There are a lot of terms that get thrown around in the cannabis growing community: DLI, Ph, RH, VPD etc. VPD is one that people see a lot but don’t know what it is or how they can use it to help them grow. VPD is the acronym for Vapor Pressure Deficit. VPD is simply the relationship between the air temperature and relative humidity. Without getting into a botany lesson, the atmosphere inside the plant tissue and the atmosphere outside the plant tissue create a process called osmosis. This is basically how the plants “breathe.” The plants’ leaves are covered with stomata, which are tiny pores in the surface of the leaf. So, keep this information in mind and let’s dive into how to properly calculate your VPD and how to use it to become a better grower.
VPD Calculator
VPD takes two variables, temperature and relative humidity (RH), and puts them into an equation. For the purposes of this article, we will refer you to a VPD calculator. Here is a very simple one that is easy to use: https://www.doctorgreenhouse.com/cannabis-vpd-calculator
This calculator allows you to input your temperature and humidity into the corresponding boxes to calculate your VPD. There is also a chart that goes along with your VPD number. The key to the chart is, keep it in the GREEN! Take your corresponding temperature and humidity, follow each until they intersect and that tells you your VPD (this is if you don’t want to use the VPD calculator) if you already have your VPD from the calculator, find where your number lands on the chart. If it’s in the green, you are good to go!
Understanding VPD
You can think of VPD as a way to increase or decrease the movement of the water inside the plant. A lower VPD will move the water inside the plant slower, and a higher VPD will result in the water moving faster. This is important because when the plant is young, it can’t move water very fast through its vascular system, as it hasn’t developed yet. So, a lower VPD at the younger stages is going to result in optimum growth at that stage. As the plant gets older and larger, it will be able to take a higher VPD.
A general rule of thumb when it comes to VPD is:
Seedling/Early veg 0.4-0.8 is ideal
Veg/Early Flower 0.8-1.1
Flower 1.1-1.3
If VPD is not something you have ever thought to use in your garden, we highly recommend it. We do understand it can be challenging to achieve the perfect environment, but this at least gives us a goal to shoot for. As always, check us out our cannabis seed bank online, and follow us on our Facebook, Reddit, Discord, and Instagram!