Grab a coffee.
VPD - It pops up as a topic from time to time. Here is a quick rundown on my understanding of it. Totally open to conversation and debate, as I haven't gone deep into it.
I researched it and thought "Holy Cow! Why isn't this more talked about?!"
Its like the one thing that unifies indoor growing in any situation, and you can download a calculator for free to remove the math!
You can literally adjust to any situation. Its a way to speed up or slow down water transpiration through the plant, and water carries food, so more food means more plant.
Vapor Pressure Defecit - VPD.
It is the difference between the internal water pressure in a plant, and the humidity in the air around the plant. Water always moves from wet to dry. In through the roots, out through the stomata (leaves).
So if your light is adjusted properly (correct rate of photosynthesis) your full leaves at the top of the plant will be 2 degrees cooler than ambient air.
So lets assume that is correct. Now the plant is photosynthesizing correctly.
VPD can speed up or slow down that correct rate of PS (photosynthesis) simply by raising or lowering the RH of the room, or by raising or lowering the temperature of the room, or by moving the light to change the rate of PS.
Its a triangle of adjustments. Its complicated without a calculator, as it has a formula which we wont bring up, because VPD calculators are free.
So if you are in Canada like me, and its colder, you could easily have a 72F environment in your tent. If you are in Florida like Jon you could easily have an 85F environment in your tent. VPD will allow both scenarios to grow weed the same.
Cold Canada
Hot Florida
As you can see, the end result of .98 kpa is the same in both scenarios. The plants are both moving water at the same speed.
So no matter where you live, you can dial in your atmosphere to make the plant happy. It makes an enormous difference. Once the atmosphere is dialed in so many tail chasing events disappear.
Its a lot deeper than that but in its basics, thats how it works.
Everyone who grows in a tent should study this a bit, then adjust your light to get a 2 degree difference, and then adjust your RH to get the desired VPD.
If nothing else at least you now know how high to hang your light
.
I have my exhaust fan hooked to an Inkbird RH controller that is wireless to my phone so I can adjust this any time I please. If temps spike while Im travelling, I adjust. If they plummet, I adjust. The food keeps flowing optimally.
If you have never used VPD, I suggest you give it a go. You will love the simplicity of it. All you need is an IR thermometer gun.
Costco sells them for about 20 dollars.
Now heres the good part... You check your leaf temps on your 4 plants.
Plant 1 - 2 degree difference.
Plant 2 - 2 degree difference.
Plant 3 - 2 degree difference.
Plant 4 - 1 degree difference.
Plant 4 is running hot. It literally has a fever. Now you can try to fix it before you see any detrimental signs. I check temps every day at about the same time. Usually if a plant runs hot it needs water.
Whooda Thunk?
We put so much blood,sweat, tears, and money into our plants, and for 20 bucks at Costco we can now improve their lives drastically, and be able to listen to them.
You can download VPD charts all over the 'net. They are all the same. Grab a chart, a calculator, and an IR thermometer, and go drastically improve your grow from day 1 to harvest...... for 20 bucks
Now when you hear someone say "I must have a problem, my plants aren't moving water" or "This grow is wicked good! These babies move so much water!" this is what they are talking about.
When I can't keep the RH in my tent up to the desired level, I know its time to water.
Proper water movement creates its own RH. A controlled fan removes the excess.
If you have powdery mildew, this is a great way out of it.