Growing Tarragon in a Greenhouse:

Growing Tarragon in a Greenhouse:

What I Learned About PAR, CO₂, and VPD Through Trial and Error

Tarragon taught me a lesson that many other herbs didn’t:
you can’t force it.

When I first grew tarragon in a greenhouse, I treated it like basil or rosemary. I increased light, dried the air slightly, and expected faster growth. Instead, the plants slowed down. Leaves became stiff, aroma weakened, and regrowth after cutting was disappointing.

That’s when I realized tarragon isn’t a “high-performance” herb. It’s a balance-driven plant, and it reacts quickly when conditions drift too far in any direction.

Here’s what I learned by adjusting PAR, CO₂, and VPD step by step, and watching how the plants actually responded.

1. Establishment & Early Growth

(Where I learned to stop pushing)

During early establishment, my instinct was to provide “enough light to get things moving.” That turned out to be a mistake.

With higher PAR and slightly dry air, the plants didn’t die — they just stalled. Leaf tips dried slightly, and roots took longer to establish.

What finally worked for me:

  • PAR: 100–180 µmol·m⁻²·s⁻¹
  • DLI: around 5–8
  • CO₂: 400–600 ppm
  • VPD: 0.6–1.0 kPa
  • Temperature: 18–22 °C

What I noticed:
At this stage, tarragon doesn’t care about high light. It cares about not losing water faster than its roots can supply it. Once I softened the light and stabilized humidity, growth became even and predictable.

2. Early Leaf Expansion

(Where CO₂ quietly starts helping)

As leaf area increased, I tried increasing PAR again. Growth didn’t improve much. But when I slightly enriched CO₂ — without changing light aggressively — things changed.

Conditions that gave consistent results:

  • PAR: 180–300 µmol·m⁻²·s⁻¹
  • DLI: 8–12
  • CO₂: 600–800 ppm
  • VPD: 0.8–1.2 kPa
  • Temperature: 18–24 °C

What I observed:
Leaves grew wider instead of longer. Texture stayed soft. Aroma began developing slowly but cleanly. This was the first time the plants looked “comfortable” rather than just surviving.

3. Main Vegetative Growth

(Where balance matters more than intensity)

This stage surprised me the most.

I expected tarragon to respond positively to higher PAR, like most herbs. Instead, when PAR went too high, leaf quality dropped. The plants didn’t burn — they just lost elegance.

The range I eventually settled on:

  • PAR: 250–400 µmol·m⁻²·s⁻¹
  • DLI: 10–14
  • CO₂: 800–1000 ppm
  • VPD: 1.0–1.4 kPa
  • Temperature: 20–26 °C

Key realization:
Tarragon doesn’t reward extremes.
It grows best when PAR, CO₂, and VPD all stay in a moderate, calm zone.

Once everything was aligned, growth wasn’t explosive — but it was steady, uniform, and repeatable.

4. Pre-Harvest & Aroma Preservation

(The difference between green leaves and real flavor)

Before harvest, I stopped thinking about growth speed and focused entirely on leaf quality.

What worked best near harvest:

  • PAR: 200–350 µmol·m⁻²·s⁻¹
  • DLI: 8–12
  • CO₂: 700–900 ppm
  • VPD: 1.2–1.6 kPa
  • Temperature: 18–22 °C

What changed:
Leaves stayed tender. Aroma became more refined instead of sharp. Post-harvest quality improved noticeably, especially when regrowth cycles followed.

Too much humidity dulled flavor.
Too much dryness hardened leaves.

How PAR, CO₂, and VPD Actually Interact for Tarragon (In Practice)

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

  • High PAR doesn’t mean better tarragon
  • CO₂ helps, but only within a narrow comfort zone
  • VPD is the silent limiter most people ignore

In real terms:

  • High PAR + high VPD → stressed, flavorless leaves
  • High CO₂ + low PAR → wasted enrichment
  • Balanced PAR + moderate CO₂ + stable VPD → consistent quality

Tarragon responds to stability, not aggression.

Practical Lessons I Took Away

  • Tarragon prefers moderation over intensity
  • It tolerates less light than basil and rosemary
  • It dislikes dry air more than thyme or oregano
  • CO₂ enrichment works only when humidity and light are balanced
  • Measuring light alone is not enough
  • VPD stability matters more than chasing “ideal” numbers

Final Thoughts

Growing tarragon changed how I think about greenhouse control.

The biggest improvement didn’t come from stronger lights or higher settings — it came from paying attention to how the plant reacted, and adjusting PAR, CO₂, and VPD together instead of in isolation.

Once I stopped forcing growth and started managing balance, tarragon finally grew the way it’s meant to:
slow, aromatic, and precise.

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