Growing Mustard Greens in a Greenhouse:

What I Learned About PAR, CO₂, and VPD by Actually Growing Them

I used to think mustard greens were almost “foolproof.” They grow fast, they tolerate cold, and they don’t look demanding on paper. But once I started growing them seriously in a greenhouse — and measuring PAR, CO₂, and VPD instead of guessing — I realized how quickly mustard greens react when conditions drift even slightly out of balance.

They don’t just grow fast.
They respond fast — both positively and negatively.

Here’s what I learned through real growing cycles, mistakes, and adjustments.

---

1. Germination & Early Establishment

(Where I learned that mustard greens dry out faster than they look)

At first, I treated mustard greens like lettuce seedlings. Same light, same humidity. That worked — but not well.

Leaves emerged quickly, but tips dried sooner than expected, especially when airflow increased.

What finally worked consistently:

• 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:
Mustard greens have thin, fast-transpiring leaves even at the seedling stage.
If VPD rises too quickly, they lose water before roots can keep up.

Soft light and stable humidity made emergence uniform and reduced early stress.

---

2. Early Leaf Development (Baby Leaf Stage)

(Where growth speed starts to explode — if conditions are right)

Once true leaves formed, mustard greens started growing noticeably faster than lettuce under the same conditions. That’s where I made my next mistake: pushing light too hard.

Growth didn’t improve — leaves just got tougher.

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: 14–20 °C

What changed:

• Leaves stayed tender
• Growth was fast but controlled
• Color deepened without bitterness

Moderate CO₂ helped here — but only when humidity stayed stable.

---

3. Rapid Vegetative Growth

(The stage where mustard greens can easily get ruined)

This is where mustard greens show their true personality.

They grow incredibly fast — but if PAR, temperature, or VPD go too high, bitterness increases and leaves toughen almost overnight.

I learned this the hard way.

The range I now aim for:

• PAR: 200–350 µmol·m⁻²·s⁻¹
• DLI: 10–14
• CO₂: 800–1000 ppm
• VPD: 0.8–1.2 kPa
• Temperature: 14–18 °C

Key realization:
Mustard greens don’t want to be pushed.
They want steady conditions, not intensity.

Once PAR, CO₂, and VPD were aligned, growth stayed fast and leaves remained soft.

---

4. Pre-Harvest Quality Control

(Where flavor is decided, not yield)

Before harvest, I stopped chasing growth rate and focused on leaf quality.

Small changes here made a big difference.

What preserved the best texture and flavor:

• PAR: 180–300 µmol·m⁻²·s⁻¹
• DLI: 8–12
• CO₂: 700–900 ppm
• VPD: 1.0–1.3 kPa
• Temperature: 12–18 °C

What I saw immediately:

• Less bitterness
• Softer leaf edges
• Better post-harvest holding

Too much dryness increased sharpness.
Too much humidity reduced shelf life.

---

How PAR, CO₂, and VPD Actually Work Together for Mustard Greens

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

• Mustard greens are fast, but not forgiving
• PAR alone does not control growth quality
• CO₂ helps only if stomata remain open
• VPD quietly decides whether leaves stay tender or turn harsh

In practice:

• High PAR + high VPD → fast growth, poor flavor
• High CO₂ + unstable humidity → uneven texture
• Balanced PAR + moderate CO₂ + stable VPD → consistent quality

---

Practical Lessons I Took Away

• Mustard greens are more sensitive than they look
• They tolerate light better than arugula, but worse than Swiss chard
• Flavor changes faster than yield under stress
• VPD stability matters more than absolute humidity
• CO₂ enrichment works best in cool conditions
• Measuring only light misses half the problem

---

Final Thoughts

Growing mustard greens taught me that speed is not the goal — control is.

Once I stopped pushing light and started watching how PAR, CO₂, and VPD interacted in real time, the crop became predictable, repeatable, and higher quality.

Mustard greens don’t need extreme settings.
They need balance, consistency, and restraint.

And once you give them that, they reward you very quickly.