In medicinal crop care and plant management, a common term is Vapor Pressure Deficit (VPD).

So what does VPD actually mean?

When I first came across the term Vapor Pressure Deficit (VPD), I found it difficult to understand. However, numerous resources are available to help us better understand VPD.

In today’s blog, I’ll explain what the term VPD means. I’ll also discuss VPD calculations and greenhouse environment considerations for medicinal cannabis cultivation operations and plant management. Reading time: 3 minutes.

Introduction to Vapor Pressure Deficit (VPD)

It is notably difficult to manage (control) VPD in a greenhouse setting when growing medicinal cannabis. Yet it is important to learn how to measure and manage vapor pressure deficit (VPD), in order to optimise yield. Discover why managing VPD is so important to cannabis plant health, growth and yield.

Definition of Vapor Pressure Deficit (VPD) | Plant Cultivation

VPD means the actual difference (in units of pressure) between the Actual Vapor Pressure (AVP) and the Saturation Vapor Pressure (SVP).

In plan language: Vapor Pressure Deficit (VPD) means the difference in vapor pressure between the inside of the leaf compared to the vapor pressure outside of the leaf. The inside of the leaf is always wet. It is drier outside of the leaf, and the leaf will try to equalize the vapor pressure through transpiration, leading to the release of water vapor through the stomata (tiny pores in the leaf).

Managing VPD in a Greenhouse Environment for Medicinal Cannabis

Measuring and Managing VPD and the impact on Transpiration

Next, we’ll discuss the importance of managing VPD in medicinal cannabis crops. It is challenging to manage vapor pressure deficit (VPD) in a greenhouse growing environment. It is important, however, to understand VPD and to successfully manage VPD to increase your yield and protect your crops.

VPD can help you identify the correct range of temperature and humidity for your plants
Managing VPD can help you achieve the best result from the grow while avoiding pests and environmental problems

Knowing your crop’s VPD will also help you control CO₂ uptake, nutrient uptake, transpiration rate, plant stress, and stomata openings, which are all vital for healthy plants and optimal growth

For this reason, it is extremely important for growers to understand the concept of VPD, how to calculate VPD and the impact of VPD on transpiration.

Calculating Vapor Pressure Deficit (VPD)

To better understand VPD, you will need to be familiar with the following terms.

Saturation vapor pressure (SVP)
Actual vapor pressure (AVP)
Relative Humidity (RH)

Saturation Vapor Pressure (SVP) | Definition and Meaning

The saturation vapor pressure (SVP) is the maximum amount of water vapor that can exist in the air at any specified temperature.

Air can only hold a certain amount of water vapor at a given temperature before it starts to condense in the form of liquid vapor (dew, precipitation).
As the air gets warmer, the saturation point increases and as it cools the saturation point will decrease.

Actual Vapor Pressure (AVP) | Definition and Meaning

Actual Vapor Pressure (AVP) is the actual humidity in the air at a given time.

Actual Vapor Pressure (AVP) is usually much lower than the (SVP).

Relative Humidity: Definition and Meaning

Relative humidity (RH) is a percentage that the Actual Vapor Pressure (AVP) is to the Saturation Vapor Pressure (SVP).

Relative humidity (RH) is expressed as a percentage.
Example:

(Actual Vapor Pressure)

(RH) = —————————– x 100%

(Saturation Vapor Pressure)

Relative humidity measures how much water vapor the air currently contains, as compared to what it would contain if saturated.

If the amount of water in the air is just half of the saturation amount, for example, the relative humidity is 50 percent.

So, when we refer to Vapor Pressure Deficit, in scientific terms, VPD means the actual difference (in units of pressure) between the Actual Vapor Pressure (AVP) and the Saturation Vapor Pressure (SVP).

This can show us how much more room there is in the air for more water vapor.
Therefore, VPD = SVP – AVP

To make this work for plants in a controlled growing environment, such as a greenhouse, there are several extra steps to consider.

