Growing Green Onions in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Speed, Texture, and Regrowth

Green onions (scallions) look simple. They grow fast, tolerate cutting, and seem much less fragile than lettuce. Because of that, I originally treated them as a “low-attention” crop.

That assumption didn’t last long.

Once I started growing green onions seriously in a greenhouse—and actually measuring PAR, CO₂, and VPD instead of relying on habit—I realized how sensitive they are to air dryness and light imbalance, especially if regrowth quality matters.

Here’s what I learned by adjusting conditions across multiple growth cycles.

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1. Germination & Early Establishment

(Where I learned not to rush them)

Green onion seeds germinate reliably, so I initially pushed light early to speed things up. The seedlings emerged quickly—but leaf tips dried sooner than expected, and growth became uneven.

What finally worked for me:

• PAR: 80–150 µmol·m⁻²·s⁻¹
• DLI: ~4–6
• CO₂: 400–600 ppm
• VPD: 0.4–0.8 kPa
• Temperature: 16–20 °C

What I noticed:
Green onion seedlings transpire more than they look like they should.
If VPD rises too early, moisture loss outpaces root uptake, even though the plants don’t immediately wilt.

Soft light and stable humidity produced uniform emergence and stronger early roots.

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2. Early Leaf Development

(Where growth speed becomes misleading)

Once true leaves appeared, green onions accelerated fast. That’s where it’s easy to make the next mistake: matching light intensity to growth speed.

I tried that. Growth didn’t improve—leaf texture just became thinner and less resilient.

The balance that worked best:

• PAR: 150–250 µmol·m⁻²·s⁻¹
• DLI: 6–10
• CO₂: 600–800 ppm
• VPD: 0.6–1.0 kPa
• Temperature: 16–22 °C

What changed:

• leaves thickened instead of stretching
• color became more consistent
• early splitting at the tips disappeared

Moderate CO₂ helped here, but only because VPD stayed stable.

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3. Rapid Vegetative Growth

(Where green onions reveal their real personality)

This is the main biomass stage—and the stage where green onions can quietly lose quality if conditions drift.

I experimented with higher PAR and warmer air to shorten the cycle. Yield increased slightly, but stems became hollow and leaves softened too much.

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:
Green onions don’t need to be pushed—they already grow fast.
What they need is controlled transpiration, not maximum light.

Once PAR, CO₂, and VPD were aligned, growth stayed fast and stem structure improved.

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4. Pre-Harvest & Regrowth Optimization

(Where green onions separate themselves from single-cut crops)

Green onions are regrowth crops. If conditions are wrong before harvest, regrowth quality drops immediately.

I used to leave conditions unchanged through harvest. Regrowth was inconsistent.

What worked better:

• PAR: 200–350 µmol·m⁻²·s⁻¹
• DLI: 8–12
• CO₂: 700–900 ppm
• VPD: 1.0–1.3 kPa
• Temperature: 16–20 °C

What improved:

• faster regrowth after cutting
• fewer yellow bases
• more uniform stem diameter

Reducing stress before harvest mattered more than pushing growth.

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How PAR, CO₂, and VPD Actually Work Together for Green Onions

After multiple cycles, one pattern became obvious to me:

• High PAR alone doesn’t improve green onion quality
• CO₂ is wasted if VPD causes partial stomatal closure
• VPD quietly controls stem thickness and regrowth speed

In practice:

• High PAR + high VPD → fast growth, weak stems
• High CO₂ + unstable humidity → uneven structure
• Balanced PAR + moderate CO₂ + stable VPD → repeatable quality

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Practical Lessons I Took Away

• Green onions tolerate light better than chives, but less than Swiss chard
• They react to dryness faster than most people expect
• Regrowth reveals environmental mistakes quickly
• CO₂ enrichment works best under cool, stable conditions
• Measuring light alone never explains texture issues
• VPD stability matters more than absolute humidity

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Final Thoughts

Growing green onions taught me that “easy crops” still demand precision.

The biggest improvements didn’t come from stronger lights or higher numbers—they came from measuring PAR, CO₂, and VPD together, and adjusting calmly instead of aggressively.

Once I stopped forcing speed and started managing balance, green onions became predictable, consistent, and far more forgiving across multiple harvests.