What is Vapor Pressure Deficit and why does it matter?

What is Vapor Pressure Deficit and why does it matter?

Posted by Celia Daly on October 12, 2017 12:00 am
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Vapor Pressure Deficit (VPD) has become a common term among cultivators. Various bloggers and cultivation experts have been weighing in on VPD and why it’s important for growing healthy, high yielding plants. Today we’re going to clarify what VPD is, why it’s important and what you can do to ensure you’re achieving the optimum environment for cultivating cannabis. 

To understand VPD, it is important to understand how humidity works. When we talk about humidity, we’re talking about the amount of water in the air in the form of vapor. There are two ways to measure humidity- absolute humidity and relative humidity. Absolute humidity tells us exactly how much water vapor is in the air while relative humidity (RH) tells us how much water is in the air as a percentage of how much water the air can hold at that temperature. This is where things can begin to get confusing, because temperature and humidity have a complicated dynamic. As temperatures increase, the air is able to hold more water vapor than at lower temperatures. This means that a room will see a drop in RH when the absolute humidity has not changed, but the temperature has increased.

 So why is this important? All cultivators know that managing humidity is important. For example, high humidity levels can lead to mold growth, damaging crops and affecting yields. However, RH levels also dictate how much a plant will transpire, and thus, how much they will grow. This is because the water we give to plants merely acts as a vehicle for nutrients. Once the nutrients have been metabolized, plants transpire water back into the air and are able to bring in more water and nutrients again. But when RH levels are high, plants have a harder time doing this.

The reason for this is that water vapor in the air creates a certain amount of pressure, pushing back on plants as they try to transpire water. As you can imagine, more water vapor in the air (i.e. higher RH levels) means more pressure, causing plants to have a harder time transpiring. This brings us to what VPD actually is. As the name indicates, this is a measure of a deficit, or difference between the pressure that could be exerted by the water vapor at 100% RH at a given temperature and the actual pressure exerted by water vapor at your current RH at the same given temperature. Essentially, this is the way the plant would feel, measuring the difference between the pressure inside the leaf and the pressure of the air outside, giving us an idea of how easy or difficult it would be for the plant to transpire. VPD is measured in units of pressure and is essentially RH and temperature in a single value.  

Much research has been done to find the ideal VPD for cannabis transpiration, ensuring plants drink slowly enough to be able to metabolize nutrients but fast enough to ensure they are taking up enough. Depending on the stage of growth, flowering plants should have a VPD of between 10kPa1 and 15.5kPa.

 It’s clear that VPD is important for healthy plants but requires a strict attention to detail and equipment that can ensure consistent humidity levels and temperatures. At Surna, we’ve been designing for precise cultivation environments for more than a decade. Unlike common HVAC systems, our climate control equipment is designed for cultivation. We understand the importance of running a facility day in and day out at a precise temperature and RH level. When you choose a Surna system, you can be confident that you’re not only getting top of the line equipment designed for cultivation, but you’re also getting the expertise that comes with years of experience. Our engineers will take time to understand your goals and design a system that will enable you to get to the right VPD, grow healthy plants and achieve a strong ROI. Contact us today to talk about your project and let’s grow together!


1kilopascal (1 kPa = 1000 Pa)

Topics: 2017 cannabis, best practices cannabis, best practices cultivation, Cannabis Basics, cannabis cultivation, cannabis cultvation, commercial cultivation, dehumidification, efficiency, humidity, vapor pressure deficity

6 responses to “What is Vapor Pressure Deficit and why does it matter?”

  1. Bart Cox says:

    While working in the oil/gas industry in the Houston area, dehumidifiers are a must in labs that have very expensive equipment used for quality assurance. Humidity is most always in the 90% range, year round. The water extracted from the air is enormous, gallons per day for a small 200 sq. foot lab, at the same time testing for vapor pressure results in gasoline. The reason for this inquiry is soon your company will be in the Houston area, for medicinal purposes, to start. I’m sure you have prepared for these conditions, and are fully capable of dealing with the heat and humidity in this area. Thanks

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  3. Moein says:

    What are your recommendation VPD for mothering room, vegetative room, and last two week before harvest? How about VPD for drying room?

  4. rite says:

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  5. You arе ѕo interestіng! Ӏ ⅾоn’t believe І have read thr᧐ugh a single thing lіke that before.
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  6. Brett says:

    Very valuable information. Would it be possible to include a conversion from VPD to relative humidity optimal percentages for flowering and vegetative growth in your article? For an example, what would “10kPa1 and 15.5kPa” be equivalent to in terms of relative humidity as displayed on a standard hygrometer.

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