Yes, I like this guy. . .thanks!
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There’s a bunch of great replies already. The “how thirsty the air” phrase by @Growgrassblowglass is a great way to look at it. @GYOweed touches on some critical aspects too.
Water movement in plants is a key aspect to a plants energy balance across it’s lifetime. VPD helps show how much water air can hold at various temperatures and relative humidities. As air temperature increases, it’s ability to hold water increases. Why does all this matter though?
When put into context, under normal circumstances, water wants to move from areas of high potential to areas of low potential. It basically wants to make drier stuff wetter. For example, when mopping up a spilled bucket of water one would find a dry mop would work better for soaking up the water than a wet mop would. And when mopping something up, as the mop starts to become saturated it’s effectiveness diminishes until it’s wrung out to remove moisture, effectively a dance of water potential.
This water potential dance happens with plants too. The surrounding atmosphere and it’s ability to hold water influence the water flow in plants which relates to it’s unique balance of energy. Differing morphology influences things like the rate at which water transpires. Things like how thick or thin the leaf is, how many leaflets along with their shape, stomal density, and such. Genetics and phenotypes will be a significant factor in which temperature ranges and relative humidities a given plant will perform best at. Just because the VPD graph says there are “acceptable” ranges down in the 60’s doesn’t mean a long flowering tropical type with high stomal density will perform best in the 60’s, but if that’s the temperature you’re working with then that’s the temperature your working with and the VPD graph is helpful for dialing in best to ones ability. Noticed @BudLarfy touch on this in their comment and think there’s a lot of wisdom in that comment. Similarly, varieties from higher latitudes that have thicker leaves with wider fuller hairy leaflets will likely have a lower tolerance threshold as temperatures increase compared to a variety from lower latitudes with thinner more slender leaves that are much less textured and basically smooth feeling. All of this is to consider plants have many leaves, all of which have slightly varying temperatures and size and shape and such. Things like this, to me, show the incredible nature and complexity of plants. Their supreme ability to grow, build, and forecast. Plants are awesome. Many blessings and much love
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Follow up question:
Indoor growers adhere to the VPD guidelines, I’m sure – but how do outdoor growers do it? Outdoors, you’d think VPD is more often than not completely “wrong.”
Kinda like darkness. Full moons don’t interfere with flower, so why does everyone lose their shit over blacking out tents 100% during the dark period?
This world of cannabis growing is a different one. . . 
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When it is humid and will be for a few days feed heavy.
When it is really dry and it will for a few days be feed light.
I don’t and I have no issues with intersexed plants either.
Why?
Because that is bro science.
Sure if you are growing plants with known intersexed traits it may help to keep the tent 100% dark.
But if you are not growing intersexed plants you should not have any issues with low light at night.
Some folks still believe plants don’t use green light, but hey do.
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I do both. Outdoors 107 avg 105-120f about 2 weeks they take it like a champ if you use aerated media in fabric pot and keep it wet, add silica, and feed it well (not less) to keep it green and the leaf temps will vary but youll literally feel your arm cool in a bushy one so now your closer to vpd range, however, i bring in the same plant even mimic the hourz with efficient leds in right vpd ill have a bigger plant.
Guidelines
P.s
Rh usually around 10% lol higher early morning a bit
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sorry, let me restate my question: how do outdoor farmers achieve a perfect (1.0) VPD? I suspect (theory only) that they don’t and they really can’t. They certainly can adapt their watering and feeding, and make other adaptations. But they can’t achieve 1.0. They can’t even get close, because it is outdoors. And that’s okay. They still grow great weed. Still with me?
IF that is all generally true, then I could (if I wanted to) let my indoor VPD go willy-nilly and I’d still grow great weed, all things being equal. (Of course, if I’m indoors, I CAN control it, so I do. I get that. I’m just posing it this way to ask my question).
Still with me?
As a beginner, I’m trying to learn, which means I want to understand exactly how much I should worry about VPD? How much should I prioritize it?
So anyway, if all of the above is GENERALLY true, then a prioritized list of growing requirements generally looks like this. (Let’s assume “basic” soil and “basic” sunshine for now. Optimization is a different level of care (water is #2, but pH’d water is #13). Make sense?)
- Soil
- Food
- Light
- Water
- Air
- Temperature
- Humidity
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Let me add one thing: this whole exercise, for me, is like when you want to really pay off debt.
You round up all your credit cards.
You rank them in order, from “highest interest rate” to lowest, and pay off your debt that way, ending your most costly debts ASAP.
Accordingly, my ranking systems is for stuff like, “Hey Self, you understand BASIC nutrients, but you should really prioritize learning more about NPK, rather than fussing over that other thing.”
It’s a way of keeping myself oriented as I learn – an answer to the inner, nagging, doubtful question, “Dude, am I even thinking about the right stuff as I try to make each grow successively better?”
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@navy66
I was with ya the whole way.
You got it right.
Get the important stuff in line like light and feed.
