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Hydroponics

VPD (vapor pressure deficit)

Calculate vapor pressure deficit from temperature and humidity. The variable serious indoor growers actually watch.

Vapor pressure deficit Temperature in °C
How much cooler the leaf is than the air. Typical 1°C when the plant is transpiring. Set to 0 if you don't know.
VPD -- kPa
Target for stage --
0 -- 2.0+
Below target: air is too humid, plants stop transpiring and nutrient uptake stalls. Above target: air is too dry, plants close stomata to conserve water and growth slows. Different stages prefer different ranges.

What VPD is

Vapor pressure deficit is the difference between how much water vapor the air could hold at saturation and how much it's actually holding. High VPD means dry, low VPD means humid. Plants transpire driven by VPD, not by humidity by itself.

The reason it matters more than relative humidity: 70% RH at 20°C is humid for plants (low VPD, around 0.7 kPa). 70% RH at 30°C is much drier for plants (high VPD, around 1.3 kPa), because warmer air can hold a lot more water before saturating. If you only watch RH you'll over-humidify a cool room and under-humidify a warm one without realizing it.

Target ranges

These ranges come from years of greenhouse and indoor growing research. They're general targets that work for most leafy greens, herbs, and fruiting crops. Strict growers tune more narrowly per crop and per cultivar.

  • Clones and seedlings: 0.4 to 0.8 kPa. Roots aren't established, plant can't pull much water. Keep it humid.
  • Vegetative growth: 0.8 to 1.2 kPa. Plant is growing fast, transpiring well, taking up nutrients.
  • Flowering / fruiting: 1.2 to 1.5 kPa. Higher VPD pushes the plant to move calcium and other minerals into developing fruits. Too low here causes calcium issues like blossom end rot in tomatoes.

Leaf temperature

This calculator uses an offset because transpiring leaves run slightly cooler than ambient air. Default is 1°C below air, which is typical when stomata are open and the plant is actively cooling itself. Use a real leaf temp reading (cheap IR thermometer pointed at the canopy) for better accuracy. In a stressed plant where stomata have closed, leaf temp will instead climb above air temp.

Further reading