Growing Zucchini in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Flowers, Pollination, and Continuous Yield

Zucchini grows fast — sometimes too fast.

When I first planted zucchini in a greenhouse, I assumed it would behave like an aggressive summer crop: lots of light, warm air, strong airflow, and everything else would take care of itself. The plants exploded with growth. Leaves were massive. Stems were thick.

And then fruits failed.

Female flowers appeared, pollination looked normal, but young fruits aborted or stalled at a few centimeters. The plants never looked stressed. That made it frustrating.

The problem wasn’t nutrition or light alone. It was air balance, and I wasn’t measuring it carefully enough.

Here’s what zucchini taught me once I started tracking PAR, CO₂, and VPD together.

---

1. Seedling & Early Establishment

(Where zucchini decides how aggressively it will transpire later)

Zucchini seedlings establish quickly and build leaf area fast. Early on, I pushed light and airflow to harden transplants.

That created hidden imbalance.

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:
Early dryness doesn’t slow zucchini growth — it programs the plant for excessive transpiration later.

Gentle light and stable humidity produced plants that handled flowering better weeks later.

---

2. Vegetative Growth (Leaf Explosion Stage)

(Where zucchini looks unstoppable)

Once established, zucchini builds enormous leaves and transpires aggressively. This stage looks successful no matter what you do.

I pushed PAR, temperature, and airflow hard.

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:

• leaf size stayed large but manageable
• internodes shortened
• plants stayed more balanced during flowering

CO₂ enrichment helped, but only when VPD stayed controlled.

---

3. Flowering & Pollination

(Where zucchini exposes air imbalance immediately)

This is where zucchini punished my early mistakes.

I treated flowering like peppers: strong airflow to move pollen and prevent disease. Flowers looked perfect.

And fruits still aborted.

What finally stabilized pollination:

• PAR: 400–650 µmol·m⁻²·s⁻¹
• DLI: 16–24
• CO₂: 800–1000 ppm
• VPD: 0.6–1.0 kPa
• Temperature: 22–28 °C

Key realization:
Zucchini flowers need higher humidity than peppers during pollination.
If the air is too dry, pollen viability drops and female flowers abort — even when pollination appears to occur.

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

---

4. Fruit Set & Rapid Expansion

(Where water balance controls yield)

Once pollination succeeded, fruits expanded rapidly — if conditions stayed stable.

The range I now aim for:

• PAR: 400–700 µmol·m⁻²·s⁻¹
• DLI: 18–26
• CO₂: 800–1100 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 24–30 °C

What I noticed:

• High VPD caused hollow or tapered fruit
• Low VPD increased fruit diameter but slowed growth
• Sudden VPD spikes caused aborted fruit tips

Zucchini translates stress into fruit abortion, not leaf symptoms.

---

5. Continuous Harvest & Late-Stage Stability

(Where consistency replaces intensity)

Zucchini is harvested continuously, which means stress accumulates.

Late in the cycle, I reduced intensity and focused on environmental stability.

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:

• steadier fruit size
• fewer aborted flowers
• improved plant longevity
• more predictable daily harvest

Too much humidity increased disease pressure.
Too much dryness reduced female flower viability.

---

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

After several cycles, one pattern became obvious:

• Zucchini tolerates high PAR — but not dry air during flowering
• CO₂ boosts growth only when transpiration is controlled
• VPD quietly determines pollination success and fruit continuity

In practice:

• High PAR + high VPD → flower abortion, hollow fruit
• High CO₂ + unstable humidity → wasted yield
• Balanced PAR + enriched CO₂ + stable VPD → continuous harvest

---

Practical Lessons I Took Away

• Zucchini is more sensitive during flowering than it looks
• Early stress shows up later as fruit abortion
• CO₂ is valuable only with proper humidity control
• VPD management matters more than airflow strength
• Excess airflow often harms pollination
• Measuring light alone never explains fruit loss

---

Final Thoughts

Growing zucchini taught me that fast crops demand even tighter control.

The biggest improvements didn’t come from brighter lights or hotter air — they came from measuring PAR, CO₂, and VPD together, and learning when to slow down a plant that appears to thrive under intensity.

Once I focused on balance instead of force, zucchini became predictable, productive, and far more consistent — harvest after harvest.