Hehe grab a coffee.
They don't really work together. I use VPD to dial my light in. I set the ambient temp, then the humidity, then I slowly add more light over the course of a few days until I have a 2 degree difference in the leaf to air temperatures. I take my readings about 10 hours after lights on. I don't use it on seedlings, they have no roots yet, so they just get warm, humid, and slow for awhile until rooted.
As I dial in the light intensity I may have to raise or lower humidity to speed up or slow down VPD until it is where I want it.
I had never used PPFD until this grow, so when Jon explained it too me, I figured why not increase the light a bit and see. It took my leaf temps up a bit high but I figured I would give it a day or 2 to see if it settles in. It didn't.
So in the end, VPD is a much more reliable safe method of dialing in your light than just arbitrarily picking a PPFD value and setting it.
VPD will tell you what light intensity is correct for the stage of the grow you are at, a number you assigned as a PPFD target.... not so much.
Now that I know Durban Poison likes 1000PPFD I have a starting point for my next DP grow, but every strain wants a different PPFD so 1000 may not work for these TMSC seeds, or your grow. Every strain that I have ever grown does like the same VPD rates through the stages of development though.
To be honest, I'm getting away from PPFD for initial setup and going back to VPD. I will track PPFD for sure, to know for future grows, but I don't want to run CO2, so going over what VPD dictates is pointless. It causes stress and deficiency.
Your environment is Earth, Wind, Fire, and Water. PPFD won't find the balancing point of the 4, but VPD sure will.
So after setting air temp and humidity, you just adjust the light to find the 2 degree difference as the grow progresses, and if ambient air temp is good, you control speed thru humidity.
So pick a room temp and pick a humidity to compliment your current stage of grow, and then slowly turn your light up until at the highest point in the day (about 10 hours after lights on) you are at a 2 degree differential between leaf and air temps.
Then take a PPFD reading so you can quickly dial in VPD next time. Then wiggle humidity to dial VPD in to perfection. If you get a temperature spike, raise humidity. If a cold snap lowers VPD, add heat or lower humidity. The light doesn't need to move unless the 2 degree rule gets compromised.
So what does VPD really do? I hear a lot of good growers toss it in the ditch saying it does nothing useful.
Well it does do 1 thing that is kinda important. It matches the amount of light needed to drive a plant under the current air temps, to the maximum speed it's rootball is capable of maintaining, without over driving the plant and causing the rootball to not keep up and form a deficiency, and on the other side of the coin, it stops you from under performance, as you may not be pushing your plants to the maximum the roots can sustain, so you are under achieving.
It also gives you a throttle. If you are going to do some stressful stuff, like topping or pruning or de-leafing (ouch!) add a bit of humidity the day before, slow them down a bit, stress them with your procedure, then a day later wind them back up.
Don't ever go as high as 1.5. You may think you are a good enough grower to handle it, and your plants are tough enough, and that could be 100% true, but one temp spike or an AC stopping, and your leaves are compromised for the remainder of the grow, so try to not exceed 1.45.
And thats in late flower when everything is rolling perfectly, not 3 weeks into veg when it should be .5. You haven't got a rootball big enough to feed high photosynthesis yet.
And the best part, you can adjust quickly, easily, and daily, to what the rootball is capable of.
So if you don't over rev your plants and a potassium deficiency shows up, its ACTUALLY a potassium deficiency, not a reaction to exhaustion. Less tail chasing.
Now you can run your plants at the proper speed and adjust future soil builds to support those proper speeds.
Higher VPD means faster transpiration. Faster transpiration means more water flowing thru the plant so more food as well.
If photosynthesis is working faster than that flow (light is too close) a problem will arise as you need supplies from the roots to photosynthesize. A deficiency occurs.
Now that the plant can't photosynthesize properly, heat builds up in the leaf and light poisoning begins, (but you will get a batch of free feminized seeds) as the plant can't defend itself at that speed.
Or it's all too slow, your soil is always wet, and your plant has bugs as a result.
A well fed plant under optimal environmental conditions won't have damaging pests.
Other than being the throttle to the entire grow, it doesn't really do much.
So in a nutshell, I will use VPD to set my light intensity, then take a PPFD measurement for a reference point. Definitely not the other way around.
This may lead to lots of questions about VPD so ask away and let's kick it around.
It's actually really simple in concept and application, but incredibly complex as to what you can do with it.
Why wouldn't you want full control? Isn't that why you came inside? for the environment?
To not optimize it?........ well outside is free.
And if you use CO2, or want to, all good CO2 meters are tied to VPD.
The wrong VPD with CO2 will yield less than no CO2 on a properly grown correct VPD plant.
If you are using CO2, you better know what your stomata are doing.
And don't panic if VPD is way out an hour after lights on. Plants raise their leaf temps in the dark to close the gap between leaf and air temps to regulate transpiration in the dark.
Check VPD any time you like, but make adjustments only on the 10 hour reading. Thats the fastest VPD of the day, the one you don't want to exceed.
VPD is THE only reason my last disaster of a grow gave me such stellar buds. Even after leaf damage I maintained the plant at it's fastest possible safe speed for the leaf damage that had occurred. That and the fact that I still had al 9,456,321 leaves left to draw from.
Never did use them all
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Gotta go bigger next time I guess, there was still gas left in the tank.
good lord this is a lot. I don't know why people use CO2 or what VPD in fact is. I'm glad in a way that my grows are so imperfectly carefree!
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