Understanding Vapor Pressure Deficit

Lately a new buzzword has permeated the upper echelons of the cannabis world. That word is VPD or Vapor Pressure Deficit. With help from Skye Hanke over at Smart Bee, we put this article together.

It is important to first understand the basics of transpiration before you can fully understand the impact of VPD on your plants. Transpiration is essentially water evaporating from a plant.

Let’s look at how this works: To begin with, the plant uptakes water and nutrients via its root system and osmosis. The plant will use what it needs from this mixture and will then release the waste via the underside of its leaves where it changes into vapor and is released into the environment of your grow.

Two main factors that affect transpiration are temperature and humidity.

This is one area where, speaking from experience, the Smart Bee system can really help you. There are many advanced monitoring systems out there, but they are cost prohibitive for most growers. The Smart Bee system is not; it’s affordable and gives the grower a tremendously valuable tool in his arsenal for creating top-of-the-line medical or recreational cannabis.

Since humidity and temperature play such an important role, as a grower, you need to have total control of those systems. In essence, we are playing the role of Mother Nature—only in this case, we can fine tune the weather to work in our advantage.

In the same way that we perspire when it gets warm out, when the temperature in your grow increases, so to will the need for your plant to transpire. When this happens, water vapor is released into the air. The hotter and drier the air, the more the plant will transpire, and in turn, more water vapor will be released in the air and the humidity will jump.

The way to control this is through an understanding of VPD.

Understanding VPD means first understanding the chart that is seen so commonly over the internet.


The chart shows you the temperature along the left axis and the humidity is on the top. The numbers that run between the two axis are the vapor pressure deficit numbers.

For cannabis, there is a sweet spot that varies through the growth cycle of the plant. Look at the plant and compare what we are trying to do with what a body builder does. We are trying to stack weight on while maintaining a beautiful structure.

By adjusting the sweet spot of your room, you can also speed or slow the uptake of nutrients to the plant. Using these little tweaks, you will optimize the environment for your plants to thrive in.

Typically in weeks one and two, the plants are drinking less as they fully enter the flower cycle. Their root mass is still growing, and they are on their way to maturation. In this stage, the VPD should be around 8.0-11 which is a temperature of 77° – 79°F and 65 – 75 percent humidity.

Once the plants are in flowering and need to uptake more nutrients, the VPD should be around 10-13.5. This would be weeks three through four. To achieve this, temps would be a bit lower around 75° – 77°F with a humidity around 55 – 65 percent.

As you move through the flower stage, the VPD would then follow to between 13-15.5 in weeks five through nine. This would mean lower temps around 74° – 76°F and a relative humidity between 42 – 51 percent. This cooler and drier condition will allow the medium to dry out more evenly so that you could have more consistency with irrigation dry downs. The cooler temps will also better preserve the terpenes on the plant.

In this cycle, you will also adjust your feeding accordingly, boosting the amount and times you feed as you run through the flower cycle. As the VPD goes up, so to does the pressure on the plants to pull water in through the roots. You will see your buds fatten up through the flower, and as you increase the watering cycle, also increase your EC. By increasing the amount of nutrients you are giving your plants at the right time in the flower cycle, you will see your yields dramatically increase.

This is why having a system that can professionally monitor and control your environment is so important.

Rather than rely on the internal temperature sensors of your AC or humidifier, I would strongly recommend using a controller with a top of the line sensor that will best allow you to perfectly dial in your environment.These sensors also override the controllers for your HVAC, giving you complete control over your rooms. You will see this will bring out the best in your cannabis.

Keep it green and keep on growing!

Any questions please email me at harryresin@hightimes.com. I can also be found on IG @Harry_resin for more information about the Smart bee system check out smartbeecontrollers.com.

  1. The perimeters used in the article to be used in the last weeks flowering look incorrect in reference to the chart or am I missing something? The VPD of 15 or so lands in the dark blue area which is an undesirable condition according to chart. Right?

  2. I see it as orange in baby stage, white line in veg and just into dark blue for flower. But the take away is: think vpd and NOT temp and humidity. Set one variable … Temp then adjust humidity to the desired vpd. Or the other way around… Right???

  3. I cant trust any of these charts…. I have looked at 10 different ones, and each one is a little different

Leave a Reply

Your email address will not be published. Required fields are marked *

Related Posts
Dry Farming
Read More

Dry Farming in Humboldt

A small region along the Eel River in Humboldt County allows cultivators to grow cannabis without ever watering their plants.
Read More

Growing for Terpenes

Increasing terpene production can result in a more flavorful, enjoyable smoke.
Read More

Chadivation or Cultivation?

The cult of great cannabis and why legacy cultivators will always produce better weed than big business.
Read More

Predictive Plant Analysis

Developers at Texas A&M University and Mariposa Technology tackle THC levels and plant sexing.