When to target temperature & humidity vs VPD - working with weather

This is a solution I recently decided to implement. I wanted to share in case it helps anyone else, and get input from all you pros if there’s way to change or improve, or anything I’ve overlooked.

Quick background / equipment
ACI 3x3 tent
ACI T3 humidifier - outside tent
ACI T4 exhaust fan w filter - inside tent
ACI controller 69 pro
Small block heater - inside tent
Small 1500 ml dehumidifier - inside tent

This set up is in my garage, which is not insulated and has no climate control of its own. I’m in SW Ontario.

Normally i don’t try to target VPD with my controller, based on advice I saw on here. I target humidity and temperature, which should hit an ideal VPD.

Right now I have a dozen new plants, about 2-3 weeks since sprouting. They’re in full on vegetative stage.

I’ve been targeting 77 F and 60% humidity. My exhaust fan is at a minimum setting of always on 1.

Last few days the humidity has been around 68-75%. No way my small dehumidifier can bring me down to target. So thought I would try to target VPD with my heater.

Humidifier is still set to a minimum of 60%.
Heater is now set to target VPD. At first I had it set for 1.1, but found it would range from 1.1 up to 1.3. I brought the target down to 1.0, and mostly it’s kept it between .9 and 1.1 for the last 2 days.

Exhaust fan is still at a minimum always on of 1. It triggers at an upper temp of 85F.

Since the humidity is set to a minimum lf 60%, I know the heater’s VPD target means it won’t get too cold.

This has worked great for the past 2 days. When humidity starts to rise, the heater kicks up. This brings up the temp up and at same time brings down the humidity, working on both sides to bring towards at an ideal VPD. It adds a bit wider range to temp and humidity than is ideal, but I’m not fighting the less than ideal weather conditions.

I haven’t seen the exhaust fan triggered on by the upper limit of 85F yet, though the tent has got real close to 85F.

I’m not saying this is the best way to go. I do think it’s a way to keep the plants happy when you don’t have absolute control over your lung room.

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You want to be adjusting your leaf VPD, target numbers are Leaf VPD targets, not Air/Room VPD targets. Although Leaf VPD can be the same as Air VPD, if the Leaf Temperature Offset is zero. You want to know your Leaf Temperature Offset ( Leaf Temp - Air Temp = Leaf Temp Offset ). Once you know that, you can pick the right static chart &/or program the offset into your controller. Otherwise you might be close, maybe not. You don’t really know until you know what your leaf temp offset is. That all being said, if the plants are young or smaller, then you probably don’t have to worry that much about it, & just keep them in ‘infirmary mode’ where -within reason- some higher RH & lower temps aren’t really going to matter that much, & just water regularly. That’s kind of what I found with my small box with a couple small plants where I try to keep the growth slower until I’m ready to use them. Also in my main box with larger plants, I’ve done what you are trying with running up the heat to combat high RH & drop the Leaf VPD number, & what I found when measuring the leaf temps was that at least to a point, the leaves were keeping themselves cool & the leaf temp offset stayed around the same or didn’t close up as much as I thought it would, meaning that the Leaf VPD didn’t end up moving all that much. Also, if you check your leaf temps you can also try to keep them in an optimal range. What is that? I don’t know. I usually think around 70 to 80f maybe.

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Thanks @PhotoFinisH, great response!

I added a guess of a -2F offset, just based on reading about LEDs and VPD. I didn’t know that the offset could change so much. Now that you say it, it makes a lot of sense to expect the plants to make adjustments to the leaves to help keep stomata open as the temps and RH vary.

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Not to say that your guess is wrong, or that your numbers didn’t end up reacting the way you wanted them to, but who knows unless you see what the offset is doing.

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I didn’t think you were saying I was wrong - though I doubt my -2 offset guess was correct.

I’m glad you pointed it out. I had assumed the offset was constant, but clearly it isn’t. That also makes sense why people say chasing VPD is tough or doesn’t seem to work as expected.


Maybe your offset is constant -2f. Easy enough to check how it runs though. I know some of those controllers have a plug-in IR sensor for it, or you can get an IR gun. Just check that the efficacy is around what you need for green leaves, or get one that adjusts to that number. I think it’s around 93, but I’ve heard a couple different numbers.

If you also use static charts, you also should match them to your leaf temp offset. Since that is a pain if the offset changes, you can use an active chart & plug in your RH air & leaf temp measurements, & it will adjust the chart according to the leaf temp offset that it calculates using your inputs.
Active VPD Chart

Don’t know what the leaf temperature offset of your static VPD chart is? Just pick a cell & plug its corresponding air temp & RH into a leaf vpd calculator, then adjust the leaf temperature until the leaf VPD matches the Leaf VPD in the cell that you picked on your chart, & the number that you moved the leaf temp away from the air temp would be the leaf temp offset of the chart. Could be a negative or positive number, or zero.
Leaf VPD Calculator

Hi there… we should be friends. New greenhouse setup this year, southern MN. Starting with a few test plants to get the kinks worked out. Wrestling with the same issues.

My heat was spiking too much through the spring so I implemented a fogging setup recently. Fogging works great! The evaporation holds the temps lower and pushes the humidity high and everything is in line, With a new 50% net on top of the greenhouse this week I think I’ll finally have a solution to most of my temp/humidity issues for most of my grow.

The real challenge is coming right now. I think I’ll still need to push my humidities high (70%) to get my VPD in line. But now I’m starting to flower and am getting really concerned about mold. Might even have some signs of it starting.
So I’m drying the whole thing up to keep that in check.

Worried… what do I do? push my vpd into the high 1’s and low 2’s, not optimal for flowering or push the humidities into 60’s and risk mold? Maybe the only solution is A/C but I’m already pushing the electrical limits.

Would want a little more info, what are your real numbers at the canopy? Leaf temp, air temp, RH? Are you tracking & adjusting your Leaf VPD or your Air VPD? One way is ‘DIR’, the other is ‘DIW’.