Understanding Vapor Pressure Deficit

By Kent Gruetzmacher
Published: April 4, 2022
Presented by AC Infinity Inc.
Key Takeaways

In learning about vapor pressure deficit (VPD), you can create conditions in your grow room which allow plants to uptake the ideal levels of water and nutrients.

Whenever you think you’ve mastered indoor cannabis growing, it’s inevitable you will uncover new things to learn. While ideal temperature and humidity levels for cannabis have long been regarded as indoor gardening basics, these important elements are being revisited by beginners and experts alike.


The concept of vapor pressure deficit (VPD) has gotten a good deal of attention in the controlled environment agriculture (CEA) space over the past few years. With VPD, scientists have discovered a more nuanced approach to regulating grow environments that better caters to plant physiology.

In learning about VPD, you can create conditions in your growroom which allow plants to uptake the ideal levels of water and nutrients. As you grow more proficient with VPD, you will help guarantee bountiful harvests long into the future.


What is Vapor Pressure Deficit?

VPD is a type of measurement for temperature and humidity in a garden that describes how these factors influence essential plant processes. VPD shows how a growroom environment affects a plant’s ability to uptake water through its roots and transpire it through its leaves.

Traditionally, cannabis growers have used relative humidity (RH) as their sole metric for understanding how humidity levels impact plant processes. While RH is an important metric for general garden planning, it still leaves much to be desired for precision growing.

RH largely fails to account for how humidity levels influence cannabis growth in a changing, dynamic environment.


When cannabis plants reach certain temperature thresholds, they rely on transpiration to cool themselves off. With transpiration, cannabis plants pull water through their roots and again release it through stomata in their leaves. In turn, this humid air is pulled away from the plants by drier air in other parts the room.

VPD refers to the difference in pressure between the current humidity level in the growroom and the dew point. Therefore, the lower the humidity, the higher the VPD.


hands holding cannabis behind glass covered in condensation

How Important is it to Get VPD Right?

It’s important to get VPD right because it directly influences how efficiently cannabis plants can uptake water and nutrients. Along with sunlight, water and nutrients are the most important elements for growing healthy cannabis crops.

Low VPD occurs in overly humid environments where transpiration processes slow to a halt. In such situations, plants stop taking up water and nutrients and grow increasingly susceptible to pathogens and diseases.

In situations with extremely high VPD, cannabis plants transpire too fast and can dry out. In these cases, leaf stomata close to try and protect water levels within the plant, halting water and nutrient uptake altogether.

The key to maximizing VPD in cannabis cultivation is balance. In other words, you need to create conditions that are dry and warm enough to stimulate transpiration, while also not going overboard and stressing plants out.

With countless factors such as irrigation systems, grow lights, and outdoor weather influencing conditions in your growroom, finding this balance is often harder than it seems. To maintain balanced VPD, you must tweak your HVAC system to account for temperature and humidity on a constantly sliding scale.

Nonetheless, when balance is achieved, using VPD gives you precise control over how quickly your cannabis plants drink water and consume nutrients.

How Do I Measure VPD?

Calculating VPD by hand is a complex process. That being said, equipment manufacturers have developed specific sensors that measure VPD, saving growers both time and energy.

For commercial producers, equipment manufacturers have developed complex VPD sensors that dynamically measure leaf temperature and humidity levels throughout the garden canopy. With such rich data in hand, producers can then dial their HVAC setting to maximize water and nutrient uptake, while also being sure not to waste money on inflated cooling costs.

Home growers needing help measuring VPD need not look any further than AC Infinity. In fact, their Controller 67 now integrates with a mobile app that measures VPD in smaller gardens such as grow tents. With the AC Infinity Controller 67, you can keep VPD dialed in your garden and monitor progress from afar on your mobile device.

cannabis leaf behind glass covered in condensation from humidity

How Do I Regulate VPD?

VPD is regulated in the same ways that you regulated temperature and humidity in the growroom. Commercial growers generally look to HVAC systems for regulating VPD, while hobbyist cultivators often use inline exhaust fan systems.

In the past, temperature and RH have been the sole factors growers have focused on with HVAC systems and exhaust setups. Yet, temperature and RH are somewhat static figures that don’t fully account for the dynamic nature of cannabis plant physiology. Namely because, temperature levels should constantly shift to account for changing humidity levels.

With traditional indoor growing practices, cultivators simply aim to keep ambient temperatures around 75°F and RH around 50 percent. Yet, when either of these factors shifts, the other doesn’t shift with it to account for the change. When temp and humidity fail to line up properly, you miss an opportunity for precision control.

When looking at a VPD chart, you will see there are ideal zones where temperature and humidity must be carefully balanced to maximize water and nutrient uptake. For example, during late flowering phase, growrooms at 77°F should have humidity levels at 35-45 percent. If the temperature shifts up to 79°F, humidity levels must shift to 40-45 percent.

When VPD sensors are integrated with your HVAC system, you can use automation to ensure that balance is constantly achieved.


As can be seen, regulating humidity levels in growrooms is more nuanced than most people originally thought. Nonetheless, putting some time and energy into learning VPD is another great way to achieve bountiful harvests. Not only does VPD maximize plant health, but it also helps reduce the amount of energy used on your indoor garden. Namely because VPD is exactly what your plants need with temperature and humidity – no more, and no less.

If VPD seems like too complex of a topic to tackle all at once, you can start learning about it by simply taking more measurements in your growroom. Using a hygrometer, pay more attention to temperature and humidity changes in your garden. Even better, take note of how things like rainy days outside influence how your plants react with the environment inside. In doing so, you will gain a much better appreciation for how VPD shapes water and nutrient uptake.

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AC Infinity is the foremost name in air delivery systems, designing and developing the latest innovations in cooling and ventilation technology. They offer a suite of quiet inline fans that automate the growing progress and track key metrics. Visit or contact [email protected] to learn more.


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Presented By

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Written by Kent Gruetzmacher | Writer, Owner of KCG Content

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Kent Gruetzmacher MFA is a Colorado-based writer and owner of the writing and marketing firm KCG Content. Kent has been working in the cannabis and hydroponics space for over a decade. Beginning in California in 2009, he has held positions in cultivation, operations, marketing, and business development. Looking specifically to writing, Kent has worked with many of the leading publications and marketing agencies in the cannabis space. His writing has been recognized by such icons as Steve D’Angelo and Rick Simpson.

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