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

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

When I first started growing kale in a greenhouse, I treated it much the way I did other leafy greens: provide bright light, consistent water, and balanced nutrients, and the plants should thrive. In the beginning that seemed sufficient — seedlings unfolded and leaves looked healthy — but as the plants matured I began to notice uneven leaf texture, variation in leaf color, and inconsistent growth rates across the greenhouse. These differences persisted even though light levels, water schedules, and feeding regimes were similar. That’s when I began recording PAR (Photosynthetically Active Radiation), CO₂ (carbon dioxide concentration), and VPD (vapor pressure deficit) together, rather than in isolation. What I discovered was that kale responds strongly to the combination of these three environmental factors at different stages of growth.

This article summarizes what I learned through repeated measurements and observing kale growth responses, and shows how monitoring these variables helped me achieve more predictable, vigorous, and consistent kale production.

Why PAR, CO₂, and VPD Are Important for Kale

Kale is a leafy green that depends heavily on efficient photosynthesis for leaf production and quality. Photosynthesis requires usable light (PAR), carbon in the form of CO₂, and stomatal gas exchange that is influenced by atmospheric conditions, which we quantify with VPD.

  • PAR tells you how much usable light the plants actually receive, in micromoles per square meter per second.
  • CO₂ is the carbon source plants use to convert light energy into sugars and biomass.
  • VPD is a measure of the difference between the moisture in the plant’s leaf environment and the air around it, and it influences stomatal opening and gas exchange.

By tracking these together, rather than guessing from light alone, I began seeing why plants in different positions or micro-zones behaved differently even under similar care.

Seedling and Early Leaf Development Stage

In the earliest stage, kale seedlings are developing roots and their first true leaves, and they respond quickly to environmental cues.

From my greenhouse measurements:

  • PAR: Midday usable light around 150–300 µmol/m²/s encouraged compact leaf formation and strong early growth. In spots where PAR remained consistently below 150 µmol/m²/s, seedlings tended to grow leggy with elongated petioles and narrower leaves.
  • CO₂: Around ambient outdoor levels (about 400–450 ppm) was adequate for steady early growth. In areas where ventilation was limited and CO₂ dipped below 350 ppm during midday, seedlings developed slightly slower and had smaller early leaves compared with better-ventilated zones.
  • VPD: A moderate VPD range of about 0.8–1.3 kPa supported active stomatal function without undue water stress. When VPD climbed above 1.5 kPa due to dry, warm air, I saw slight leaf curl and slower leaf expansion even with adequate light.

Recording these measurements helped me understand why seedlings in one corner of the greenhouse seemed more vigorous than those in another, even under similar care.

Vegetative Growth: Leaf Production and Canopy Formation

Once kale entered vigorous vegetative growth, its demand for usable light and CO₂ increased, and atmospheric conditions played a larger role.

In this phase:

  • PAR: Midday usable light around 300–500 µmol/m²/s supported broader leaves and rapid leaf count increase. Areas where midday PAR rarely exceeded 250 µmol/m²/s produced thinner, smaller leaves and slower canopy expansion.
  • CO₂: During peak photosynthesis, CO₂ levels sometimes dropped in less ventilated areas. Maintaining CO₂ closer to 450–600 ppm by increasing air exchange correlated with faster leaf production and richer green color. Spots with fluctuating or low CO₂ tended to have slower growth despite similar light.
  • VPD: I observed that VPD values around 1.0–1.8 kPa supported active gas exchange without premature water loss. When VPD soared above 2.0 kPa — common mid-afternoon in hot conditions — leaves appeared slightly stressed, and growth slowed.

Balancing ventilation, temperature, and humidity helped me keep VPD in a range where stomata could remain open for efficient CO₂ uptake without excessive transpiration.

Pre-Harvest and Maturation Stage

As kale approached full size and leaf production plateaued, the interplay between usable light, carbon availability, and atmospheric demand influenced leaf thickness, color depth, and uniformity of growth.

From my greenhouse logs:

  • PAR: Usable light near 450–650 µmol/m²/s during midday supported healthy, fully developed leaves that were thicker and richer in color. Days where DLI stayed below about 22–25 mol/m²/day often saw lighter, thinner leaves.
  • CO₂: Maintaining midday CO₂ near 500–650 ppm during active photosynthesis helped maintain consistent leaf quality and uniform growth across plants. In areas where CO₂ continually dipped below 400 ppm, leaf texture was sometimes slightly tougher and plants showed signs of slower carbohydrate accumulation.
  • VPD: Moderate VPD (about 1.2–1.8 kPa) helped sustain stomatal function without water stress. When VPD dropped too low — often due to very high humidity — stomatal motion became sluggish, slowing gas exchange and limiting CO₂ uptake even though usable light was high.

In practice, this meant using shade cloth or ventilation at specific times of day to prevent extreme peaks in PAR or VPD that could counteract steady growth.

How I Monitor and Adjust the Environment

In my greenhouse I take measurements several times throughout the day — early morning, close to midday, and in the late afternoon — to capture how PAR, CO₂, and VPD change over time. Recording these variables rather than relying on single spot measurements gave me insights into patterns that influenced plant growth.

Ventilation

Good air exchange prevents CO₂ depletion and helps stabilize VPD. In areas with stagnant air, CO₂ often dropped during peak photosynthesis, and stomatal exchange became limited. Increasing ventilation in those zones made a noticeable difference in growth consistency.

Temperature, Humidity, and Shading

Temperature and humidity directly influence VPD. On hot, dry days, VPD spiked and grew faster than usable light levels increased, which stressed plants. Using shade cloth during peak heat and managing humidity through fogging or venting helped keep VPD in a more favorable range.

Light Distribution

Measuring usable light at canopy height showed me where PAR was being blocked or diffused more than expected, even in spots with good overall daylight. Those measurements helped me adjust plant placement and supplemental lighting to ensure more uniform conditions across the crop.

Practical Tips for Everyday Growers

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

  • Take multiple measurements throughout the day rather than relying on one snapshot. Light and atmospheric conditions change, and trends matter for plant response.
  • Balance fresh air exchange with thermal control to maintain CO₂ and VPD in ranges that support active gas exchange.
  • Moderate midday extremes — very high PAR without supportive CO₂ or balanced VPD can stress plants rather than help them grow.
  • Watch plant responses as feedback — leaf thickness, expansion rate, color intensity, and turgor tell you a lot about whether conditions are aligned.

Final Reflection

Growing greenhouse kale taught me that plants don’t respond to a single factor in isolation. They integrate usable light, carbon availability, and atmospheric demand across time to drive leaf production, canopy formation, and quality. Usable light provides energy, CO₂ provides the carbon source for building biomass, and VPD governs how freely stomata can open for gas exchange.

By tracking and adjusting PAR, CO₂, and VPD together, I gained a much clearer understanding of what was actually limiting growth at different stages. Instead of guessing about conditions based solely on how bright the greenhouse looks, I learned to interpret the environment through measurable variables that directly influence plant physiology.

For everyday growers who want leafy greens like kale to grow vigorously and consistently from seedling to maturity, thinking in terms of usable light intensity, carbon availability, and atmospheric conditions offers a practical framework for better decisions and better results.

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