Vapor Pressure Deficit (VPD)

Cannabis cultivators growing their plants in indoor grow tents or rooms with special, artificial lighting take into consideration many different parameters, including temperature, humidity levels, etc., to maximize growth.

This is all a part of achieving an abundant harvest with top-quality flowers that promise taste and potency.

what is VPD mearing(Vapor Pressure Deficit)

While it is true that these factors play an essential role in nutrient uptake of the plants and their blooming abilities, there is one factor that growers often forget, the VPD, the shortened form of Vapor Pressure Deficit.

Now we will cover everything a cannabis grower should know about VPD and why it matters. Since vapor pressure deficit directly correlates to transpiration rates, it can be used to fine-tune water flow through the plant.

This gives growers an additional ‘control knob’ to balance the environment for optimal plant growth and proliferation.

What is VPD

VPD is the abbreviation of Vapor Pressure Deficit. VPD is the difference between moisture present in the air and the potential for your indoor environment to hold more moisture.

Our indoor air is a mixture of many gases, including nitrogen, oxygen, and water vapor. This vapor can be measured as a part of the total air pressure, helping us keep track of just how much water is present in the air in gas form.

Build VPD chart weed can help us how much the actual vapor pressure is short of reaching its saturation point.

VPD chart for cannabis growers

If you have any questions about VPD 

Why Vapor Pressure Deficit Matters Cannabis Grow

VPD for Cannabis

Since VPD cannabis cultivation involves moisture transpiration in your plants. It is important to first go back to what the transpiration rate is.

A plant’s transpiration rate is the rate at which it loses moisture as water vapors from the stomata. This allows plants to gain access to CO2, making it a critical part of the photosynthesis process.

Growing cannabis plants transpire between 95% to 99% of their total water intake. This leaves all the expelled vapor floating about in the indoor air of your grow room, increasing its humidity levels.

Due to the high transpiration rate of your cannabis plant, it is critical to keep a strict check on VPD regularly.

Lets watch the video to learn more about transpiration.

With too much moisture trapped in your grow room, the higher humidity levels will result in mold and milder. Moreover, it can also increase the chances of your plants developing bud rot, causing your entire yield to be severely affected.

You could find more about "Bud Rot"

VPD for Flower

Another important factor that offers insight on why VPD matters is the different levels required in different stages of plant growth.

In the initial growth stages, your cannabis plants require low VPD as it keeps the stomata open. Allowing plants to increase their CO2 absorption rates for better growth.

However, once your plants have reached the flowering stage, it is better to maintain a higher VPD level. This prevents the growth and development of mold and the spread of other pathogens to your established crop.

So, the best practice is to start with a low VPD to allow your plants better access to nutrients and then move to a higher level as the plants start flowering to prevent undesirable diseases.

Important Note

The VPD, in general, should be maintained at a range of 0.8 to 102 kPa according to the different temperature and humidity level.

Understanding Vapor Pressure In-Depth

To better understand how Vapor Pressure Deficit affects your plants, it is important to study it in depth. There are two terms that directly control the VPD of your grow room;

Saturation Vapor Pressure (SVP) 

Your grow room’s indoor air can only contain a certain amount of water vapor at one time, at a given temperature. Once this amount is exceeded, the vapor quickly starts condensing back to water in the form of dew. This maximum level of water vapor that the air can hold is called its saturation vapor pressure.

Actual Vapor Pressure (AVP)

On the other hand, the actual amount of water vapor present in your grow room’s air environment at any given time is its actual vapor pressure. This increases and decreases as the transpiration rates of your plants change.

AVP and SVP are interlinked, as the maximum AVP that your grow room can tolerate is its SVP. It is important to fully understand these terms, as it will help you calculate your grow room’s VPD below.

How to Calculate Vapor Pressure Deficit

VPD Calculator

Now, we know that the maximum amount of vapor the air can hold is its SVP. And the actual amount of vapor currently in the air is its AVP. Therefore, we simply need to subtract the maximum value from the actual value to find out our deficit value.

Here is a vpd calculator, formula about how to calculate Vapor Pressure Deficit:


P.S. Keep in mind, you can calculate the air VPD and leaf VPD separately. Using the grow room’s temperature and humidity levels. Leaf VPD will help you understand the cooling effect of water evaporation on plants. Thus, it will help you to find ways to better control their growth.

You will need an IR temperature to calculate leaf VPD.

Vapor Pressure Deficit and HVAC System

Most grow farmers wonder what is the point of adding this complexity to the mix instead of simply using relative humidity percentage.

VPD is so important because it directly relates to our plant’s transpiration rates, whereas relative humidity does not. The vapor pressure is the driving force behind moving water vapor out of the leaves and opening the stoma pores for nutrient uptake.

We know that VPD is highly influenced by growing room temperature and humidity levels. Therefore, it is important to maintain these factors at the right levels to ensure maximum transpiration for growing buds.

Grow room hvac - GA function

This can easily be achieved using the right from Altaqua grow room HVAC system

Altaqua grow room HVAC system - Cooling
Altaqua grow room HVAC system - Heating


Cold temperature below 65F slows down plant growth due to a reduced rate of photosynthesis. Moreover, anaerobic pathogens seek this opportunity to attack your weaker roots, resulting in mold and disease.

Similarly, if the temperature goes above 80F, that’s no good for your cannabis crop either, as the plant would need to use part of its energy to dissipate heat.

Altaqua grow room HVAC system - Dehumidifying

Relative Humidity

On the other hand, if your grow room is at 100% relative humidity. No further water can be absorbed by the air, and the plant’s transportation process will completely stop. This also indicates that SVP has been reached.

But, if the relative humidity percentage is too low, the stomata will close in an effort to preserve the plant’s moisture content. Also stunting the transpiration process.

Control VPD Well for Cannabis Grow

While maintaining other environmental conditions in your indoor grow room is essential, you also need to keep an eye on the Vapor Pressure Deficit.

Through careful controlling of the room temperature and humidity levels, along with regular monitoring and calculation of the VPD, you can create an ideal environment to maximize your cannabis yield.

Control cannabis VPD well forcannabis grow

Your plants will conduct transpiration at an optimal level at different growth cycles, thus taking in a good amount of nutrients and offering you potent and better-tasting buds.

Please contact our team of engineers. We will provide a free grow room air conditioning solution to create the best VPD for every stage of cannabis.

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