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

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

When I first started growing rosemary in my greenhouse, I treated it like most herbs I’d grown before: give it light, water appropriately, and let nature take its course. In the very early stages that seemed to work — seedlings sprouted and developed their first aromatic leaves. But as the plants matured, I began noticing differences in leaf thickness, aroma strength, and overall plant vitality that could not be explained by light or nutrients alone. Plants in some areas developed dense foliage and deep scent, while others, under similar light levels, remained less vigorous.

Those inconsistencies inspired me to start monitoring not just light, but usable light (PAR), carbon dioxide (CO₂), and vapor pressure deficit (VPD) together across different growth stages. Over several seasons of tracking these variables and observing plant responses, I discovered that rosemary’s performance depended on how these environmental factors interacted over time.

Below is a detailed, experience-based guide that explains how PAR, CO₂, and VPD influence rosemary growth from seedling through maturity.

Why PAR, CO₂, and VPD Matter for Rosemary

Rosemary is a perennial herb with dense, evergreen foliage and a strong aromatic profile. Because its leaves are relatively thick and its growth habit can be both vertical and bushy, environmental balance is especially important.

  • PAR (Photosynthetically Active Radiation) refers to usable light within the 400–700 nm spectrum that plants use for photosynthesis, measured in micromoles per square meter per second (µmol/m²/s).
  • CO₂ (Carbon Dioxide) provides the carbon skeletons that plants fix into sugars and biomass during photosynthesis.
  • VPD (Vapor Pressure Deficit) reflects the atmospheric demand for moisture and influences how open stomata can remain for gas exchange and transpiration.

Tracking these three together revealed patterns that weren’t obvious when looking at any one factor in isolation.

Seedling and Early Leaf Development Stage

At the earliest stage, rosemary seedlings are establishing roots and the first true leaves. How they respond to light, carbon supply, and atmospheric conditions sets the tone for their future growth.

From my greenhouse measurements:

  • PAR: Usable light around 150–300 µmol/m²/s at midday supported compact, healthy early foliage. In areas where midday PAR stayed consistently below 150 µmol/m²/s, seedlings tended to produce thinner leaves and longer internodes, as if reaching for more usable light.
  • CO₂: During active light hours, maintaining CO₂ near ambient outdoor levels (about 400–450 ppm) helped early leaf expansion. In stagnant zones where midday CO₂ dipped below 350 ppm, seedlings expanded more slowly and appeared less robust.
  • VPD: A moderate VPD range of about 0.8–1.3 kPa supported stomatal function without causing excessive water loss. On warm, dry afternoons when VPD spiked above 1.5 kPa, early leaves showed slight curling and expansion slowed even when PAR and CO₂ were adequate.

Recording these values helped me understand why seedlings in some micro-zones seemed more vigorous than others, even when they were given similar care.

Vegetative Growth: Leaf Expansion and Canopy Development

Once rosemary passed the seedling stage and entered the vegetative phase, its demand for usable light and carbon increased, and atmospheric conditions became more critical in determining growth rate and leaf quality.

In this phase:

  • PAR: Midday usable light near 300–500 µmol/m²/s supported robust leaf expansion and denser foliage. In spots where midday PAR rarely reached 250 µmol/m²/s, plants developed thinner, less aromatic leaves and expanded more slowly.
  • CO₂: As leaf area increased and photosynthesis became more active, CO₂ levels sometimes dipped during peak light periods in less ventilated zones. Improving airflow to keep CO₂ closer to 450–600 ppm correlated with more even and vigorous leaf production. In areas where CO₂ consistently dropped below 400 ppm, growth was less uniform.
  • VPD: Moderate VPD values — typically 1.0–1.8 kPa — allowed stomata to remain open for efficient gas exchange without provoking undue water stress. On especially hot, dry afternoons when VPD climbed above 2.0 kPa, I observed leaf stress symptoms such as slight wilting and slower expansion, even though usable light and CO₂ were adequate.

Balancing ventilation, shading, and humidity helped keep VPD in a range that supported steady gas exchange and leaf production.

Mature Growth and Aroma Development

As rosemary plants approach full size and focus more on foliage density and essential oil synthesis, the interplay between usable light, carbon availability, and atmospheric demand continues to shape plant performance.

From greenhouse logs:

  • PAR: Usable midday light near 450–650 µmol/m²/s supported full canopy development and deeper green foliage. In zones where the daily light integral (DLI) stayed below about 18–22 mol/m²/day, leaves were paler and less dense.
  • CO₂: Maintaining midday CO₂ near 500–650 ppm during active photosynthesis helped sustain carbohydrate production that contributes to both biomass and essential oil synthesis. In areas where CO₂ frequently dipped below 400 ppm during peak light periods, leaves grew more slowly and aromatic intensity was less pronounced at harvest.
  • VPD: Moderate VPD (around 1.2–1.8 kPa) facilitated consistent stomatal conductance. When VPD remained very low due to high humidity, stomatal responsiveness slowed and growth lagged. When VPD spiked above 2.0 kPa on hot, dry afternoons, leaf edges exhibited slight stress signs and growth was uneven.

By adjusting shade, humidity control, and fresh air exchange during midday heat, I was able to keep VPD within a range that allowed rosemary to use both sunlight and carbon efficiently.

How I Monitor and Adjust Conditions

Effectively managing these variables in my greenhouse required tracking them over the course of the day, rather than relying on individual snapshot measurements. I measure PAR, CO₂, and VPD at key times — early morning, midday, and late afternoon — to see how conditions change and how plants respond.

Ventilation and Airflow

Ensuring fresh air exchange, especially during peak photosynthesis, prevents midday CO₂ drawdown and helps stabilize VPD. On calm or hot afternoons I increase circulation to maintain consistent airflow.

Humidity and Temperature Management

Temperature and humidity together determine VPD. On hot, dry days I use shade cloth and increased airflow to prevent midday VPD spikes. On humid days, boosting circulation improves stomatal performance without excessive moisture buildup.

Usable Light Distribution

Measuring PAR at canopy height revealed where shading from greenhouse structure or neighboring plants reduced effective light. Adjusting plant placement and supplemental lighting helped ensure more uniform PAR distribution.

Practical Tips for Everyday Growers

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

  • Take repeated measurements throughout the day rather than relying on a single reading. Trends reveal how conditions interact with plant physiology.
  • Balance fresh air exchange with humidity control to maintain CO₂ and VPD in ranges conducive to active gas exchange.
  • Moderate midday extremes — very high usable light without supportive atmospheric conditions can stress plants and slow growth.
  • Observe plant behavior as feedback — leaf density, posture, aroma intensity, and growth rate often reflect how environmental conditions are affecting physiology.

Final Reflection

Growing greenhouse rosemary taught me that plants do not respond to single variables in isolation. Instead, rosemary integrates usable light energy, carbon availability, and atmospheric demand across the day to determine growth rates, foliage density, and aromatic quality. Usable light provides the energy needed for photosynthesis, CO₂ supplies the carbon framework, and VPD influences how freely stomata can open for gas exchange without undue water stress.

By tracking PAR, CO₂, and VPD together rather than in isolation, I gained a much clearer understanding of what my rosemary plants were actually experiencing. That understanding helped me adjust greenhouse conditions for more predictable, vigorous, and aromatic growth at every stage. For everyday greenhouse growers who want rosemary with fuller foliage and stronger scent, thinking in terms of these interacting variables provides a practical, evidence-based framework for better outcomes.

View on Amazon

Amazon is a trademark of Amazon.com, Inc. or its affiliates.