The importance of VPD in Cannabis cultivation

What is VPD (Vapor Pressure Deficit)?

VPD is the amount of water vapor needed at a given time to saturate the atmosphere. It refers to the evaporative capacity of the air.
The VPD value represents the difference between the amount of water vapor that the atmosphere can retain (which depends on temperature) and the amount of water vapor contained in it at a given time (relative humidity). It is normally measured in kilopascals (kPa).

When growing indoors, basically every aspect of the growing conditions, such as relative humidity and temperature, is controlled. These elements can end up affecting the way your plants perform their basic processes and that's where VPD comes in. Vapor pressure deficit is a technique used to adjust temperature and humidity to optimal values, increasing the performance of your plants and achieving maximum growth.
There is a common misconception that the transpiration rate of an indoor cannabis grow (or any other) can have a direct effect on the relative humidity of the grow space. In fact, it is the opposite: once we can fully control the environmental conditions of our growing area, we can control its rate of transpiration.

What we can say for sure is that the transpiration of plants has a direct impact on their growth, and VPD has a special impact on the transpiration rate of plants.
With an optimal VPD, the plants release more water in the form of vapor, trying to saturate the environment. Greater transpiration means greater absorption of nutrients and with it increases photosynthesis and therefore production.

With too high a VPD, plants close their stomata in order to avoid excessive water loss or dehydration by transpiration, affecting photosynthesis and yield. With a VPD that is too low, the environment is saturated with water and the plants do not transpire.

We can influence VPD by influencing temperature and relative humidity.
The movement of water through a cannabis plant's system is determined by its rate of evaporation from the surface of the leaves. This loss of moisture is known as perspiration. The three main factors that influence transpiration rate are RH (Relative Humidity), VPD (Vapor Pressure Deficit), and ambient temperature at deck level.

Once the water is in the leaves, it is slowly but surely transpired through their pores, also known as stomata. This process is necessary in the metabolism of cannabis plants, since the opening of the stomata also allows the release of the waste products of photosynthesis, the main by-product being C02.

OPENING OF STOMATAS

As VPD increases, the plant's stomata get smaller as they try to reduce water loss. This means that they will not die of dehydration due to excessive perspiration, but photosynthesis may be slower.