Get VPD kinda close for the time being.
Once everything is perfect or near perfect move forward making everything else perfect.
Most of the stuff like VPD works toward increasing yield.
So if that is important to you and you are squeezing every last bit you can out of your grow, then VPD is probably more important to you than it is to me so to speak.
So that should be considered also.
Something else to consider here.
I like a 1-2 day dry back cycle.
I must assume the air needs to be quite dry for that.
I personally would stick to my dry back cycle rather than chase VPD.
If the plants don’t dry fast enough I crank up the dehumidifier.
If they dry too fast I turn it down some.
To chase VPD I will need to constantly adjust the temp to meet the numbers on the chart.
Now what happens in a tent?
If you exhaust constantly like I do the temps and humidity reflect the climate outside of the tent.
If you do not constantly vent, then when your fan shuts off your humidity and temps go nuts.
When the lights go off the temps change dramatically and humidity will rise.
So now that needs to be considered in the equasion also.
For me it is a lot of extra stuff to think about for a little extra weed.
Now if you are doing huge rooms of course you should get your VPD dialed in.
Not doing so could cost you big in the long haul.
But how much yield would you lose in a 10x10 area, I would say not that much if everything else is dialed in properly.
But that could be very important to you, and I take no issue there.
Food, dry-back, light ect. are more important to the little guy.
Large scale cash croppers play by different rules.
We can adopt most of their ideas but some of them don’t transfer well to a tent ect.
Think about things like this.
First you grow roots.
Then you grow leaves and branches.
Then you grow flowers.
Then you ripen.
If you adjust your feed a bit to meet the needs of the plant at each cycle you should be rewarded much better than if you kept the VPD perfect and neglected the plants needs concerning food and light requirements.
For reference, I have been growing since the late 80’s.
But only small rooms and tents, never a commercial grow etc.
As I said… that is a whole different animal.
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I have a friend that runs a grow in a huge corporate facility in a desert. He says his VPD is all over the place and he he sees very little difference running thousands of cloned plants in different scenarios over time in regard to VPD. We just talked about the other day oddly enough.
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With great mass of plants comes great humidity 
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With thousands of clones we can even speak about a microclimate ^^
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Dam @fuel you beat me to it, you are fast on the draw today. 
BTW
I think @Fuel said pretty much the same thing I said but in a much more intelligent manner. 
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God for one time i succeed at it shortly, i will print the post ^^
You’re disadvantaged, full time OG today. I’ve to keep the soul busy.
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Every time i go down to my basement grow i keep forgetting to take some pictures of the 2 4x2 tents hooked to a wooden box over an oil heater sucking heat into the intakes, 2 humidifiers running in seed starting/veg tent, humidifier in the basement lung room, and a 4x2 heat mat on the floor of the flower tent to keep both tents happy in a cold dry basement 
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In one basement lab far and deep in the mountains, i poured epoxy all over the floor and up to one feet in the walls with rollers. It changed totally the game. Cheap and fast. 3 millimeters thickness is enough.
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Non important post
Thought you meant Vancouver Police Department. Bahaha
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I don’t pay VPN any mind.
I feed 3 times a day no matter how much my plants are transpiring or eating.
I do start to pay attention to humidity during the last month of flower and I shoot for the lowest % I can.
During winter it stays below 15% lights on and off.
During the summer it runs at about 35% during lights on and about 45% lights off which gets a bit dicey for keeping bortyris at bay.
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If I never told you, thank you for this. Great reply.
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One aspect of VPD that doesn’t get the attention it deserves is the part where the rubber meets the road: Leaf temp.
Charts and calculators rarely take in to account this 3rd parameter, arguably the one that really matters. Here is a calculator that does use leaf temp:
Play with the leaf temp slider and watch what happens. What might seem like an ideal VPD can be really far off.
That leaf temp shows us the end resulting effectiveness of the entire transpiration chain. Just like our skin cools when it sweats, leaves cool as they ‘sweat’ water out of their stomata. A previous description of VPD as being “how thirsty the air is” is excellent! Another way to think of it is how hard the air pulling at the stomata for water. Remember the plant can’t push water out, all it can do is open the windows and wait for it dry out.
At some point, the plant will close stomata if the air is pulling too hard. When that happens, leaf temp will often be at or slightly above room temp.
All plants will shut the windows at some point to protect themselves from drying out. Where that point is varies incredibly wildly from cultivar to cultivar. Using a 30 dollar laser thermometer, I have seen girls shut down around 5 degrees below ambient, and have seen others go past 15 degrees and still act like they want more.
Just something to consider when using VPD to make decisions. Consult with the plants by checking their leaf temp, you know, get their side of the story.
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Thanks! The information I’m getting on these responses really confirms what I initially suspected: people make WAY too big of a deal about VPD. I get your point about ensuring we measure properly, but since now I’ll not be worrying too much about VPD overall, I can’t ever see myself losing sleep about VPD at the leaf level.
Which is good, since my old brain is already full. 
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