Growing Scallions in a Greenhouse:

What I Learned About PAR, CO₂, and VPD Through Real Growth Cycles

Scallions are often treated as a low-effort crop. They grow quickly, tolerate cutting, and rarely collapse dramatically when something is off. Because of that, I used to think they didn’t need much environmental attention.

That assumption turned out to be wrong.

Once I started growing scallions in a greenhouse while actively tracking PAR, CO₂, and VPD, I realized they respond very clearly to imbalance — not by failing outright, but by quietly losing quality: thinner stems, uneven regrowth, and softer tissue than expected.

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

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1. Germination & Early Establishment

(Where scallions taught me not to rush light)

Scallion seeds germinate reliably, so my instinct was to push light early to speed up establishment. Germination was fast — but leaf tips dried slightly, and early growth wasn’t as uniform as I wanted.

What eventually worked best for me:

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

What I noticed:
Young scallions transpire more than they appear to.
If VPD climbs too quickly at this stage, moisture loss outpaces root uptake — even though the plants don’t visibly wilt.

Once I softened the light and stabilized humidity, emergence became more even and early root development improved.

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2. Early Leaf Development

(Where fast growth can hide early stress)

As soon as true leaves appeared, scallions accelerated rapidly. That’s where it became tempting to increase PAR to “match” the growth speed.

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

The balance that gave the best structure:

• 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 uniform
• early tip splitting disappeared

Moderate CO₂ enrichment helped, but only because VPD stayed within range.

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3. Rapid Vegetative Growth

(Where scallions show their real limits)

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

I experimented with higher PAR and warmer air to shorten the cycle. Yield increased slightly, but stem structure suffered: hollower shafts and softer tissue.

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:
Scallions 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 firmness improved noticeably.

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4. Pre-Harvest & Regrowth Optimization

(Where scallions reveal whether conditions were right)

Scallions are regrowth crops. Any environmental stress before harvest shows up immediately in the next cut.

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

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, more uniform regrowth
• fewer yellow bases
• more consistent stem diameter

Reducing stress before harvest mattered more than pushing growth speed.

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

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

• High PAR alone doesn’t improve scallion 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

• Scallions tolerate light better than chives, but less than Swiss chard
• They react to air dryness faster than most people expect
• Regrowth cycles expose environmental mistakes quickly
• CO₂ enrichment works best under cool, stable conditions
• Measuring light alone never explains texture problems
• VPD stability matters more than chasing ideal humidity numbers

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Final Thoughts

Growing scallions reminded me that “easy crops” still demand precision.

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

Once I stopped forcing speed and started managing balance, scallions became predictable, consistent, and far higher quality across repeated harvests.