Vapor Pressure Deficit (VPD) is a measurement of the drying power of air and is widely used in greenhouses to optimize plant transpiration, nutrient uptake, and growth. Unlike relative humidity (RH), VPD considers both air temperature and humidity, making it a more accurate indicator of plant environmental conditions.

1. Measure Temperature and Relative Humidity

To calculate VPD, you need two measurements:

Air temperature (°C or °F)

Relative humidity (%RH)

Use a reliable environmental sensor placed near the plant canopy.

Recommended sensor locations:

At plant height

Away from direct airflow from fans

Away from heating/cooling equipment

In representative growing areas

Avoid placing sensors:

Near greenhouse walls

Near irrigation systems

Directly under ventilation outlets

2. Calculate VPD Using Temperature and RH

The basic formula is:

VPD = Saturation Vapor Pressure (SVP) × (1 − RH/100)

Where:

SVP = maximum amount of water vapor air can hold at a given temperature

RH = relative humidity

Where:

T = air temperature in °C

SVP = kPa

Example:

Greenhouse conditions:

Temperature: 26°C

Relative humidity: 60%

At 26°C:

SVP ≈ 3.36 kPa

VPD:

3.36 × (1 − 0.60)= 1.34 kPa

This means the air has a moderate drying capacity suitable for many vegetative growth stages.

3. Use a VPD Meter or Environmental Controller

Instead of manual calculation, growers commonly use VPD sensors to measure Temperature, RH, and VPD automatically. Use smart greenhouse controllers can display real-time VPD, control dehumidifiers, adjust ventilation, control heating and cooling, and maintain target VPD ranges.

4. Recommended Greenhouse VPD Ranges

VPD targets depend on plant growth stage:

Growth Stage	Temperature	Target VPD
Seedlings / clones	22–26°C	0.4–0.8 kPa
Vegetative growth	24–28°C	0.8–1.2 kPa
Flowering / fruiting	24–28°C	1.0–1.5 kPa
Late flowering	20–26°C	1.2–1.6 kPa

For many greenhouse crops, a VPD around 0.8–1.2 kPa is a common target during active growth.

5. How Dehumidifiers Help Maintain Greenhouse VPD

A greenhouse often has high humidity because plants release water through transpiration. When RH becomes too high:

VPD decreases

Transpiration slows

Nutrient uptake declines

Mold and fungal risks increase

A greenhouse dehumidifier helps by:

Removing excess moisture

Increasing VPD to the optimal range

Reducing condensation on leaves

Improving disease prevention

Refrigerant dehumidifiers are commonly used when:

Temperature is above 18°C

RH is high

Large moisture removal is required

Desiccant dehumidifiers are preferred when:

Very low humidity is required

Greenhouse temperatures are low

VPD must be maintained during cold periods

6. Practical VPD Monitoring Setup

A good greenhouse monitoring system includes:

Canopy-level temperature/RH sensor

VPD calculation software or controller

Dehumidifier with humidity control

Air circulation fans

Data logging system

For commercial greenhouses, multiple sensors are recommended because humidity can vary significantly between:

Upper canopy

Lower canopy

Near irrigation zones

Different greenhouse sections

Summary

To measure greenhouse VPD:

Measure temperature + relative humidity at canopy height

Calculate VPD using a formula or use a VPD sensor

Compare the value with the crop’s ideal range

Adjust humidity using ventilation, heating, or dehumidification

Maintaining the correct VPD helps plants achieve better transpiration, nutrient absorption, growth rate, and disease resistance.