VPD explained
Vapour pressure deficit (VPD) describes how strongly the air pulls moisture out of a plant. It combines temperature and humidity into one number that is often more useful than either alone.
VPD is the gap between how much water vapour the air could hold at its current temperature and how much it actually holds. It is measured in kilopascals (kPa).
- Low VPD: the air is nearly saturated. Plants transpire slowly and leaves stay wet for longer, which favours disease.
- High VPD: the air is dry and pulls water out quickly. Plants may close their stomata to save water, which slows growth.
Relative humidity depends on temperature: 70% humidity at 15 °C is a very different condition for a plant from 70% at 30 °C. VPD takes temperature into account, so a single target keeps its meaning throughout the day.
CropWatch calculates VPD from the temperature and humidity readings of an air sensor.
Many greenhouse growers aim for roughly 0.8 to 1.2 kPa, but the right range depends on your crop and its growth stage. Follow the guidance for your own crop.