keep VPD in a range where coriander leaves could grow actively without closing stomata to conserve water.

Pre-Harvest and Maturation Stage

In later stages, as plants mature and approach full leaf density, the interplay between usable light, carbon availability, and atmospheric conditions influences leaf thickness, aroma intensity, and overall plant vitality.

From greenhouse tracking:

  • PAR: Usable light near 450–650 µmol/m²/s at midday generally produced fuller, richer foliage. Days where the daily total of usable light (daily light integral, or DLI) stayed below 18–22 mol/m²/day tended to produce lighter, thinner leaves.
  • CO₂: Maintaining midday CO₂ near 500–650 ppm helped sustain strong photosynthetic rates and supported fuller leaf sets. In zones where CO₂ dipped below 400 ppm during active light, leaves were smaller and aromatic intensity was less pronounced at harvest.
  • VPD: Moderate VPD — around 1.2–1.8 kPa — helped stomata remain sufficiently open for consistent gas exchange. Too low VPD (often due to high humidity) led to sluggish stomatal movement and slower growth. Too high VPD (on hot, dry days) caused stress responses that reduced leaf turgor and slowed final expansion.

Managing shading, humidity, and airflow during peak times of heat helped keep VPD in a favorable range while still providing ample usable light and stable carbon availability.

How I Monitor and Adjust Conditions

Managing PAR, CO₂, and VPD effectively in a greenhouse requires regular observation and adjustment:

Ventilation and Airflow

Ensuring fresh air exchange during times of peak PAR prevents midday CO₂ from dropping and helps stabilize VPD. On still days when CO₂ tended to drop, I increased circulation or opened vents to bring in fresh air.

Temperature and Humidity Control

Temperature and humidity together determine VPD. On hot, dry afternoons, using shade cloth and increasing airflow reduced midday peaks in VPD. On humid days, increased circulation prevented stagnation and maintained better stomatal function.

Usable Light Measurement

Measuring usable light at canopy height — rather than assuming brightness based on appearance — showed me where shading or greenhouse structure reduced effective PAR. Adjusting plant placement or supplemental lighting helped ensure more uniform PAR distribution.

Practical Tips for Everyday Growers

Here are some practical takeaways from my experience that helped align PAR, CO₂, and VPD for stronger coriander growth:

  • Take repeated measurements throughout the day instead of relying on a single reading. Observing trends over time reveals how conditions interact with plant physiology.
  • Balance ventilation with humidity control to maintain CO₂ and VPD in ranges conducive to stomatal gas exchange.
  • Moderate midday extremes — very high usable light without supportive atmospheric conditions can stress plants more than help them grow.
  • Watch plant behavior as feedback — leaf size, posture, aroma intensity, and growth rate often reflect how environmental conditions are interacting with plant physiology.

Final Reflection

Growing greenhouse coriander taught me that plants do not respond to a single factor in isolation. Instead, they integrate usable light energy, carbon availability, and atmospheric demand over the course of the day to determine how vigorously and uniformly they grow. Usable light supplies the energy needed for photosynthesis, CO₂ provides the carbon building blocks, and VPD influences how freely stomata can open for gas exchange without creating water stress.

By tracking PAR, CO₂, and VPD together instead of in isolation, I gained a clearer understanding of what my coriander plants were actually experiencing — and how to adjust greenhouse conditions for more predictable, vigorous, and aromatic growth at every stage. For everyday greenhouse growers who want leafy herbs like coriander to thrive, paying attention to these interacting variables offers a practical framework for better outcomes.

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