Coffee time, go grab a big mug. For those who click the link in the middle of this long winded post, everything is about to fall into place.
As you guys n gals know, I use VPD a lot. I try not to talk about it too much because it's mysterious to many, and quite often scoffed at as a secondary thing you can do to take your plants from 90% to 99%, or something like that, and then there is the misunderstanding arguments that arise when someone thinks they understand it and they don't and and get defensive when you try to help them.
If you try to explain it, eyes gloss over quickly because it runs on physics, which are both confusing and concrete. Once you get it, concrete makes it so easy.
Well the truth is, VPD IS what grows a plant. People think it's light, which is definitely a part of it, but VPD to a plant is what a heart is to a human. Literally.It is what runs a plants vascular system.
So for anyone who wants to know what VPD's effects are on a plant,
Here is a pretty good summary of all the parts.
It's a fairly long read but it repeats itself a lot to remind you of what's going on as VPD interacts with the plant, so be patient if you decide to read it, and please read it all.
Pay very close attention every time carbon dioxide is mentioned, or stomata, because CO2 to a plant is the same as O2 is to us.
Without intaking it the plant will die, and if it's partially compromised the plant will suffer. Unknown deficiencies you can't fix no matter what you try will arise.
VPD controls stomata. Stomata control all intakes except for light, so it controls CO2 intake too.
If VPD goes out of whack then CO2 is automatically compromised, as is nutrient circulation, and light is the photosynthetic driver that requires nutrients and CO2 to work, so light is intimately linked to it.
VPD matches light and nute intake so you don't get deficiencies or nute burn.
If you are going to read it then please remember that increasing light raises leaf temperature without raising air temperature, and decreasing light lowers leaf temperature without lowering air temperature. The radiation in light does this.
Leaf temperature is a value that you input into a free VPD calculator, and if you have one, or you go get a VPD calculator, make sure you get the right calculator. You need the one with 3 inputs. Air temp, leaf temp, and RH. The 2 input calculator is for weather. It uses a different VPD formula.
For those of you who have been following along for the last year or so, you are ready for this now. It's a long learn, but once you get it it's amazingly simple and you can fix many deficiency/issues simply by altering room temp or RH or light intensity (leaf temp). You are about to become an elite grower. It's why we came indoors.
It is entirely possible and very common to have deficiencies arise even tho PH is perfect and food is abundant, and you are about to find out exactly why.
It doesn't touch on light much but light is the easiest part, it's only used to raise or lower leaf temp. When leaf temp is optimal so is light. You strive for a leaf that is 2 degrees F lower than air temp as the physics hiding in the background performs most efficiently there.
This will also allow you to match your PPFD to your nutrient flow for the stage of the grow your at and keep it dialed in perfectly as the grow progresses. From start to finish
Even if you don't want to use VPD, just knowing it will help you when you can't figure out why things suck.
It's also why and how plants pray, so thats always fun.
We can talk about it all winter long to get it straight, but please read this article 1st, and then any others you wish, as there are lots of great ones. This one breaks it down better is all. It's also amazingly adaptable so if you have high RH or low RH or whatever, VPD doesn't care you just raise or lower room temp and/or light intensity and it all settles back to the sweet spot.
If you use CO2 you absolutely need to know this, or your plants may not be using the CO2 you are supplying.
.
.