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

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

When I first transitioned from casual container gardening to growing tomatoes in a greenhouse, I approached it the same way I had treated other plants: give them plenty of light, water, and nutrients, and they should thrive. For the first few weeks that worked well, but as the plants matured I began seeing inconsistency in flowering, fruit set, and overall vigor that couldn’t be explained by light intensity or watering alone. That observation pushed me to start measuring not just PAR (Photosynthetically Active Radiation), but also CO₂ concentration and vapor pressure deficit (VPD) throughout the tomato growth cycle.

Over multiple seasons I learned that tomatoes respond to a combination of usable light, available carbon, and atmospheric conditions in very predictable ways. Tracking PAR, CO₂, and VPD together gave me a much clearer understanding of how to manage greenhouse conditions at each stage of growth — from seedlings to harvest — and led to more consistent yields.

This article shares what I learned about the requirements and interactions of these three environmental factors for tomatoes at different growth stages.

Why PAR, CO₂, and VPD Matter for Greenhouse Tomatoes

Tomatoes are light-demanding crops. They use usable light (PAR) to drive photosynthesis, CO₂ as the carbon source for building sugars and other biomolecules, and VPD governs transpiration and gas exchange. Each element influences not just growth rate but plant form, stress responses, flower set, and fruit quality.

When I first measured PAR in my greenhouse using a handheld meter, I was surprised how much usable light changed through the day — even when the greenhouse looked uniformly bright. When I paired that with CO₂ measurements and VPD data from a simple environmental monitor, patterns emerged that explained many of the inconsistent outcomes I had seen.

Together, PAR, CO₂, and VPD are not independent conditions; they interact and influence how effectively tomatoes use light, open their stomata, and build biomass.

Seedling Stage: Establishing Healthy Roots and Leaves

At the seedling stage, tomatoes are building roots and first true leaves. Light, carbon, and atmospheric conditions all influence early vigor.

From my observations:

  • PAR: Seedlings developed optimal leaf structure and compact growth when midday PAR measured around 150–250 µmol/m²/s. Lower usable light led to stretched stems and smaller leaves.
  • CO₂: At this stage, maintaining ambient CO₂ near outdoor levels (about 400–450 ppm) supported steady leaf expansion. In poorly ventilated sections of the greenhouse where CO₂ dipped below about 350 ppm during active photosynthesis, seedlings grew slower.
  • VPD: I found that a moderate VPD range (not too dry or too humid) around 0.8–1.2 kPa supported robust stomatal activity and steady water uptake without excessive transpiration stress. Too high VPD meant stomata partially closed, slowing CO₂ uptake; too low VPD slowed transpiration and nutrient movement.

A typical morning in my greenhouse during early tomato growth looked like this: PAR climbed gradually with the sun, CO₂ stayed reasonably stable, and VPD rose with daytime temperature. Recording these three together showed me why seedlings in one corner grew faster — they combine sufficient usable light, steady CO₂, and balanced atmospheric demand.

Vegetative Growth: Leaf and Stem Expansion

Once seedlings had several true leaves and moved into vigorous vegetative growth, usable light and carbon availability became more critical.

During this phase:

  • PAR: I targeted midday usable light around 300–450 µmol/m²/s near the canopy. Readings below about 250 µmol/m²/s consistently correlated with slower expansion and sparser leaf canopies.
  • CO₂: In my more enclosed greenhouse areas, midday CO₂ sometimes dropped as plants rapidly consumed carbon for photosynthesis, especially when supplemental lighting was used. By ensuring periodic fresh air exchange or gentle ventilation, I was able to keep CO₂ closer to 400–500 ppm through the main light period, which visibly improved growth rate.
  • VPD: VPD became a decisive factor in how much stomata could stay open for CO₂ uptake. When midday VPD exceeded about 1.8–2.0 kPa, plants under otherwise strong light and adequate CO₂ still showed slowed growth, as partial stomatal closure limited gas exchange. Keeping VPD in the 1.0–1.8 kPa range through balanced air movement and humidity control supported more consistent vegetative development.

In practice this meant adjusting side vents and fans on warm, dry days to avoid overly high VPD, and adjusting shading or misting on calm, humid days to avoid overly low VPD — both of which influence how effectively the plant uses light and CO₂.

Flowering and Fruit Set

When tomatoes begin to flower, their carbon demand rises sharply. At this stage, good usable light, sufficient CO₂, and balanced VPD can mean the difference between abundant fruit set and blossom drop.

