Growing Bok Choy in a Greenhouse:

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

Bok choy is often described as an easy, fast leafy green. It grows quickly, looks sturdy, and rarely collapses when conditions drift. Because of that, I initially treated bok choy as a “low-risk” crop and focused mainly on light and harvest timing.

That approach worked—until I paid attention to quality.

Once I started measuring PAR, CO₂, and VPD together, I realized bok choy reacts very quickly to environmental imbalance. It doesn’t fail loudly. Instead, it quietly loses texture, develops hollow petioles, or regrows unevenly after cutting.

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

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1. Germination & Early Establishment

(Where bok choy looks tough but behaves delicately)

Bok choy seeds germinate fast, so my instinct was to give them moderate light right away. Emergence was quick—but leaf edges dried slightly, and early growth wasn’t uniform.

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: 16–22 °C

What I noticed:
Bok choy seedlings lose water faster than they appear to.
If VPD rises too early, dehydration shows up later as uneven leaf size and weaker stems.

Soft light and stable humidity produced much more uniform starts.

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2. Early Leaf Development

(Where growth speed can hide stress)

Once true leaves appeared, bok choy accelerated quickly. This is where it’s easy to make a mistake: matching light intensity to growth speed.

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

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: 16–24 °C

What changed:

• leaves thickened instead of stretching
• petioles stayed firm
• overall structure improved

Moderate CO₂ helped, but only because humidity stayed stable.

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3. Rapid Vegetative Growth

(Where bok choy quietly reveals its limits)

This is the main biomass stage. Bok choy grows so fast here that it looks like it can handle anything.

It can’t.

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: 250–450 µmol·m⁻²·s⁻¹
• DLI: 10–16
• CO₂: 800–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 18–26 °C

Key realization:
Bok choy doesn’t need to be pushed—it already grows fast.
What it needs is controlled transpiration, not maximum light.

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

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4. Pre-Harvest Quality Control

(Where texture and shelf life are decided)

Before harvest, I stopped chasing size and focused on leaf firmness and post-harvest performance.

Small adjustments here made a noticeable difference.

What worked best near harvest:

• PAR: 200–350 µmol·m⁻²·s⁻¹
• DLI: 8–14
• CO₂: 700–900 ppm
• VPD: 1.0–1.4 kPa
• Temperature: 14–20 °C

What I saw:

• firmer leaves
• stronger petioles
• improved shelf life
• reduced disease pressure

Higher VPD near harvest helped reduce excess moisture and leaf softness, as long as it wasn’t pushed too far.

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How PAR, CO₂, and VPD Actually Work Together for Bok Choy

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

• Bok choy grows fast, but quality is fragile
• PAR alone never explains success or failure
• CO₂ is wasted if stomata close
• VPD quietly controls everything

In practice:

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

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Practical Lessons I Took Away

• Bok choy tolerates more light than lettuce, less than Swiss chard
• It reacts to air dryness faster than most people expect
• Hollow stems appear before visible stress
• CO₂ enrichment works best under cool, controlled conditions
• VPD stability matters more than absolute humidity
• Measuring light alone never explains texture problems

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Final Thoughts

Growing bok choy taught me that speed hides mistakes.

The biggest improvements didn’t come from stronger lights or faster cycles—they came from measuring PAR, CO₂, and VPD together, and resisting the urge to push a crop that already grows aggressively.

Once I focused on balance instead of speed, bok choy became predictable, consistent, and far higher quality—cycle after cycle.