The Importance of VPD - Hydrohobby

Vapor Pressure Deficit is more often overlooked in growing environments - and yet is the cause of many commonly found indoor gardening problems. Today we explain VPD and why its important to monitor and adjust accordingly to ensure healthy yields every harvest - via humidity control.

Knowing your VPD is important as it assists you to gauge plant water consumption - changing the VPD will bring you better results - and recommended all grow room environments.

There are many variables affecting the VPD such as lighting conditions or additional co2 supplementation. The day VPD for example is usually higher - than the night time optimal, it is better to have a drop during the night period. Increasing the VPD during the night period will correlate declines in the canopy carbon dioxide rates.

Calculating VPD

So to how do you work out the VPD?  You will need to know your Leaf Temp, Room Temp & Relative Humidity inside your growing environment - and then use the the complicated Arrhenius equation :) - which will provide which will provide you with the VPD!

Or you can simply find out your VPD by referring to the chart below plotting your growroom's Humidity and Temperature - aiming to be in the GOLD Zone values for the best results.......

Dangers of High and Low VPD 

When the VPD is too high your plants will transpire a lot more. Because the air is holding additional water your plants will lose more water through through their stomata. This will equate to a loss of water, it also puts extra pressure on the plants roots systems -  as they will now need to transpire more water to compensate the loss.

Your plants will also face other issues if the plant root mass is not large enough or there isn't enough water available - plants will wilt - due to the air taking away more water - than the plant can transport via its tissues.

It's very difficult for any organism to evaporate water when the VPD is Low - close to 100% humidity - for humans its the same as wearing a sweat soaked t-shirts - but for your plants - they can not transport nutrients to their leaves or cool their surfaces. Low VPD is very stressful for plants as it causes plants to be unable to transport water.

Here I have listed some of the common symptoms associated with low and high VPD :


MIneral deficiencies, Guttation, Diease, Soft Growth


Wilting, Leaf Roll, Stunted plants, Leathery/crispy leaves

Managing Humidity Levels

With more than 15 years in the hydroponics  industry - you can rely on our range of Environmental Products to assist you in controlling your humidity levels.

Our full range of Humidity Control products include the latest Quest DehumidifiersQuest CDG 74 Portable Dehumidifier - making it simply to move the professional dehumidifier exactly where its needed. Other models include : Quest 70 Overhead DehumidifierQuest Dual 155 Dehumidifier units and Quest F9 Air Mover  all offering the perfect to solution to high humidity in the grow environment.  Equally our professional Faran HR-50 Humidifiers with Humidistat and Faran HR-15 Humidifiers with Humidistat - all units will ensure your environment is not dry but - perfect for your plants to thrive and your yields to increase!

* Remember air humidity is closely related to the VPD - see the ideal humidity range ranges below reference to your plants stage of development :

  • 70-80% after you see the first leaves
  • 40-70% during the vegetative stage
  • 40-50% during the flowering stage
  • <40% in the last week before harvest