Growing Escarole in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Leaf Thickness, Mild Bitterness, and Balance

Escarole fooled me at first.

Compared with endive, escarole looks forgiving: broader leaves, faster growth, and a reputation for being less bitter. When I first grew escarole in a greenhouse, I assumed it would tolerate stronger light and drier air — closer to romaine than to endive.

It tolerated those conditions.
But quality quietly declined.

Leaves thickened too quickly, bitterness became uneven, and inner leaves lost tenderness. Nothing looked “wrong” at first glance, which made it even harder to diagnose. The issue wasn’t nutrients or variety — it was environmental balance, and I wasn’t measuring it carefully enough.

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

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1. Germination & Early Establishment

(Where escarole quietly decides how tough it will become)

Escarole germinates evenly and establishes faster than endive. Because seedlings looked sturdy early on, I pushed light sooner than I should have.

That decision showed up weeks later.

What finally worked for me:

• PAR: 90–160 µmol·m⁻²·s⁻¹
• DLI: ~4–6
• CO₂: 400–600 ppm
• VPD: 0.4–0.7 kPa
• Temperature: 14–18 °C

What I noticed:
Escarole seedlings respond to early dryness by thickening leaf tissue, not by wilting.
That early structural response later limits tenderness and increases background bitterness.

Gentle light and stable humidity produced softer outer leaves and a more flexible plant structure.

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2. Early Leaf Expansion

(Where bitterness level and leaf texture are quietly set)

As true leaves developed, escarole expanded outward rapidly. This stage felt productive — until I compared flavor and texture between batches.

When I increased PAR to speed growth, leaves grew larger but thicker, and bitterness became more pronounced.

The balance that worked best:

• PAR: 160–260 µmol·m⁻²·s⁻¹
• DLI: 6–10
• CO₂: 600–800 ppm
• VPD: 0.6–1.0 kPa
• Temperature: 14–20 °C

What changed:

• leaves stayed thinner and more pliable
• bitterness remained mild and consistent
• canopy developed more evenly

Moderate CO₂ helped expansion only when humidity stayed stable.

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3. Main Vegetative Growth

(Where escarole looks strong but quietly records stress)

This is the stage where escarole appears very forgiving. Plants look large, upright, and healthy even when conditions drift — which makes this stage deceptive.

I treated escarole like a high-light leafy green and pushed PAR and airflow. Biomass increased, but leaf quality declined.

The range I now aim for:

• PAR: 260–420 µmol·m⁻²·s⁻¹
• DLI: 10–16
• CO₂: 700–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 16–22 °C

Key realization:
Escarole converts excess stress into leaf thickness and uneven bitterness, not into better yield.
High PAR or high VPD hardens leaves long before visible stress appears.

Once PAR, CO₂, and VPD were aligned, growth slowed slightly — but texture and flavor improved dramatically.

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4. Inner Leaf Development & Blanching Phase

(Where escarole separates itself from other lettuces)

As escarole matured, inner leaves began forming the heart. This stage exposed every earlier imbalance.

If the air was too dry, inner leaves hardened and bitterness intensified.
If light was too strong, the heart stayed loose and uneven.

What finally stabilized inner leaf quality:

• PAR: 220–360 µmol·m⁻²·s⁻¹
• DLI: 8–14
• CO₂: 600–900 ppm
• VPD: 0.7–1.0 kPa
• Temperature: 12–18 °C

What I noticed:
Escarole prefers slightly lower VPD during inner leaf development than during outer leaf growth.
Cooler, more humid air allowed inner leaves to stay paler, softer, and less bitter.

This was the stage where precise VPD control mattered more than light.

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5. Pre-Harvest Quality Control

(Where texture, bitterness, and shelf life are finalized)

Before harvest, I stopped pushing growth entirely and focused on stability and disease prevention.

What worked best near harvest:

• PAR: 200–320 µmol·m⁻²·s⁻¹
• DLI: 7–12
• CO₂: 600–800 ppm
• VPD: 1.0–1.3 kPa
• Temperature: 10–16 °C

What I saw:

• cleaner cuts
• improved shelf life
• less post-harvest bitterness increase
• more consistent head quality

Too much humidity increased disease pressure.
Too much dryness caused rapid quality loss.

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

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

• Escarole quality is controlled by stress history, not final size
• PAR alone increases leaf thickness, not eating quality
• CO₂ helps only when stomata stay open
• VPD quietly determines whether bitterness stays mild or becomes uneven

In practice:

• High PAR + high VPD → thick leaves, uneven bitterness
• High CO₂ + unstable humidity → inconsistent inner leaf quality
• Balanced PAR + moderate CO₂ + stable VPD → tender, predictable heads

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Practical Lessons I Took Away

• Escarole is more tolerant than endive, but not careless
• Leaf thickness increases before visible stress appears
• CO₂ enrichment works best at moderate PAR
• VPD stability matters more than airflow strength
• Strong airflow often worsens texture
• Measuring light alone never explains bitterness variability

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Final Thoughts

Growing escarole taught me that “less bitter” doesn’t mean “less sensitive.”

The biggest improvements didn’t come from stronger lights or faster cycles — they came from measuring PAR, CO₂, and VPD together, and learning how small shifts in air balance quietly shape texture and flavor.

Once I stopped forcing growth and started managing balance, escarole became predictable, tender, and consistently high quality — harvest after harvest.