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

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

When I first started growing strawberries in a greenhouse, I treated them the way I did most fruits: good soil, regular water, and plenty of light should be enough. In the early weeks that seemed to work, but as plants entered flowering and fruiting stages I began noticing inconsistent bloom set, variation in fruit size, and sporadic ripening timing. That pattern didn’t make sense based on heat, water, or fertilizer alone. To understand what was really limiting growth, I began systematically recording three environmental variables together — usable light (PAR), carbon dioxide (CO₂), and vapor pressure deficit (VPD) — throughout the crop cycle.

Over multiple seasons of tracking these conditions and observing plant performance, I found that strawberries respond strongly to the combined effects of usable light, available carbon, and atmospheric demand. Managing these variables together gave me a much clearer picture of what strawberries actually need at each growth stage, from establishment through harvest.

This article shares insights from that experience and how everyday greenhouse growers can apply the same principles.

Why PAR, CO₂, and VPD Matter for Strawberries

Strawberries are not only light-demanding but also sensitive to atmospheric conditions that influence how they use that light. Here’s why the three variables matter:

  • PAR (Photosynthetically Active Radiation) provides the usable light energy for photosynthesis. It’s measured in micromoles per square meter per second (µmol/m²/s).
  • CO₂ (Carbon Dioxide) is the carbon source plants fix into sugars during photosynthesis. Indoor or greenhouse CO₂ can fluctuate significantly without proper ventilation.
  • VPD (Vapor Pressure Deficit) describes the drying power of the air and affects stomatal opening, transpiration, and ultimately how much CO₂ a plant can take in.

Focusing on any single factor in isolation oversimplifies how plants respond. Light without adequate carbon or with extreme atmospheric demand can leave plants unable to use the available energy effectively.

Seedling and Young Plant Stage

In the earliest stage, strawberries are developing roots and their first set of functional leaves. Conditions during this phase influence how well later growth unfolds.

From my greenhouse measurements:

  • PAR: Young strawberry plants developed compact leaves and steady early growth when midday usable light was around 150–300 µmol/m²/s. Too little usable light led to longer, weaker petioles.
  • CO₂: At this stage, maintaining CO₂ near ambient outdoor levels (around 400–450 ppm) was sufficient for steady establishment. In corners with poor airflow, midday CO₂ sometimes dipped below 350 ppm, and plants in those spots appeared slower to expand leaf area.
  • VPD: A moderate VPD range (approximately 0.8–1.3 kPa) supported active stomatal behavior without excessive transpiration or slowed gas exchange. When VPD spiked above 1.5 kPa under hot, dry conditions, leaves appeared slightly curled or wilted despite the same light and water, indicating physiological stress.

I learned that even at this early stage, balancing usable light with stable CO₂ and moderate atmospheric demand helped seedlings establish a strong foundation.

Vegetative Growth: Leaf Expansion and Canopy Development

Once strawberries passed their seedling phase and began forming more leaves and runners, usable light and carbon demand increased significantly.

In this mid-growth stage:

  • PAR: Midday usable light near 300–500 µmol/m²/s supported more expansive leaf canopies and runner formation. Locations where PAR rarely reached this range often produced thinner foliage and slower runner growth.
  • CO₂: With more leaves photosynthesizing actively, midday CO₂ began showing more noticeable dips in poorly ventilated zones. I found that maintaining CO₂ near 450–600 ppm during peak photosynthetic hours correlated with stronger leaf expansion and earlier runner development.
  • VPD: I monitored VPD to ensure stomata could remain open for CO₂ uptake. When VPD was too low (<0.8 kPa) due to excessive humidity, stomatal movement became sluggish and growth slowed. When VPD climbed above 1.8–2.0 kPa in hot, dry air, plants again exhibited signs of stress. Balancing VPD around 1.1–1.8 kPa supported vigorous canopy expansion during this stage.

Balancing airflow, shade cloth timing, and humidity control helped me keep usable light and VPD aligned so that CO₂ uptake remained efficient.

