VPD - Overview
Until the last couple years, Vapor Pressure Deficit, was measured mostly by research universities and agricultural laboratories. The tech available to growers these days has enabled us to dial a grow room in very accurately with regards to heat and RH. We all know that cannabis can benefit from an environment between 70F- 85F or 20C - 30C. We also can benefit from slightly colder temps at night and this is encouraged as it promotes maximum growth during lights out and the potential for certain genetic properties of a particular chemovar to manifest such as purple or pink buds, etc. Temperature and Relative Humidity will allow you to grow cannabis, and grow it well, if the parameters are maintained. When we keep our plants in the "sweet spot" of VPD, the stomata of the leaves are manipulated in a way that allows maximum performance from our plant. One analogy is that the VPD happy plant is like an athlete, whereas a plant outside of the ideal VPD zone is just strolling through the park.
I'd like to first state that I was urged to look into this information from fellow growers @Pennywise and @InTheShed . I am sharing what I learned, in my own words, based off the research I did and then began implementing in my own grow. After dialing in the environmental numbers, my plants looked completely different in less than 24 hours. They were perkier, healthier looking, and just plain looked ready to burst with growth. All anecdotal, of course, but if you have the capability to manipulate your temps and RH, you need to try this!
In order to understand what VPD is, we need to define the variables that go into calculating the VPD. Water vapor is the gas form of water. Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature.
The air inside a grow space can hold a given amount of water vapor relative to the grow space temperature before it condenses to the liquid form of water. As temperature increases the SVP rises and as temperature falls, so does the SVP.
This is why we get dew in the morning after a cool night. The SVP is low, because the temps are low, but the Actual Vapor Pressure ( AVP) is higher than the SVP and boom! We have dew all over everything as the water vapor turned to liquid form!
Did you know that Relative Humidity or RH is actually a comparison of the amount of water in the air relative to the maximum amount the air can hold? That's why we measure it in percentages. 50% RH means that at a given temperature, the air is holding 50% of its maximum amount of moisture possible.
So if Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature, and we know Relative Humidity, we can determine the Vapor Pressure Deficit.
Put simply, Vapor Pressure Deficit is the difference, or deficit, between SVP and AVP
VPD= SVP - AVP.
But Archiweedies, I dont want to math! Fear not fellow growers, you only need a couple things to start dialing in that environment so that you can take full advantage of this technique.
What you need:
Laser Temp Gun I got mine at Harbor Freight for quite cheap. We need this to determine our leaf temps at the top of the canopy. We can't assume that ambient is the same as leaf temps. Remember, the point is to get the stomata working so we care about the leaf temps at the top of the canopy.
Hygrometer - You should have one already! It would be nice to have one with a probe that can be placed at canopy level to get accurate RH measurements.
VPD chart This chart is courtesy of Just4Growers.com There are many online and I suggest you find one you like and print it out for your grow room. The example below is a VPD chart that assumes a Leaf Temperature Deficit of -2 degrees Celsius below ambient.The chart you will need is going to be based on how hot your lights are and what the difference is between ambient grow room temps and leaf surface temps. Ive found, with my LED COBS that they run roughly 2 degrees Celsius cooler.
Now that we have our leaf temperature and our canopy RH, we simply cross reference the chart of your choice. If we are in the red zones above we are outside of the sweet spot for VPD. Once outside of the sweet spot and your plants are not transpiring as efficiently as they could be.
If you are outside the acceptable range, simply determine if it would be easier to manipulate the RH or temperatures to get back in line.
Example: I check my hygrometer and it says 25C. I check the leaf temps they are at 23C. RH is at 37%.
This places us just outside the VPD and it is up to you to determine if adjusting your RH up or your temps down makes more sense for you. For me, and this depends on your unique environment at the time which of course changes daily, I would bump RH up to 45% if I was in late veg/preflower. In flower I would likely choose to maintain the RH and lower temps a couple degrees Celsius to bring it back into line.
Summary
Dialing in the VPD allows us to turbocharge plant growth by allowing the plant to transpire efficiently. When the plant is transpiring more efficiently, it is drinking more nutrients from the roots. I think that is the goal of growing cannabis. Grow roots in veg and use those roots during bloom to produce flowers. By giving the plant a happy environment with the perfect VPD balance, we accelerate the plants growth across all phases of growth.
Thank you to the wonderful community here at
. I hope that by presenting this information, I have in some way given back to the community that has given me so much over the last few years.
but I think I know which one is the most accurate 
That’s a good damn question... I don’t have a good reason. 
. The only thing I got is that by forcing myself to practice Celsius I’m breaking down American stereotypes. I’m a package deal, Shed 
If you look into ways to maintain a certain RH in your tent through some humidistat or environmental controller, you will be closer to this than you think. Cheers!