Growing Eggplant in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Flowers, Fruit Shape, and Nighttime Stress

Eggplant taught me patience.

Compared to tomatoes or peppers, eggplants grow with confidence — thick stems, large leaves, and a willingness to tolerate heat. When I first grew eggplants in a greenhouse, I assumed they would thrive under strong light and aggressive airflow.

They grew fast.
But flowers aborted, fruit set was uneven, and shapes varied wildly. Nothing looked obviously wrong during the day.

The problem wasn’t daytime conditions.
It was what happened when the lights went off.

Once I started tracking PAR, CO₂, and VPD together — including at night, eggplants exposed how sensitive they are to nighttime humidity and stress recovery.

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

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1. Seedling & Early Establishment

(Where eggplants learn how to handle stress later)

Eggplant seedlings are slower than tomatoes but sturdier than peppers. Early on, I pushed light to speed up transplant readiness.

That created hidden stress.

What finally worked for me:

• PAR: 150–260 µmol·m⁻²·s⁻¹
• DLI: ~8–12
• CO₂: 400–600 ppm
• VPD: 0.4–0.8 kPa
• Temperature: 22–26 °C

What I noticed:
Eggplant seedlings tolerate early stress without showing it.
But that stress reduces flower stability weeks later.

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

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2. Vegetative Growth (Canopy Expansion)

(Where eggplants look unstoppable)

Once established, eggplants build massive leaf area. This stage feels safe because plants rarely collapse.

I pushed PAR, temperature, and airflow aggressively.

The balance that worked best:

• PAR: 350–600 µmol·m⁻²·s⁻¹
• DLI: 14–22
• CO₂: 800–1100 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 24–30 °C

What changed:

• leaves stayed broad and upright
• internodes shortened
• plants supported fruit weight better later

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

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3. Flowering & Fruit Set

(Where eggplants expose nighttime problems)

During flowering, eggplants looked perfect during the day. Flowers were large and vibrant.

And then fruit set failed intermittently.

What finally stabilized fruit set:

• PAR: 400–650 µmol·m⁻²·s⁻¹
• DLI: 18–24
• CO₂: 900–1200 ppm
• VPD (day): 0.7–1.1 kPa
• VPD (night): 0.3–0.6 kPa
• Temperature (day/night): 22–28 °C / 18–22 °C

Key realization:
Eggplants are extremely sensitive to night VPD.
If nighttime air is too dry, flowers abort — even when daytime conditions are perfect.

Lowering night airflow and maintaining humidity improved fruit set more than increasing light ever did.

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4. Fruit Development & Shape Control

(Where stress shows up as misshapen fruit)

Once fruit set stabilized, I assumed the hard part was over.

It wasn’t.

The range I now aim for:

• PAR: 400–700 µmol·m⁻²·s⁻¹
• DLI: 18–26
• CO₂: 800–1100 ppm
• VPD: 0.9–1.3 kPa
• Temperature: 24–30 °C

What I noticed:

• High VPD produced narrow, elongated fruit
• Low VPD increased fruit diameter but slowed growth
• Sudden VPD swings caused curved or uneven shapes

Eggplants translate stress into fruit shape variation, not leaf damage.

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5. Ripening & Late-Stage Stability

(Where consistency replaces speed)

As fruit ripened, I reduced intensity and focused on stability across day and night.

What worked best late-stage:

• PAR: 350–550 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:

• more uniform color
• reduced flower abortion on upper nodes
• improved fruit firmness
• steadier harvest rhythm

Too much humidity increased disease pressure.
Too much dryness triggered late flower drop.

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How PAR, CO₂, and VPD Actually Work Together for Eggplant

After several cycles, one pattern became clear:

• Eggplants tolerate strong light — but not dry nights
• CO₂ boosts growth only when transpiration is controlled
• VPD — especially at night — controls fruit set and shape

In practice:

• High PAR + dry nights → flower abortion, uneven fruit
• High CO₂ + unstable humidity → wasted potential
• Balanced PAR + enriched CO₂ + stable day/night VPD → consistent yield

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Practical Lessons I Took Away

• Eggplants are more sensitive at night than during the day
• Early stress shows up later as flower instability
• CO₂ is valuable only with proper humidity control
• Nighttime VPD matters as much as daytime VPD
• Strong airflow at night often hurts more than it helps
• Measuring light alone never explains fruit set problems

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Final Thoughts

Growing eggplants taught me that nighttime conditions matter as much as daytime ones.

The biggest improvements didn’t come from brighter lights or hotter days — they came from measuring PAR, CO₂, and VPD across the full 24-hour cycle, and learning when to let plants recover instead of pushing them constantly.

Once I started managing balance instead of intensity, eggplants became predictable, productive, and far more consistent — exactly what a long-cycle crop demands.