Growing Sweet Peppers in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Flowers, Fruit Set, and Real Yield

Sweet peppers look sturdy. Thick stems, large leaves, long growing season — they give the impression that once they’re established, they can handle almost anything.

That assumption cost me yield.

When I first grew sweet peppers in a greenhouse, I focused heavily on light intensity and temperature. The plants grew tall and healthy, but flower drop, uneven fruit set, and slow sizing kept showing up. It wasn’t until I started measuring PAR, CO₂, and VPD together that the real limiting factors became obvious.

Here’s what I learned by growing sweet peppers across multiple greenhouse cycles.

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1. Seedling & Early Establishment

(Where root balance matters more than speed)

Sweet pepper seedlings are slower than tomatoes, which made me want to push light early to accelerate development. That backfired.

The plants looked fine above ground, but roots lagged behind, and later stress showed up during flowering.

What finally worked for me:

• PAR: 120–220 µmol·m⁻²·s⁻¹
• DLI: ~6–10
• CO₂: 400–600 ppm
• VPD: 0.4–0.8 kPa
• Temperature: 20–24 °C

What I noticed:
Sweet pepper seedlings need time to build roots.
If VPD rises too early, transpiration outpaces root uptake, creating hidden stress that shows up weeks later.

Gentle light and stable humidity produced sturdier transplants with thicker stems.

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2. Vegetative Growth (Canopy Development)

(Where leaf area sets the ceiling for yield)

Once established, sweet peppers build leaf mass steadily. This stage determines how much energy the plant can support later — but only if conditions stay balanced.

I tried pushing PAR and CO₂ aggressively here. Leaves grew fast, but internodes stretched and plants became less stable.

The balance that worked best:

• PAR: 300–500 µmol·m⁻²·s⁻¹
• DLI: 12–18
• CO₂: 700–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 22–26 °C

What changed:

• leaves thickened instead of stretching
• canopy became denser
• stems supported fruit load better later

CO₂ enrichment helped significantly here — but only when VPD stayed in range.

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3. Flowering & Fruit Set

(Where peppers quietly punish imbalance)

This is the most sensitive stage for sweet peppers — and the stage where I lost the most yield early on.

I initially treated flowering like a continuation of vegetative growth. The plants stayed healthy, but flowers dropped, and fruit set was inconsistent.

What finally stabilized fruit set:

• PAR: 400–650 µmol·m⁻²·s⁻¹
• DLI: 18–24
• CO₂: 900–1200 ppm
• VPD: 0.6–1.0 kPa
• Temperature: 20–24 °C

Key realization:
Sweet peppers need slightly lower VPD during flowering than tomatoes.
If the air is too dry, pollen viability drops and flowers abort — even when everything looks fine visually.

Once humidity and VPD were controlled, fruit set improved dramatically without increasing light.

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4. Fruit Development & Sizing

(Where yield is built slowly, not forced)

After fruit set, my instinct was to push PAR and temperature to speed up sizing. The fruit grew faster — but walls were thinner, and shape became inconsistent.

The range I now aim for:

• PAR: 350–600 µmol·m⁻²·s⁻¹
• DLI: 16–22
• CO₂: 800–1000 ppm
• VPD: 0.9–1.3 kPa
• Temperature: 20–26 °C

What improved:

• thicker fruit walls
• more uniform shape
• better color development

Moderate VPD helped transpiration without stressing flowers or young fruit.

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5. Ripening & Late-Stage Production

(Where quality and shelf life are decided)

During ripening, I stopped chasing size and focused on consistency and stress reduction.

What worked best late-stage:

• PAR: 300–500 µmol·m⁻²·s⁻¹
• DLI: 14–20
• CO₂: 700–900 ppm
• VPD: 1.0–1.4 kPa
• Temperature: 18–24 °C

What I saw:

• better color uniformity
• firmer fruit
• reduced blossom-end issues
• improved shelf life

Too much humidity increased disease pressure.
Too much dryness reduced fruit quality.

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How PAR, CO₂, and VPD Actually Work Together for Sweet Peppers

After multiple cycles, one pattern became very clear to me:

• Sweet peppers tolerate high PAR — but only with correct VPD
• CO₂ boosts yield only when stomata stay open
• VPD quietly controls flowering success and fruit quality

In practice:

• High PAR + high VPD → flower drop, thin fruit
• High CO₂ + unstable humidity → wasted potential
• Balanced PAR + enriched CO₂ + stable VPD → consistent yield

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Practical Lessons I Took Away

• Sweet peppers are more sensitive than tomatoes during flowering
• Root balance early affects fruit set later
• CO₂ is most valuable from vegetative growth through fruit set
• VPD control matters more than absolute humidity
• Pushing growth often reduces yield instead of increasing it
• Measuring light alone never explains flower drop

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Final Thoughts

Growing sweet peppers taught me that yield is built through balance, not intensity.

The biggest improvements didn’t come from stronger lights or higher temperatures — they came from measuring PAR, CO₂, and VPD together, and adjusting conditions gently at each stage instead of forcing speed.

Once I stopped pushing and started managing balance, sweet peppers became far more predictable, productive, and consistent across long harvest cycles.