Humidity can be a tricky thing to understand when you are first starting to grow cannabis plants. After setting up the lights, watering system, and air handling system, humidity might not be at the forefront of the beginning growers' attention. But experienced growers know that humidity is really the key to getting the best yields from the healthiest plants if you can just get it right.

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Vapor Pressure Deficit, VPD for short, is another term and concept that gets thrown around a lot nowadays. Currently, it's become something of a buzzword in the cannabis-growing community. But what is it really and how can understanding it help you grow better cannabis? To better understand how humidity and VPD play a role in plant development, let’s review some of the key concepts.

Relative Humidity vs. Absolute Humidity

To really grasp the fundamentals needed to understand how and what VPD is, we need to start with the difference between Relative Humidity (RH) and Absolute Humidity (AH).

A green cannabis plant with colorful hues displayed from light reflection Every grow should watch their humidity and VPD within their grow whether simple or advanced. photo credit

RH is what you read on a hygrometer, it's shown as a percent along with the temperature. RH tells you how much water vapor is in the air relative to what the current temperature is. RH changes with temperature, as warmer air holds more moisture than colder air. As temperature rises, the RH percentage lowers due to the air’s capacity to hold more moisture. Air with 100% relative humidity is considered “saturated”, holding as much water as physically possible at that temperature.

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Absolute humidity refers to the total amount of moisture in a given quantity of air. Absolute humidity does not change with temperature. To help us understand this, let's visualize a glass box with a hygrometer inside it. Looking at the hygrometer, it says the temperature is 60 degrees with 100% RH.

If we raise the temperature in the box to 70 degrees, the RH gets lower. The air can hold more water now that it is warmer, but the same amount of water is still in the box, it did not disappear with the increase in temperature. This amount of moisture is the absolute humidity which is usually expressed as a weight per volume.

VPD Explained

Most growers rely on relative humidity to tell them how much moisture is circulating in the air of their grow room. While knowing the RH is useful, reading and adjusting the Vapor Pressure Deficit is a much better, more plant-oriented way of dealing with humidity. Vapor pressure refers to the total weight of the moisture in the air exerting force on an area.

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In a high humidity environment, the vapor pressure will be high. In a low humidity environment, vapor pressure will be low. The inside of a plant's leaf has its own vapor pressure that is different from the outside environment. VPD is a measure of the difference of the vapor pressure in the leaf and the vapor pressure outside the leaf.

An up-close image of green cannabis leaves To figure out the VPD, you'll need a hygrometer and infared laser thermometer to measure the temperature of the leaf. photo credit

This pressure difference is extremely important to the basic functions of all plants. In cannabis, on the underside of every leaf there are tiny, microscopic pores called stomata. These pores open up to transpire moisture from the plant, which keeps water and nutrients running through the plant like a wick.

In a high vapor pressure environment (high humidity) these stomata are forced closed, limiting the amount the plant can transpire and slowing the plant’s metabolism. In a low vapor pressure environment, the stomata are open, increasing transpiration and speeding up the plant’s metabolism.

To figure out what the VPD is in a grow room you’ll need a hygrometer and infrared laser thermometer to measure the temperature of the leaf. All you need is the room temperature, the room’s relative humidity and temperature of the surface of the leaf.

There is a somewhat complicated formula to plug these numbers into, but for the sake of simplicity we’ll just link one of the handy dandy VPD calculators available online. Since VPD is the measure of weight over an area, it can be measured in pounds per square inch, but it's more often measured in kilopascals (kPa).

If you don’t have an infrared thermometer, there are many VPD Charts available online as well that will give you a pretty good guess as to what the VPD is based on just temperature and relative humidity. For cannabis it's recommended to have a VPD of about 0.8 kPa, but there are many other things to consider.

Looking at the chart, if a grow room is sitting at 73℉, to get a VPD of 0.8 you’d need 70% relative humidity in the room, which is high enough to start causing mold problems. At most temperatures, the optimal VPD for cannabis requires high humidity, which really isn’t optimal for producing a clean final product.

A green cannabis plant that is in its vegetating stage, planted in a white pot Vegetating cannabis plants need more humidity than flowering plants. photo credit

For this reason, it's recommended to try and optimize the VPD during the beginning vegetaive cycle, but stray away from optimal VPD during late flowering. Younger plants tend to do better with high humidity environments, so optimizing the VPD in the early veg stage helps get vigorous growth. Dialing back the humidity by the third week of flower helps to increase transpiration and move nutrients through the plant, increasing yield. This also helps reduce the chance of molds and mildew forming on the precious buds.

Managing VPD

Having the ability to lower or heighten the humidity when necessary in your grow area is key to controlling VPD. In dry climates, you might not need to dehumidify as much and can get away with just using a humidifier to increase humidity.

Vice versa with wet climates, you might just need to dial in a dehumidifier to get the desired relative humidity. Keep in mind that air conditioning units by nature dehumidify the air, so if you are using lots of them you might need to add moisture to the room with a humidifier.

Also having a hygrometer is essential to getting your desired VPD right. A simple cheap hygrometer from your local grow store will do just fine, so if you don’t have one just grab one of those. More advanced models can connect to a smartphone and have features that allow you to track temperature and humidity over weeks and months. Using this data you can dial in your equipment to efficiently and constantly keep your desired environmental conditions.

A hand, wearing a green glove, is touching a purple cannabis cola Master cultivators know the importance of VPD and humidity. photo credit

Vapor Pressure Deficit is obviously a much more complex subject than this article touches on, we’ve only tried to simplify the concept to make it easier to start to understand. If you are interested in learning more, there are many great educational resources and videos on VPD available with a quick search. Learning about VPD, and how it affects plant metabolism is a keystone to becoming a modern master cultivator of all plants.

Photo Credit: Shutterstock

Frequently Asked Questions

How Can I Make My Grow Room Less Humid?

Proper air exchange can help keep humidity in check. Standard air conditioning units will help to dehumidify, as well as dedicated dehumidifiers. Having an exhaust set up to a humidity-controlled rheostat can act as a fail-safe when humidity levels spike. 

How Do You Control Humidity in a Grow Tent When the Lights Are Off?

Use a dehumidifier to keep the humidity from getting too high when the lights are off.

What is the Best Way to Control Humidity?

The best way is to use a dehumidifier if the tent is too humid, and a humidifier if the tent is too dry.

Do Fans Lower Humidity?

Fans alone will not lower the humidity, they only circulate the air.

Can you Dehumidify Too Much?

Yes, plants need some humidity to function properly. Growing in an area with little to no humidity will stunt development.

Does Heating a Room Reduce Humidity?

It will not reduce the amount of water in the air. Warmer air can hold more water, but it doesn’t lose the water it already has. Heating a room will reduce the relative humidity but not the absolute humidity.