Don’t Understand VPD or what to do about it?

I am in mid flower and maintaining ~80 degree temp with ~40% humidity. However my VPD is over 2.

I have an ACI 2x4 tent with ACI exhaust and oscillating fans (3). The controller sensor is canopy height.

I’ve read a lot of old posts and googled it. I find VPD confusing and I don’t know how to drop it and still maintain (roughly) 80 degrees and 40% humidity.

My plant is growing like crazy and no issues. Should I just ignore the VPD?

Any advise?


I ignore VPD and keep it simple by shooting for >50% in veg and < 50% in flower.


:point_up_2: @Doug79 you’ll drive yourself crazy chasing VPD numbers :love_you_gesture:


I tried to keep up with VPD manually for about three days before I realized that if you stay in the commonly recommend temperature and rh ranges it’s close enough. Its definitely not something you have to worry about, but if you get down all the other aspects of growing then a smart exhaust is the most reasonable way to dial in VPD. I just got an AC Infinity smart exhaust and it keeps VPD pretty close to perfect.


A lot of good info in here:
VPD Guide

It sounds like you are confusing Room VPD numbers with Leaf VPD numbers/ target numbers.
The 2.1 VPD at 80f & 40%RH is your Room VPD. But VPD target numbers are (should be) Leaf VPD numbers. The target numbers for veg/flower, & the VPD numbers in the cells on the charts should all be Leaf VPD numbers. The only time Room VPD & Leaf VPD are the same, is if the leaves are the same temp as the air. But the leaves are most likely running cooler than the air temps. You need to know your leaf temps, your air temps, & your RH to calculate your Leaf VPD. You can plug all of those numbers into a Leaf VPD calculator. Get an IR temp gun & start measuring your leaf temps across the canopy. Get your RH & your air temps from around the hottest leaf temp areas. Some calculators ask for your leaf temperature ‘offset’ instead of the leaf temperature. Subtract your air temp from your leaf temp to get your leaf temp offset number. Example: Subtract 80f air temp from 70f leaf temp, & you get a leaf temp offset of -10f. You would also use the leaf temp offset number to select a correct VPD chart to use. Each chart is generated based on one leaf temp offset number. If your leaf temp offset is -10f, you would need a VPD chart created with a -10f offset. A -0f offset chart would be a ‘Room VPD’ chart. If you are presented with a chart with an unknown offset, you can run the numbers through the calculator to determine it.
Personally I avoid charts, & I stick to a calculator:

Adjusting any or all of the three factors (Air temp, Leaf temp, RH) will change the Leaf VPD, although you can end up on the same Leaf VPD number as before, even after changing a couple factors around. The math is the math. But eventually you realize that the calculator is telling you where to put the other factor(s) when making changes to one or two of them, when you are moving numbers around trying to calculate for a target Leaf VPD number. This is an intriguing aspect since you could avoid having to worry about ‘where should my air temp be’ & ‘where should my RH be’.
That being said, hitting Leaf VPD targets can be tough or impossible. But following where your Leaf VPD really is & where your leaf temps really are, are two good reasons to start tracking leaf temps & calculating the Leaf VPD. You might find that your leaf temps & Leaf VPD are right around where you want them to be.


Thanks for taking the time to explain this. I get what you are saying and will give it a shot.


If youre using the controller 69 i can show you my automations. They let the exhaust control the rh/vpd in your tent hands free. I started chasing vpd and practically ruined 2 grows chasing numbers. The controller is designed to do this for you.


Being a simple grower I just dont chase perfection in many places. Vpd is 1 of them. There is definitley an overthinking and controlling issue when starting out. Best advice I have is get where you can easily and soldier on. Cannabis is a hearty plant and Mother Nature has not 1 single control in place for environment stability. Never has and never will. If your plants have good soil/nutes/ airflow and light you are usually golden. Ph issues occur far more often than a vpd issue.


Yes, please share your controller setting. I do have the ACI 69’controller


I take the approach, as most others do, higher temps and higher humidity at the beginning and lower temps and more importantly, lower humidity at the end are preferred. I see the most difference in growth when temps & RH are higher during seedling through veg stages.


@Doug79 here ya go. Hope it helps

Wrong post one sec

1 Like

Ok here ya go. Its the basic setup for running automations. My system just updated again so i imagine i have some settings to play with whenever i get them.

The only thing to do other than whats on this is set your times on and off. The system will adjust everything else. Also dont bother setting up your fans for automation. I just run mine around setting 4, 24/7.