Vapor Pressure Deficit - The Hidden Force on your Plants

Once you understand what vapor pressure deficit is, all those environmental factors you’re trying to juggle in your mind suddenly click into place and you start to think and feel like a plant. Take a few minutes to understand why VPD management is key to creating the perfect indoor growing environment! Your plants will thank you for it!

Words: Gareth Hopcroft and Everest Fernandez (thx)

Humidity is HUGE when it comes to growing plants. An important milestone in becoming a competent and responsive grower is developing an understanding of what humidity is, how plants respond to it, and how you can manage and manipulate it

Firstly, let’s make sure we’re all on the same page. When we speak of the “humidity” of or in the air we are basically referring to the amount of water in the air. “In the air?” What do we mean? Well, water can only truly stay in the air when it is in gas form - aka “water vapor”. We’re not talking about tiny droplets of water in the air here (e.g. fog or mist.)

Unsurprisingly, temperature plays a crucial role when it comes to humidity. The warmer the air, the more water vapor it can potentially hold. As the amount of water air can hold constantly changes with temperature it can be difficult to get a handle on what we need to measure. Fortunately an answer comes in the form of the concept of “Relative Humidity” (RH) - this is a measurement in terms of percentage, of the water vapor in the air compared to the total water vapor potential that the air could hold at a given temperature.

So, when we say there’s a relative humidity of 50% - we mean “At this specific temperature, the air is carrying half the potential water vapor possible.”

The Effect of Relative Humidity on Your Plants

RH can be easily measured using digital or analogue meters called “hygrometers.” They are available for around $15 at your local indoor gardening store. But what do the readings mean for your plants?

Turns out-they mean a great deal! While many novice growers focus solely on keeping temperature in range, many take their eye off the ball as far as RH is concerned-perhaps because they don’t fully understand what it is or how to manipulate it to their advantage.

Have you ever been to Florida in July? You’ll know that it’s not just the heat that’s oppressive, it’s the humidity! You feel constantly wet with sweat - the whole place feels like a sauna you can’t escape from! (Sorry Floridians!)

RH has an ever more direct effect on plants. Plants need to “sweat” too - or rather, they need to transpire (release water vapor through their stomata) in order to grow.

The amount of water plants lose through transpiration is regulated, to a point, by opening and closing their stomata. However, as a general rule, the drier the air, the more plants will transpire.

Under Pressure

All gasses in the air exert a certain “pressure.” The more water vapor in the air the greater the vapor pressure. What does this mean? Well, in high RH conditions (think of Florida again) there is a greater vapor pressure being exerted on plants than in low RH conditions. From a plant’s perspective, high vapor pressure can be thought of as an unseen force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air making it harder for the plant to ‘push back’ by losing water into the air by transpiration. This is why with high RH plants transpire less. Conversely, in environments with low RH, only a small amount of pressure is exerted on the plants’ leaves, making it easy for them to lose water into the air.

What is Vapor Pressure Deficit (VPD)?

Okay, so now that you have RH firmly implanted into your conceptual map, we move on to Vapor Pressure Deficit or VPD. As implied by the word “deficit” we’re talking about the difference between two things. In this case, it’s the difference between the theoretical pressure exerted by water vapor held in saturated air (100% RH at a given temperature) and the pressure exerted by the water vapor that is actually held in the air being measured at the same given temperature.

The VPD is currently regarded of how plants really ‘feel’ and react to the humidity in the growing environment. From a plant’s perspective the VPD is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If we look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So, in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself.

Another way of thinking about VPD is the atmospheric demand for water or the ‘drying power’ of the air. VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant. As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is it’s difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes it’s not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.

Managing Humidity

Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant’s water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth. So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it’s not the whole story as it fails to take into account the air temperature.

By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)

The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crop’s ideal VPD range, in the example it would be 64-79°F (18-26°C.)

The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress. Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. What’s important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can. If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:

Measure the leaf temperature and look up the vapor pressure at 100% RH on the table below.

Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on the above table.

Subtract the air vapor pressure from the leaf vapor pressure:

Example: Leaf Temperature = 24°C (100% RH)
Leaf VP: 29.8 Air Temperature = 25°C @ 60% RH
Air VP: 19.0
VPD= 10.8

Humidity’s Effect on Plants

Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.

When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.

For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavorable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant - the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.

Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.



Thanks for posting this… It is essential for the grower to understand plant’s transpiration. Also experienced growers can compensate for increased need for transpiration (in dry and hot grow room) by additional watering without nutes.

Some techniques, for example cloning, are virtually impossible to do properly without understanding role of humidity.


Yes… It helped me clarify a lot of things about plants life.

It’s well written … one learns why and how. Not only that it is best to maintain a certain value (even if in practice it already is).

Forunately in my neck of the woods, when ambient outdoor temps rise, so does the humidity so no real attention to this factor is needed until winter. The air gets very dry with decreased temps in winter so I augment day time blooming with a vaporizer in my ‘lung room’. Increasing RH to 40-50% during winter helps to eliminate PM as well, which loves 20-30% in a cooler room. I would never run a room higher than 60% RH regardless of temps as that is just asking for trouble, IMO. thanks for the article, good work crazyben!!


this has helped me adjust my indoor tent, but outside my temps can hit 115 in july so… probably not VPDable.

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Excellent, thanks @crazyben


I resemble that statement :laughing:



Wow. I’ve only become aware of VPD in the last couple days, but now I’m doing a ton of reading! This might be what has been causing issues with my grows all winter.
In winter, our RH% drops so low it’s like growing in the desert.

My current Temp/RH settings are causing my VPD to be way off into the danger zones and, some of the photos I’ve seen, of plants looking very sick due to severe VPD imbalance… look just like most of my plants have been all winter. I’ve been chasing down pH and Nutrient issues since last autumn. But… now I’m thinking it’s been VPD all along.

I’m adding code into my automation system, to auto calculate the VPD and add it to the graphs, so I can keep adjusting things until I get into the 0.8 - 1.0 range (Since this morning, I’ve already tweaked it down from 1.6 to 1.2, so its getting closer)


I think you’ll be shocked at how they take off once you get the VPD dialed!

Also consider getting a cheap laser gun, knowing the difference between your leaf and room temps is important for calculating


An ultrasonic humidifier is your friend :+1:


I’ve been able to stabilize the VPD in my tents to between 0.9 and 1.2 (varies throughout the hour), not always 100% spot on, but SO much closer to what is should be.

Hoping for great things now!


Sharing with the community…recent email from Pulse

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If you’re a hacker or tinkerer make sure to check out the documentation here:

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  • Pete & the Pulse Team
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