Growing Leeks in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Slow Crops, Stem Thickness, and Patience

Leeks are slow.
Not “a bit slower than green onions” slow — genuinely slow. When I first decided to grow leeks in a greenhouse, I underestimated just how different they are from scallions or chives. I assumed similar light levels and a longer timeline would be enough.

That assumption cost me quality.

Leeks don’t fail loudly. They stretch, thin out, and lose stem density quietly if conditions are even slightly off. It wasn’t until I started measuring PAR, CO₂, and VPD together that I understood why some batches looked tall but never developed the thick, solid shafts I wanted.

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

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1. Germination & Early Establishment

(Where leeks demand patience immediately)

Leek seeds germinate slowly and unevenly compared to most leafy crops. Because emergence took time, my instinct was to increase light early to encourage stronger seedlings.

That backfired.

What finally worked for me:

• PAR: 60–120 µmol·m⁻²·s⁻¹
• DLI: ~3–5
• CO₂: 400–600 ppm
• VPD: 0.3–0.6 kPa
• Temperature: 16–20 °C

What I noticed:
Young leeks have very limited root systems.
If VPD rises early, transpiration outpaces root uptake, leading to thin, weak stems that never fully recover.

Lower light and higher humidity produced sturdier seedlings with thicker bases.

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2. Seedling Development

(Where stem thickness is quietly decided)

Once true leaves appeared, leeks still grew slowly — and that’s where I made another mistake: treating slow growth as a problem to solve.

I increased PAR to “push” development. Height increased, but stem diameter didn’t.

The balance that worked best:

• PAR: 120–220 µmol·m⁻²·s⁻¹
• DLI: 5–8
• CO₂: 600–800 ppm
• VPD: 0.5–0.9 kPa
• Temperature: 16–22 °C

What changed:

• leaves stayed shorter and thicker
• stem bases widened gradually
• plants looked sturdier, not taller

Moderate CO₂ helped biomass accumulation — but only because humidity stayed stable.

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3. Vegetative Growth (Stem Building Stage)

(Where leeks finally start to look like leeks)

This is the longest stage and the most deceptive one. Leeks can look healthy here even when stem quality is already compromised.

I tried pushing PAR and temperature to accelerate growth. The plants grew faster — but stems stayed hollow and thin.

The range I now aim for:

• PAR: 250–400 µmol·m⁻²·s⁻¹
• DLI: 10–14
• CO₂: 800–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 18–24 °C

Key realization:
Leeks don’t respond to intensity with thickness — they respond with time and balance.

Once PAR, CO₂, and VPD were aligned, stem diameter increased slowly but consistently.

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4. Shaft Thickening & Pre-Harvest Control

(Where density and texture are finalized)

Before harvest, I stopped chasing size and focused on stem density and texture.

What worked best near harvest:

• PAR: 200–350 µmol·m⁻²·s⁻¹
• DLI: 8–12
• CO₂: 700–900 ppm
• VPD: 1.0–1.3 kPa
• Temperature: 14–20 °C

What I saw:

• firmer, denser shafts
• improved uniformity
• less internal hollowing
• better post-harvest handling

Too much humidity increased disease pressure.
Too much dryness reduced stem quality and caused premature leaf aging.

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

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

• Leeks reward consistency, not intensity
• PAR alone increases height, not shaft diameter
• CO₂ helps only when transpiration is controlled
• VPD quietly determines stem density and long-term quality

In practice:

• High PAR + high VPD → tall, thin leeks
• High CO₂ + unstable humidity → uneven growth
• Balanced PAR + moderate CO₂ + stable VPD → thick, dense shafts

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Practical Lessons I Took Away

• Leeks are far slower than green onions, and that’s normal
• Early stress shows up months later as thin stems
• CO₂ enrichment is useful, but only over long, stable periods
• VPD stability matters more than absolute humidity
• Pushing growth rarely improves shaft quality
• Measuring light alone never explains poor leek structure

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Final Thoughts

Growing leeks taught me that some crops simply cannot be rushed.

The biggest improvements didn’t come from stronger lights or warmer air — they came from measuring PAR, CO₂, and VPD together, and accepting that leeks build quality slowly, over time, under stable conditions.

Once I stopped forcing growth and started managing balance, leeks became predictable, dense, and consistently high quality — exactly what I was aiming for.