Growing Chili Peppers in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Flower Drop, Heat, and Real Yield

Chili peppers taught me restraint.

I had already grown sweet peppers successfully, so when I moved to chilies, I assumed I could simply push things a little harder: more light, warmer air, higher CO₂. The plants looked vigorous. Leaves were dark green. Stems were strong.

And then the flowers dropped.

Not all at once. Quietly. In waves. The plants never looked stressed, which made it worse. It wasn’t until I started tracking PAR, CO₂, and VPD together that the pattern became obvious: chilies don’t forgive imbalance, especially during flowering.

Here’s what I learned from growing chili peppers across multiple greenhouse cycles.

---

1. Seedling & Early Establishment

(Where future stress tolerance is quietly set)

Chili seedlings are slower than tomatoes but faster than eggplant. Early on, I pushed light to shorten the nursery phase.

That decision came back later.

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: 22–26 °C

What I noticed:
Early dryness doesn’t stunt chili seedlings visibly.
It programs them to drop flowers later under mild stress.

Gentle light and stable humidity produced thicker stems and more resilient transplants.

---

2. Vegetative Growth (Canopy Building)

(Where chilies look unstoppable)

Once established, chili plants grow aggressively. This stage is deceptive because they tolerate a wide range of conditions without obvious symptoms.

I pushed PAR and CO₂ hard here, chasing faster canopy development.

The balance that worked best:

• PAR: 300–550 µmol·m⁻²·s⁻¹
• DLI: 12–20
• CO₂: 700–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 24–28 °C

What changed:

• canopy filled in without excessive stretch
• internodes shortened
• plants supported flowers better later

CO₂ enrichment helped significantly — but only because VPD stayed controlled.

---

3. Flowering & Fruit Set

(Where chili peppers expose every mistake)

This is where I lost yield early on.

I treated chili flowering like sweet pepper flowering: high PAR, enriched CO₂, and strong airflow. Flowers looked healthy — and then fell.

What finally stabilized fruit set:

• PAR: 350–600 µmol·m⁻²·s⁻¹
• DLI: 16–22
• CO₂: 800–1100 ppm
• VPD: 0.6–1.0 kPa
• Temperature: 22–26 °C

Key realization:
Chilies need lower VPD during flowering than sweet peppers.
If the air is even slightly too dry, pollen viability drops and flowers abort — even when leaves look perfect.

Reducing airflow and slightly lowering temperature improved fruit set more than increasing light ever did.

---

4. Fruit Development & Heat Accumulation

(Where flavor and pungency are shaped)

Once fruit set stabilized, I assumed I could push intensity again. The plants tolerated it — but flavor changed.

The range I now aim for:

• PAR: 350–650 µmol·m⁻²·s⁻¹
• DLI: 16–24
• CO₂: 800–1000 ppm
• VPD: 0.9–1.3 kPa
• Temperature: 24–30 °C

What I noticed:

• Higher VPD increased pungency but reduced fruit size
• Lower VPD produced thicker walls but milder heat
• Extreme PAR hardened skins and reduced uniformity

Chili peppers translate stress into heat, not yield.

---

5. Ripening & Late-Stage Production

(Where consistency matters more than speed)

As fruit ripened, I stopped chasing size and focused on stability.

What worked best late-stage:

• PAR: 300–500 µmol·m⁻²·s⁻¹
• DLI: 14–20
• CO₂: 600–900 ppm
• VPD: 1.0–1.4 kPa
• Temperature: 20–26 °C

What I saw:

• better color uniformity
• reduced fruit drop
• improved shelf life
• steadier harvest rhythm

Too much humidity increased disease pressure.
Too much dryness caused flower abortion even late in the cycle.

---

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

After several cycles, one pattern became impossible to ignore:

• Chili peppers tolerate high PAR — until flowering
• CO₂ boosts growth — until stomata close
• VPD quietly controls fruit set and pungency

In practice:

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

---

Practical Lessons I Took Away

• Chilies are more sensitive than sweet peppers during flowering
• Early stress shows up weeks later as flower loss
• CO₂ is valuable only when humidity is controlled
• VPD management matters more than airflow strength
• Stress increases heat, not yield
• Measuring light alone never explains flower drop

---

Final Thoughts

Growing chili peppers taught me that yield and heat are not the same thing.

The biggest improvements didn’t come from stronger lights or hotter air — they came from measuring PAR, CO₂, and VPD together, and learning when to ease off instead of pushing harder.

Once I stopped forcing growth and started managing balance, chili peppers became predictable, productive, and far more controllable — both in yield and in heat level.