Flowering and Fruit Set

Flowering is a pivotal phase for strawberries; it determines how many potential fruits will develop and how well they will mature. This stage also reveals how well the plant is using its light and carbon resources.

From my observations:

  • PAR: Usable light in the 450–650 µmol/m²/s range around midday supported abundant flower formation and strong initial fruit set. Days where the integrated daily usable light (DLI) stayed below about 20–22 mol/m²/day tended to correlate with uneven flowering and lower fruit counts.
  • CO₂: In flower-forming zones, maintaining midday CO₂ near 500–650 ppm often produced fuller flower clusters and higher fruit set. In stagnant air zones where CO₂ fell below 350–400 ppm, flowers were fewer and fruit set was sporadic even with similar light levels.
  • VPD: Moderate VPD (about 1.2–1.8 kPa) helped flowers stay hydrated without suppressing stomatal exchange. When VPD stayed too high — especially on hot, sunny afternoons — some blossoms wilted or dropped early, indicating that the plant could not maintain balance between water loss and gas exchange.

In practice, this stage required vigilant ventilation and sometimes shading during peak heat to prevent VPD spikes that compromised flower retention.

Fruit Development and Ripening

Once fruit set begins, the plant shifts carbon and energy into developing and maturing berries. The balance of usable light, CO₂, and atmospheric demand affects size, sweetness, and consistency of ripening.

From greenhouse tracking:

  • PAR: Peak usable light near 600–800 µmol/m²/s supported robust fruit growth, provided that extreme midday peaks did not coincide with excessive heat and VPD spikes.
  • CO₂: During active photosynthesis, keeping CO₂ closer to 550–700 ppm supported steady carbohydrate production that contributed to fruit size and consistent ripening. When CO₂ dropped below 400 ppm during long light periods, fruit size stagnated even though light was abundant.
  • VPD: Moderate VPD around 1.3–1.8 kPa helped maintain stomatal conductance for efficient carbon uptake. When VPD exceeded about 2.0 kPa, older leaves showed heat stress and transpiration imbalance, which in turn slowed fruit growth and delayed maturation.

In a few controlled weeks where ventilation and misting were optimized to avoid extreme VPD, I recorded not only larger average berry size but also more uniform ripening across plants in the same greenhouse.

How I Monitor and Manage Environment Conditions

Managing these variables together requires thoughtful monitoring and occasional adjustment:

Routine Monitoring

I take measurements at multiple points during the day — morning, midday, and late afternoon — to capture how PAR, CO₂, and VPD change through the growth cycle. Logging these trends helps identify whether environmental conditions are tracking with plant responses.

Ventilation and CO₂ Balance

Good airflow prevents CO₂ depletion during peak photosynthetic periods. I use vents, circulation fans, and periodic fresh air intake to keep CO₂ more stable, especially when supplemental heating or lighting is used.

Heat Management and VPD Control

I manage shading, humidity, and airflow to avoid extreme midday VPD spikes. On hot days, even with strong usable light, unchecked VPD can lead to stress responses that diminish fruit set and quality.

Final Reflection

Growing greenhouse strawberries taught me that a plant’s environment is not defined by light alone. Strawberries integrate usable light energy, carbon availability, and atmospheric conditions across time. Usable light (PAR) provides the energy. CO₂ provides the carbon needed to build sugars and fruit biomass. VPD shapes how effectively the plant can exchange gases and regulate water loss.

Treating these as interconnected variables allowed me to see patterns that explained why plants sometimes underperformed even when light and water seemed adequate. Recording PAR, CO₂, and VPD together gave me a clear sense of what the plants actually experienced — and how to adjust my greenhouse environment to support consistent growth and high-quality fruit.

For everyday greenhouse growers who want strawberries that grow vigorously and fruit reliably, understanding how usable light, carbon dioxide, and atmospheric demand work together provides a practical, evidence-based approach that goes beyond simply “bright light” or “more water.”

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