You cannot simply use the air temperature alone.
You also need to consider the leaf temperature in order to calculate the leaf SVP.
This is because the leaf has a saturated (100% RH) inner environment.

Calculating VPD for indoor growing environments

Therefore, when working out your VPD for an indoor growing environment, the correct VPD to be looking at is the leaf temperature SVP minus the product of the air temperature SVP.

The calculation will look like this:

VPD = LEAFsvp – (AIRsvp x AIR%RH)

The reason why we also consider this calculation is because plants have a natural cooling effect of evaporating water, as water exits the plant leaves through the stomata (transpiration).
It is important to know the air temperature at the canopy level as well as the temperature of the leaves.
The best way to do this is by using an infrared Thermometer tool.
You should also bear in mind that each leaf temperature may vary due to shading or location from the light source (closer or further away from the light source).
It is recommended to take various readings from a plant; then average those readings out and use the average of those readings as your leaf temperature.

How important is VPD in plant management and crop yield?

When we look at how we can influence plant growth and optimisation using VPD, we can quickly identify three (3) areas of importance for cannabis cultivation operations.

There are 3 main variables that we can influence which are specific to VPD (Vapor Pressure Deficit):

Temperature
Humidity
Light intensity

With regards to the temperature, when you increase the temperature of your growing environment, you will increase the VPD.

As the temperature decreases, so will the VPD.

When we look at humidity, you will see that:

if you increase the humidity, your VPD will decrease;
if the air becomes dryer, your VPD will increase.

Impact of Artificial Lighting on VPD

In terms of the facility placement of artificial lighting, the distance between the lights and the plants can greatly affect your VPD.

If you move the lights closer to the plants (increasing intensity), you will increase the leaf temperature, which will increase your VPD.
You will achieve the opposite effect if you raise the lights (decreasing light intensity).

Impact of extreme VPD on leaf transpiration rates

When using VPD as a growing tool, it is important to know the risks of extreme VPD levels.

If your VPD levels are too high or too low, this will negatively affect plant growth because the plants can’t (e.g., won’t be able to) transpire as they should for optimal growth.
If the air around your plants is very dry with a low VPD, the plants may transpire too quickly. This situation runs the risk of too much nutrient salts building up in the leaves, often referred to as leaf burn or nutrient burn.
If the air is very humid with a very high VPD, the plants will struggle to release moisture through transpiration resulting in a poor carbon dioxide and nutrient uptake.
Furthermore, excess moisture in the air and on your plants increases the risk of mould and powdery mildew.

Cannabis Cultivation Operations: Managing VPD and Transpiration Rates

Ideal VPD levels can be difficult to achieve.

The only way to follow optimal VPD parameters closely would be in a traditional indoor (temp/humidity controlled) cannabis growing environment.

In conclusion, the ideal VPD, as a general rule for plant growth, is around 0.8-1.2kPa (kilopascals).
As your plants go through the different stages of growth, the VPD required will vary during different stages.
The younger (clones, seedlings) your plants, the lower the VPD required.
The ideal VPD when in flower is closer to the top end of the range 1.2-1.5kPa.

Additional Definitions that may be of interest to Cannabis Cultivation Employees or Crop Management Personnel

Vapour pressure is the point where liquid becomes vapour.
Transpiration is the process where water and nutrients move through a plant’s cells. Transpiration processes help cannabis plants regulate their own temperatures and obtain adequate carbon dioxide levels to survive and/or thrive. VPD drives transpiration and nutrient uptake from roots to plant leaves. Plants then release water vapor through openings called stomata (underneath the leaves).
Wet bulb vs dry bulb readings: if the difference between dry and wet bulb temperatures is minimal or small, there is a large amount of moisture in the air. If the difference is large, the air is considered very dry.

Publications and Resources: VPD and transpiration rates

Want more?

If you want to read more you may also be interested in the following blogs:

Get in Touch

If you would like to learn more about our services, request a quote or ask any questions please fill out our enquiry form here.