Demystifying the Climate: What is VPD?

- How to grow indoors and control the vpd climate - what is vpd (vapor pressure
deficit)
You have the best lights and the best nutrients, but your plants are still growing slowly. The culprit is likely VPD (Vapor Pressure Deficit). It sounds like complex physics, but it is
actually the master key to indoor growing.
What is VPD?
VPD is not just humidity. It is the specific relationship between Temperature and Humidity.
In simple terms, VPD measures the drying power of the air. It tells us how strongly the
atmosphere is pulling water out of your plants.

Low VPD (Air is wet): The air is saturated. The plant cannot evaporate water
from its leaves (transpiration). If the plant can´t sweat, it can´t pull water and
nutrients up from the roots. Growth stops.
High VPD (Air is dry): The air is thirsty. It sucks water out of the plant faster
than the roots can drink. The plant closes its pores (stomata) to save water,
which means it stops absorbing CO2. Growth stops.

The Goldilocks Zone
You want a VPD where the plant transpires freely but isn´t stressed. This is measured in
Kilopascals (kPa).
- Clones/Seedlings: 0.4 – 0.8 kPa (High humidity, warm air)
- Vegetative Stage: 0.8 – 1.1 kPa (Moderate humidity)
- Flowering Stage: 1.2 – 1.5 kPa (Lower humidity to prevent mold)
How to Control VPD Indoors
To hit these targets, you cannot just look at a humidity meter. You need to adjust based on
temperature.
1. Use a VPD Chart: Print one out and stick it on your grow tent.
2. The Tools: You will likely need an exhaust fan to remove humidity and a
humidifier to add it.
3. Automation: The best way to manage this is with a smart controller that
calculates VPD for you and turns your fans on/off automatically.