VPD - Vapour Pressure Deficit
For plants to move nutrients and minerals effectively around their branches and leaves, understanding VPD (Vapour Pressure Deficit) is essential. This principle is the holy grail of growing not just chilli plants but all plants.
What is VPD?
VPD is the result of temperature and humidity combining to create a pressure gradient. Think of visiting a tropical country with high humidity — you quickly start sweating as your body struggles to cool itself. Plants operate similarly.
When heat and humidity are in balance, VPD sits at an optimal rating of 0.95–1.1, providing the best conditions for growth.
How VPD Affects Your Chilli Plants
High VPD (1.2–1.5):
Occurs when temperatures are high, but humidity is low.
The pressure forces oxygen and water molecules to be expelled too quickly, preventing proper deposition.
Symptoms: Curling leaves and stunted growth.
Low VPD (0.4–0.6):
Occurs when both temperature and humidity are low.
The plant struggles to push oxygen and nutrients up through its veins.
Symptoms: Yellowing leaves and stunted growth.
In both scenarios, the chilli plant is considered stressed.
Managing VPD for Healthy Plants
To monitor and maintain optimal VPD:
Invest in a Temperature and Humidity Sensor — affordable and widely available.
Many sensors come with free apps that calculate VPD automatically.
In our grow room, we use 10 sensors to map the room’s temperature and humidity levels, ensuring plants remain in ideal conditions.
Optimal VPD at Different Growth Stages
Early Growth (Seedlings in a Propagator):
Temperature: 21°C
Humidity: 80%
This creates a low VPD ideal for rapid early growth while the first true leaves develop.
Post-Seedling Stage:
Once the first true leaves appear, aim for a VPD rating of 1.0 to promote steady, consistent growth.
Summer Growing Conditions:
In the height of summer (UK/North EU), temperature and humidity naturally diverge, raising VPD.
A higher VPD at this stage is ideal for mature plants as they focus on podding and ripening chillies.
Stress and VPD
Most growing problems are caused by a combination of:
Watering issues.
Nutrient imbalances.
Poor VPD management.
Using a temperature and humidity sensor will help you identify and address stress conditions early, ensuring strong, healthy, and rapid growth.
VPD Reference Chart
As shown in the chart below, a temperature of 25°C with 65% humidity gives a VPD rating of 1.0 — the sweet spot for consistent growth. Adjust temperature and humidity levels to maintain optimal VPD for each growth stage.