Growing Radish in a Greenhouse:

What PAR, CO₂, and VPD Taught Me About Speed, Roots, and Quality

Radish is one of those crops that looks incredibly simple.
Short cycle. Fast growth. Strong flavor. Because of that, I originally treated radish as a “no-problem” crop and focused mostly on timing.

That was a mistake.

Once I started growing radish in a greenhouse and actually tracking PAR, CO₂, and VPD, I realized how quickly radish reacts to environmental imbalance — not just above ground, but below the surface, where the real harvest is forming.

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

---

1. Germination & Early Establishment

(Where radish shows its sensitivity to dryness)

Radish seeds germinate quickly, so my instinct was to give them moderate light right away. Germination was fast — but early leaf edges dried, and root swelling was uneven later on.

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:
Radish seedlings transpire more than they look like they should.
If VPD rises too early, water stress doesn’t always show in the leaves — it shows later as poor root formation.

Soft light and stable humidity gave me much more uniform roots at harvest.

---

2. Early Leaf Development

(Where leaf growth sets up the root)

Once true leaves appeared, radish growth accelerated rapidly. This is where I learned that leaf growth and root quality are tightly linked.

I tried pushing PAR early to “build more energy.” Leaves grew fast — but roots became elongated instead of round.

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 grew steadily, not explosively
• root swelling started earlier
• fewer misshapen radishes

Moderate CO₂ helped, but only when humidity stayed stable.

---

3. Rapid Vegetative & Root Bulking Stage

(Where radish quality is decided)

This is the most critical stage for radish. The plant may still look healthy even when conditions are wrong — but the root quality tells the real story.

I experimented with higher PAR and warmer air to shorten the cycle. The plants grew faster, but the radishes became hollow and overly pungent.

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:
Radish doesn’t want speed — it wants even growth.
High PAR or high VPD shifts energy into leaves and stress responses instead of root bulking.

Once PAR, CO₂, and VPD were aligned, roots became rounder, denser, and more consistent.

---

4. Pre-Harvest Quality Control

(Where texture and flavor are locked in)

Before harvest, I stopped pushing growth completely. I focused on maintaining stability, not increasing size.

What worked best near harvest:

• PAR: 180–300 µmol·m⁻²·s⁻¹
• DLI: 8–12
• CO₂: 700–900 ppm
• VPD: 1.0–1.3 kPa
• Temperature: 12–16 °C

What I saw:

• crisper texture
• less bitterness
• fewer hollow centers
• better shelf life

Too much dryness increased pungency.
Too much humidity reduced firmness.

---

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

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

• Radish quality is decided below ground, but controlled above ground
• High PAR alone does not produce better roots
• CO₂ helps only when transpiration is balanced
• VPD quietly controls whether roots swell evenly or stress

In practice:

• High PAR + high VPD → fast tops, poor roots
• High CO₂ + unstable humidity → uneven bulking
• Balanced PAR + moderate CO₂ + stable VPD → uniform, high-quality radish

---

Practical Lessons I Took Away

• Radish is faster than lettuce, but more sensitive than it looks
• Root quality suffers before leaf quality shows stress
• CO₂ enrichment works best under cool conditions
• VPD stability matters more than absolute humidity
• Measuring only light explains leaf growth, not root success
• Radish exposes environmental mistakes very quickly

---

Final Thoughts

Growing radish in a greenhouse taught me that speed hides problems.

The biggest improvements didn’t come from stronger lights or higher temperatures — they came from measuring PAR, CO₂, and VPD together, and slowing the system down just enough to let roots develop evenly.

Once I stopped forcing growth and started managing balance, radish became predictable, consistent, and much higher quality — cycle after cycle.