In my measurements:

  • PAR: Midday usable light near 450–650 µmol/m²/s during flowering supported steady flower and fruit development. When PAR stayed consistently below 350 µmol/m²/s across multiple days, flower production was reduced and fruit set became irregular.
  • CO₂: In flower and fruit set conditions, I saw better outcomes when midday CO₂ stayed above ambient levels — around 450–600 ppm — especially in dense plantings. In areas where CO₂ dropped below 350–400 ppm during peak photosynthesis, blossom drop increased.
  • VPD: Fruit set and early fruit development often responded best when VPD stayed in moderate ranges (about 1.2–1.8 kPa). When VPD was too high (>2.0 kPa), stress responses increased and flower retention decreased. When VPD stayed very low (<1.0 kPa), stomata were sluggish and nutrient uptake slowed, also reducing fruit quality.

In a greenhouse trial where I adjusted ventilation timing and shading based on recorded PAR, CO₂, and VPD, I saw noticeably more consistent fruit set compared with areas where I only monitored light intensity.

Fruit Development and Maturation

As fruits grow and mature, the plant system shifts more carbon into fruit tissue. Balancing usable light and carbon supply through this period influences size, firmness, and sugar content.

From my tracking:

  • PAR: Usable light near 600–800 µmol/m²/s at peak midday hours helped fruit development without burning foliage in full sun — provided VPD and CO₂ were managed.
  • CO₂: Maintaining CO₂ near 450–650 ppm during peak light hours through enhanced airflow or periodic fresh air intake helped sustain carbohydrate production. When CO₂ dipped below 350 ppm during long light periods, fruits were smaller and ripened more slowly.
  • VPD: Balanced VPD (approximately 1.2–1.8 kPa) helped maintain stomatal function during peak photosynthesis without excessive water loss. On very hot days when VPD spiked above 2.0 kPa, fruit quality and leaf turgor suffered even when light and CO₂ levels were favorable.

In dry, high-light conditions without air exchange, I noticed that stomata began to close midday in response to high VPD, reducing CO₂ uptake and slowing photosynthesis. This pattern reinforced that tomatoes need the whole environmental picture — not just strong light — to develop high-quality fruit.

How I Use These Measurements Practically

Tracking PAR, CO₂, and VPD together in my greenhouse changed how I schedule ventilation, shading, and supplemental lighting:

  • Ventilation timing: I open side vents or use low-speed circulation midway through the light period when CO₂ tends to dip, which prevents carbon starvation during strong light.
  • Shading decisions: Instead of reacting only to midday brightness, I look at how usable light curves interact with VPD trends — shading earlier if VPD climbs too quickly.
  • Humidity and airflow: On humid days, slight airflow prevents stomatal sluggishness. On dry days, gentle misting or coolant nets help moderate VPD so stomata can stay open longer.

Because tomatoes integrate environmental conditions over many hours each day, I focus on the pattern of conditions rather than single snapshot measurements.

Final Reflection

Growing greenhouse tomatoes taught me that strong usable light (PAR) alone is not enough. To maximize plant response at different growth stages — from seedling through fruiting — tomatoes need a combination of sufficient carbon, balanced atmospheric demand, and consistent usable light.

PAR provides the energy plants use to drive photosynthesis. CO₂ supplies the carbon they build into sugars and biomass. VPD affects how open stomata stay, which governs how effectively plants can take in light and CO₂.

By monitoring these three environmental factors together, I gained insight into why plants sometimes underperformed even when light seemed adequate, and how to adjust ventilation, shading, and plant placement to achieve more predictable results.

For greenhouse growers who want reliable growth from tomatoes at every stage, thinking in terms of PAR, CO₂, and VPD — and how they interact — provides a practical, evidence-based framework for better decisions and better yields.

Final Takeaway

Tomatoes are CO₂-hungry, light-driven crops — but only when their physiological capacity matches their environment.

  • Seedlings: gentle environment
  • Vegetative: maximum CO₂ + PAR
  • Flowering: peak light & peak CO₂
  • Fruiting: balanced light and moderate CO₂
  • All stages: VPD must remain optimal

This is why using a multi-parameter device such as AH-200 — which simultaneously records PAR, CO₂, temperature, humidity, and VPD — allows growers to fine-tune greenhouse conditions with scientific precision.

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