Why do I need to read this?

Unlike humans, plants don’t have a heart for circulation. They use transpiration for the uptake of water and for the transport of nutrients. Too little transpiration means the plant will stop growing due to lack of nutrients. Too much transpiration will put the plant in “survival mode” meaning it will only transpire and evaporate water to stay alive and to minimise the vapor pressure deficit. The uptake of CO2 will be minimal to nothing because of the closed stigmata.

What is Vapor Pressure Deficit (VPD)?

Water vapor creates pressure, inside the plant and in the air. This vapor pressure is related to the temperature and the relative humidity (RH). Inside the plant the humidity is 100% and we can measure the leaf temperature with an infrared thermometer. In the air we can measure both using a thermo/hygrometer.The difference between these 2 pressures is the Vapor Pressure Deficit. VPS (vapor pressure saturated) inside the leaf – VPA (Vapor Pressure Air) = Vapor Pressure Deficit

What VPD do I want in my growroom?

Danger Zone <0,4

Low transpiration 0,4-0,6 KPa (Propagation, Early vegetative, wk 1,2)

Healthy transpiration 0,7-1,0 KPa (Late vegetative, early flower stage, wk 3,4,5,6)

High transpiration 1,1-1,3 KPa (Late Flower stage wk 7,8,9)

Danger Zone >1,5

(Indica strains prefer slightly lower vpd than sativa strains)

What will be the effect of a correct VPD?

  • Better uptake of CO2
  • Improvement in yield quality & quantity
  • The plants metabolic rate will stabilise
  • Less plant stress
  • Decreasing pathogen stress resulting in less need for pesticides
  • More adequate use of irrigation resulting in lower use of fertilisers

How to change the VPD?

Short version: VPD too high? -> lower leaf and ambient temperature and/or raise Relative Humidity

VPD too low? -> raise ambient temperature and/or lower humidity

In general this means that in the “lights-on” period you will need to use a high capacity humidifier and in the night time use a de-humidifier (and possibly a heater)