VPD - Summary
VPD describes the interplay between temperature and humidity. Anyone who knows the target range for each phase and measures correctly can set up their indoor climate more calmly, more stably, and more reproducibly.
Temperature and humidity are often considered separately in indoor setups. In practice, however, both values always work together. This is exactly where VPD comes in: it shows how strongly the air can absorb moisture – making climate relationships tangible.
The foundation of any VPD-based approach is clean measurement directly in the setup. What matters is not the room value, but the microclimate at plant height. You can find suitable devices at hygrometers & climate sensors.
What is VPD – simply explained
VPD stands for “Vapor Pressure Deficit”. Put simply, the value describes how “receptive” the air is to moisture. Warm air can hold more water vapour than cold air – which is why the VPD value changes even with small temperature fluctuations.
While relative humidity only provides a percentage value, VPD additionally takes temperature into account. This makes VPD better suited as a control variable when it comes to stable conditions.
Why VPD is so helpful in everyday indoor growing
Many typical climate problems are not caused by a single “wrong” value, but by an unfavourable combination of temperature and humidity. VPD helps to evaluate this combination and correct it in a targeted way.
A persistently very low VPD often goes hand in hand with heavy, humid air. A very high VPD tends to feel dry and can lead to significant fluctuations. The goal is not a theoretical ideal value, but a stable range that can be reliably maintained in everyday use.
Practical values by phase (no calculations needed)
The following ranges serve as guidelines. The important thing is to stay within a corridor and avoid abrupt jumps.
Early phase
Character: gentle climate, rather low VPD.
Practical tip: slightly higher humidity at moderate temperatures, without stagnant air.
Tip: Always place sensors at plant height, not in the direct airflow. Sensors with an external probe are particularly helpful here.
Vegetative phase
Character: mid-range VPD.
Practical tip: consistent temperatures and moderately managed humidity ensure smooth progressions.
Later development phase
Character: slightly higher VPD, stable and even.
Practical tip: tend to keep humidity reduced, avoid strong fluctuations – especially during day/night transitions.
Key takeaway: Stability is more important than hitting an exact number.
Controlling VPD – the key adjustment levers
Temperature
Temperature directly affects the VPD value. Even small changes can noticeably shift the climate. That is why temperature should always be considered in conjunction with humidity.
Humidity
If natural air exchange is not sufficient, additional equipment can help. In rooms with high baseline humidity, dehumidifiers are a sensible addition.
Air exchange (exhaust)
Exhaust air is the most important lever for humidity regulation. Modern axial and radial fans with integrated temperature or speed control automatically adjust their output to changing conditions – without the need for separate controllers.
Air circulation
Circulation does not directly change the VPD value, but ensures uniform conditions at the plant surface. Circulation fans help to avoid microclimate zones.
Setup check: measuring correctly
- Always measure at plant height
- Do not place the sensor directly in the airflow or too close to the lighting
- Use min/max values, not just snapshots
- Consider day and night values separately
Common issues & fixes
Humidity too high despite exhaust
The cause is often high room humidity or unfavourable ducting. Insufficient fresh air intake can also slow down air exchange.
Strongly fluctuating values
Excessive fan power or hard on/off cycles cause jumps. Fans with integrated, stepless speed control enable smoother progressions.
Readings look fine, but the climate feels wrong
Usually the sensor is incorrectly positioned. Relocating it and comparing trends often helps more than adding more equipment.
FAQ – VPD explained in brief
What is a good VPD value?
A good value is a stable range that suits the respective phase. Consistency is more important than perfection.
Do I need to calculate VPD?
No. Practical ranges are sufficient as long as measurements are taken correctly.
Why do values change so much at night?
Falling temperatures increase relative humidity. Day/night differences should be consciously planned for.
Is exhaust with integrated control sufficient?
In many setups, yes. Additional devices are usually only needed under extreme conditions.
Conclusion
VPD helps to better understand climate relationships. With reliable sensors and exhaust systems with integrated control, the target range can be easily and stably maintained in practice.