VPD (vapor pressure deficit) is one of the key climate parameters for controlled and reproducible plant development in indoor cultivation.
While temperature and humidity are often considered in isolation, the VPD value describes their functional effect on the plant:
The value shows how much water the ambient air can absorb from the leaf surface. This is how transpiration, nutrient transport, and cooling of the plant take place. The value directly shows how much water the air absorbs from the leaves. And thus how actively water and nutrients are transported through the plant.
In contrast to individual values, the VPD provides an interpretable climate signal. That is why professional cultivation companies and associations use this value as a control variable to coordinate climate, light, irrigation, and CO2.
Both too high and too low VPD causes the transpiration rate to drop. This disrupts the plant's gas exchange and impairs photosynthetic performance. The ambient air must not be too dry or too humid in order to keep the plant within the respective VPD target range. The goal is to achieve controlled VPD ranges for each growth phase of cultivation (mother plant, clones, vegetative, generative). Within these sweet spots, stomatal conductance is high, water and nutrient flow is stable, and CO2 uptake is optimal.
The GrowControl VPD Master Guide explains the physical principles, relationships, and chains of effects of this value in the system. It also provides practical use cases and visual examples of the processes.
Unfortunately, the document is currently only available in German.
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