PAR, CO₂, and VPD Requirements for Greenhouse Peppermint at Different Growth Stages

Peppermint is a fast-growing perennial herb prized for its essential oil concentration, strong aroma, and robust growth. It prefers cooler temperatures than basil, moderate PAR, and a balanced humidity level that supports leaf oil production and prevents stress.

---

1. Rooting & Young Plant Establishment

Plant state:
Root initiation, small leaves, minimal CO₂ uptake.

Parameter	Range
PAR	100–200 µmol·m⁻²·s⁻¹
DLI	6–10
CO₂	400–600 ppm
VPD	0.5–0.9 kPa
Temp	18–22 °C

Why:
Young peppermint plants are moisture-dependent and prone to drying under high VPD or intense light.

---

2. Juvenile Leaf Development (early expansion)

Plant state:
Leaf surface increases; photosynthesis accelerates.

Parameter	Range
PAR	200–350 µmol·m⁻²·s⁻¹
DLI	10–14
CO₂	600–800 ppm
VPD	0.7–1.1 kPa
Temp	18–24 °C

Why:
Peppermint begins converting CO₂ into biomass effectively and oil biosynthesis begins in small amounts.

---

3. Vigorous Vegetative Growth (oil-rich stage)

Plant state:
Maximum leaf development and essential oil production.

Parameter	Range
PAR	300–600 µmol·m⁻²·s⁻¹
DLI	12–18
CO₂	800–1200 ppm
VPD	0.9–1.3 kPa
Temp	18–22 °C

Why:
This is the stage where peppermint benefits most from increased CO₂.
However — peppermint prefers cooler air than basil.
Higher VPD keeps stomata open for CO₂ intake, while PAR drives oil concentration.

This stage influences:

leaf size

menthol oil concentration

aroma strength

---

4. Pre-Harvest Quality Stage

Plant state:
Final chemical maturation; oil concentration peaks.

Parameter	Range
PAR	250–450 µmol·m⁻²·s⁻¹
DLI	10–14
CO₂	700–1000 ppm
VPD	1.0–1.4 kPa
Temp	16–20 °C

Why:
Cooler temperatures significantly increase menthol and essential oil quality.
Moderately high VPD helps prevent fungal stress while promoting gas exchange.

---

Interaction of PAR, CO₂, and VPD for Peppermint

High PAR + High CO₂ + Cool Temperature
Best oil content, maximum aroma

High PAR + High VPD + Low Temp
Firm leaves, crisp texture, strong mint scent

High PAR + Low CO₂
good color but limited biomass

High CO₂ + Low PAR
lowered productivity due to insufficient light

VPD too low (humid)
disease risk, lower oil concentration

VPD too high (dry)
leaf edge drying & aroma loss

---

Practical Grower Advice

Peppermint prefers cooler climate than basil
CO₂ enrichment works best during vigorous vegetative growth
Avoid excessive humidity — peppermint is mold sensitive
High light is beneficial but not as high as basil
Strong airflow is beneficial — peppermint is tolerant to airflow
Cooling at the final stage improves aromatic quality

---

Final Takeaway

Peppermint thrives under:

gentle light & high humidity at rooting

increasing light & moderate CO₂ in leaf development

stronger PAR + high CO₂ during vegetative acceleration

cooler temperatures & stable VPD before harvest

By monitoring PAR, CO₂, VPD, temperature, and humidity using tools such as the AH-200, growers can maximize peppermint’s growth speed, leaf integrity, and essential oil density — producing a stronger, fresher, more aromatic crop.