Understanding Vapor Pressure Deficit (VPD)

Vapor Pressure Deficit, also known as VPD is a very useful tool for growers. However it is a variable growers sometimes forget, do not know about its existence or do not understand. 

VPD is a variable related to humidity. Humidity in growing systems can be expressed as: Relative humidity, one of the most common variables used in greenhouses and is expressed as percentage; Vapor concentration, a variable used to express the amount of water vapor in a specific volume of air and Vapor pressure deficit

Vapor pressure deficit can be defined as the amount of vapor that can still be stored in the air until reaching saturation point, under the same temperature. This variable can be calculated as the difference between the actual vapor pressure and the saturation vapor pressure.

Why can VPD be helpful for growers?

We know humidity in growing systems is important to keep plant health and development. But too much humidity can be risky inside growing systems. For growers VPD can give an indication of the possibilities of the crop to keep transpiring at a given temperature without the danger of condensation. Condensation is not good inside growing systems, it can trigger fungi and bacteria growth. In addition the process of condensation releases heat into the systems making environmental management more challenging and can even affect plant nutrient uptake.

When we increase cooling maintaining vapor pressure at the same level, at some point the gas mixture will be saturated and condensation will take place. Therefore, it is important to track VPD inside growing systems in order to identify and maintain healthy air moisture conditions at different temperatures. 

When working only with relative humidity we are able to know the relative humidity percentage inside our system. However when we maintain relative humidity levels but temperature changes, vapor pressure inside the system will be affected, impacting also plant health. 

Relation between Relative humidity and VPD

If you are used to working with relative humidity and are now willing to understand VPD. We need first to understand the relationship between these two variables:

  • Higher VPD levels mean more moisture can be held in air before condensation. Meaning and environment where relative humidity is in low levels.
  • On the other hand low VPD levels are related to an environment where less moisture can be held in air before condensation occurs. Meaning relative humidity levels are higher.

Relative humidity is a good measurement. But knowing VPD we can get to know more information about our system. But, how can we know if VPD levels are correct?

Optimum VPD levels can be crop specific and even fluctuate across the crop cycle. General recommendations for VPD levels inside growing systems goes from 0.3 to 1 kPa. 

VPD also correlates with transpiration. Transpiration can affect nutrient uptake, water usage and general environment in our greenhouse. Another important reason to maintain optimum VPD levels in our greenhouses.


High VPD levels above 2 kPa can cause high transpiration rates and low water potential in leaves. High transpiration will affect the environment, water use and nutrient uptake.

Both high and low VPD levels can lead to heat injury, increasing leaf temperature.

Low VPD levels can suppress transpiration impacting energy balance and movement of passive nutrients within the plant. 


VPD can be calculated using a psychrometric chart providing information about actual vapor pressure and saturated vapor pressure. However the most easy way to track this variable is by installing the proper sensors inside our system. In Hort Americas we offer the most complete systems to monitor our growing systems! Contact us today to get to know the best options to optimize environmental management inside your greenhouse or plant factory!