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

Strawberries are light-responsive, humidity-sensitive crops that require proper environmental balance to achieve strong vegetative growth, flower initiation, and high fruit quality. Here’s how PAR, CO₂, and VPD should be managed at each stage of greenhouse cultivation.

1. Early Establishment — Young Plants & Rooting Stage
   Plant state: Small leaf surface, fragile root system, low photosynthetic capacity.
   PAR: 120–250 µmol·m⁻²·s⁻¹ (gentle light)
   DLI: 6–10 (just enough for leaf initiation)
   CO₂: 400–600 ppm (elevated CO₂ not useful yet)
   VPD: 0.5–0.8 kPa (higher humidity supports rooting)
   Temp: 18–22 °C (slightly cool, stable climate)
   Why: Early strawberry plants prioritize root growth, not rapid leaf expansion. Too much light or high VPD at this stage leads to dehydration.
2. Vegetative Growth — Leaf Expansion & Crown Development
   Plant state: Leaf area increases; plants prepare for reproductive development.
   PAR: 300–600 µmol·m⁻²·s⁻¹ (light drives crown development)
   DLI: 12–18 (supports leaf expansion)
   CO₂: 700–1000 ppm (boosts leaf and crown biomass)
   VPD: 0.8–1.2 kPa (promotes transpiration and CO₂ intake)
   Temp: 18–24 °C (ideal for biomass formation)
   Why: Strawberries respond strongly to enriched CO₂, moderate VPD, and steady PAR, resulting in fuller leaf mass, stronger crowns, and better fruiting potential later.
3. Flowering & Pollination Stage
   Plant state: Plants switch to reproductive mode; energy shifts to flower production.
   PAR: 450–700 µmol·m⁻²·s⁻¹ (strong light increases flower count)
   DLI: 16–22 (high daily light demand)
   CO₂: 900–1200 ppm (enhances reproductive energy)
   VPD: 0.7–1.0 kPa (slightly humid helps pollination)
   Temp: 20–24 °C (good pollen viability)
   Why: Good VPD control is critical. Too humid and pollen clumps causing poor pollination. Too dry and flowers desiccate. Strawberries need slightly lower VPD during flowering than tomatoes or cucumbers.
4. Fruiting & Ripening — Yield Production Stage
   Plant state: Sugars are transported into fruit; flavor and firmness stabilize.
   PAR: 350–600 µmol·m⁻²·s⁻¹ (supports fruit sweetness)
   DLI: 14–20 (balanced and consistent)
   CO₂: 700–1000 ppm (moderate enrichment)
   VPD: 1.0–1.4 kPa (reduces fungal risk)
   Temp: 18–22 °C (cooler conditions make berries firmer and sweeter)
   Why: Higher VPD during fruiting reduces botrytis, decreases powdery mildew, and prevents berry softness.

Understanding the Interaction Between PAR, CO₂, and VPD in Strawberries
High PAR + High CO₂ + Correct VPD = more leaves early, more flowers later, more berries ultimately
High PAR + Low CO₂ = light wasted due to carbon limitation
High CO₂ + Low PAR = CO₂ unused due to lack of photon energy
Wrong VPD = stomata close, CO₂ intake blocked, photosynthesis inhibited

Practical Grower Advice
Strawberries prefer cooler conditions than tomatoes.
They thrive under stable, moderate humidity.
CO₂ is valuable during vegetative and flowering stages.
Avoid extreme VPD during flowering to maintain pollination quality.
During fruiting, manage humidity carefully to prevent disease.

Final Takeaway
Strawberries require optimized conditions: lower light at seedling, higher light and CO₂ during vegetative growth, controlled humidity during flowering, and slightly reduced temperature with higher VPD during fruit maturation. Monitoring PAR, CO₂, temperature, humidity, and VPD with precise instruments such as the AH-200 enables growers to optimize growth, increase yields, and produce high-quality fruit.