Growing Tarragon in a Greenhouse:

What I Learned About PAR, CO₂, and VPD at Each Growth Stage

Tarragon looks like a simple herb, but growing it well in a greenhouse taught me a lesson I didn’t expect:
tarragon hates extremes.

I initially treated it like basil and rosemary — pushing light and drying the air — and the plants clearly pushed back. Growth slowed, leaves stiffened, and aroma weakened. Only after dialing things back and paying closer attention to PAR, CO₂, and especially VPD did tarragon start behaving like the elegant herb it’s known to be.

Here’s what I learned from actually growing it, stage by stage.

1. Establishment & Early Growth

(Where I made my first mistake)

When young tarragon plants first established, I assumed they would appreciate strong light. They didn’t.

What I saw instead:

slow root establishment

thin, stressed leaves

uneven growth across the tray

What finally worked:

PAR: 100–180 µmol·m⁻²·s⁻¹

DLI: ~5–8

CO₂: 400–600 ppm

VPD: 0.6–1.0 kPa

Temperature: 18–22 °C

Why this matters:
At this stage, tarragon is far more sensitive to water balance than light availability.
If VPD drifts too high, leaves lose moisture faster than the roots can support.

Once I softened the light and stabilized humidity, growth immediately became more uniform.

2. Early Leaf Expansion

(Where CO₂ starts to matter — quietly)

As leaf area increased, I noticed something subtle: growth speed didn’t improve just by increasing light, but it did respond when CO₂ rose slightly — as long as humidity stayed stable.

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 became broader, not longer

stems stayed flexible, not woody

aroma developed slowly but cleanly

Pushing PAR higher at this stage didn’t help — it actually stressed the plants.

3. Main Vegetative Growth

(Where balance matters more than power)

This is the phase where I expected tarragon to behave like other herbs and reward higher light levels. It didn’t.

When PAR went too high:

leaf texture hardened

aroma flattened

growth became uneven

The sweet spot I 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 is not a “push crop.”
It grows best when nothing is extreme — not light, not CO₂, not dryness.

Once PAR, CO₂, and VPD were aligned, growth became steady and predictable rather than fast-and-fragile.

4. Pre-Harvest & Aroma Preservation

(The difference between “green” and “fragrant”)

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

What gave the best results:

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:

aroma became more distinct and refined

leaves stayed tender

post-harvest quality improved noticeably

Too much humidity dulled aroma.
Too much dryness stiffened leaves.

How PAR, CO₂, and VPD Actually Interact for Tarragon

From repeated cycles, one thing became very clear to me:

High PAR does not equal better tarragon

CO₂ helps, but only within a narrow comfort zone

VPD is the silent limiter most people overlook

In practice:

High PAR + high VPD → stressed, flavorless leaves

High CO₂ + low PAR → wasted enrichment

Balanced PAR + moderate CO₂ + stable VPD → consistent, aromatic growth

Practical Lessons I Took Away

Tarragon prefers moderation, not intensity

It tolerates less light than basil and rosemary

It dislikes dry air more than thyme or oregano

CO₂ enrichment works, but only when everything else is calm

Measuring light alone gives an incomplete picture

VPD stability matters more than chasing ideal numbers

Final Thoughts

Growing tarragon taught me that “stronger” is not always “better” in a greenhouse.

The biggest improvement didn’t come from new equipment or higher settings — it came from listening to how the plant responded 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 elegant.