Is VPD a catch 22??

So I’m in the last weeks of my first ever total indoor grow. I’ve grown plenty outside and only used fluorescents to start things off before moving outdoors. I’m just learning about VPD and how to get my space dialed in for the best conditions.
I’m using a HLG 320xl LED and it really shines...pardon the Pun.....it’s a great light and I’ve not heard a bad word about it.
I’ve always thought to run comfortable temps with higher RH for seedlings, maybe a tad lower for veg, then reduce RH to <50% for flower. I’m told the lower humidity decreases chance of mold or rot, and I’ve always heard from successful growers that it was beneficial for the buds and possibly helped with density, or something to that affect. What I do know is that every source I’ve seen has maintained that lower humidity in flowering is the unquestioned standard.
I am now being told by several trustworthy folks that my 70-72f degrees (with lights on) would be much better 8-10degrees warmer. Perhaps it’s the LED that performs better with higher temps? You tell me... I did raise my temps and they are hovering around 76f...and I have to work to get my RH up to 42-44%.
So the issue that I’m facing is to achieve an optimal VPD with 80degrees temps, a much higher RH is required. If I flower with a RH of 40-45% the VPD chart says that RH corresponds to a temp of much cooler than 80degrees.....actually below 70f. In turn, if I heat my space up to 80degrees, optimal VPD is achieved at around 60% RH......which is higher than I’ve ever seen suggested for flowering.
The chart tells me that my VPD under my current settings (76f degrees and 42-44RH) is on the high side. I think the number was around 1.7-1.8.
So what gives ?? Run the higher temps/higher RH and just make sure your airflow is really moving??
My space is a closet in a small guest house on my property. It’s tough to keep those conditions from bouncing around. Maybe I should list my questions:
1. Do plants do better under LEDs with higher temps?? Is 80f the benchmark?
2. Does the lower RH have any benefit other than decreasing mold/rot?? That’s a pretty good benefit by itself if you ask me.....bud rot sucks.
3. would increasing or making sure your airflow is spot on allow you to safely run your RH at higher levels (during flower)??
4. Which of these conditions is most important?
My little grow of 3 autos is turning out decent....the yield will be ok, with the quality looking excellent.
But if it could be much better by ignoring generalities and targeting the best VPD, period, then that’s what I’ll do. I mean just b/c I’ll harvest some really high quality buds doesn’t mean there isn’t a ton of room for improvement.
Thanks for the input and sorry for the long winded post.

I can tell you that I ran 64-65% Rh with lights on, and 72-80% at its highest at time and had no issues with bud rot. I did have a ton of air flow in there. I think that’s the key along with fresh air exchange to maintaining the high Rh Without any issues.

You need to be concerned with leaf surface temp, air temp is largely irrelevant to the plant.

The leaf surface temp will be LOWER (usually 3-5f) than ambient air temp with LEDs due to transpiration, so the air temperature can be higher while maintaining a good leaf temp.

Low-mid 80s air temp under LEDs is optimal.

Billy the Mountain said:

You need to be concerned with leaf surface temp, air temp is largely irrelevant to the plant.

The leaf surface temp will be LOWER (usually 3-5f) than ambient air temp with LEDs due to transpiration, so the air temperature can be higher while maintaining a good leaf temp.

Low-mid 80s air temp under LEDs is optimal.

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That’s been about where I sit as far at temps.. leaf temps sitting at 78-79 with 82f temps.. even in late flower, and they don’t flinch.

I try to keep my temps in the high 70's. Managing temps and humidity while in a tent isnt the easiest thing to do.
For me its a compromise both in veg and in flower. I have no desire to keep my house temps and humidity ideal for growing.
So in my case, if i want lower humidity i have to deal with lower temps. Higher temps and higher humidity

Boatguy said:

I try to keep my temps in the high 70's. Managing temps and humidity while in a tent isnt the easiest thing to do.
For me its a compromise both in veg and in flower. I have no desire to keep my house temps and humidity ideal for growing.
So in my case, if i want lower humidity i have to deal with lower temps. Higher temps and higher humidity

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For me it’s higher temps results in lower RH.....my little guest house has elec baseboard heaters and I have to crank them up where the main room is boiling to get my closet up to 80....and my humidity plummets..at least this time of yr. I run a humidifier in the closet and one directly beside the door where the inflow goes into the closet.
It’s so telling that my buds are taking forever to ripen. I’m on day 82 since they popped out of the soil, and they need a couple more weeks minimum...just barely progressing. They’re autos and the breeder’s 9-10week timeline is gonna end up 15weeks if I’m lucky. Maybe it’s just the pheno and I know their timelines are iffy at best, but I do wonder if such a high VPD is making them sluggish and slow moving??

I guess I’ll make an effort to bring up both conditions and maybe that will speed em up.

HydroKid239 said:

That’s been about where I sit as far at temps.. leaf temps sitting at 78-79 with 82f temps.. even in late flower, and they don’t flinch.

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Where do you keep your RH?? The chart says around 60% is best at those temps.

Most home hobbyist growers can really only do so much when it comes to 'perfect' conditions. We have tents and cobbled together gear rather than ground-up designed growing environments. When I was struggling, I asked a few long time people and their advice was 'yeah, in a perfect world'...but you do what you can and the temperature side of the equation is the one I was told to focus on and to just 'do my best' with the humidity side.

My VPD during lights out is .8-1.1 and with lights-on peaks at 1.7 (rarely) and averages out across a week to 1.2. At this point I shrug because you can only do what you can do in a tent or closet when you're also heating/humidifying a dwelling space (which really is the priority for most of us). I also know a few people who grow in super humid climates and super dry climates, their VPD is fairly well fucked and they still do it.

Bullmark said:

For me it’s higher temps results in lower RH.....my little guest house has elec baseboard heaters and I have to crank them up where the main room is boiling to get my closet up to 80....and my humidity plummets..at least this time of yr. I run a humidifier in the closet and one directly beside the door where the inflow goes into the closet.
It’s so telling that my buds are taking forever to ripen. I’m on day 82 since they popped out of the soil, and they need a couple more weeks minimum...just barely progressing. They’re autos and the breeder’s 9-10week timeline is gonna end up 15weeks if I’m lucky. Maybe it’s just the pheno and I know their timelines are iffy at best, but I do wonder if such a high VPD is making them sluggish and slow moving??

I guess I’ll make an effort to bring up both conditions and maybe that will speed em up.

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Need a stronger humidifier and/or less exhaust cfm leaving the grow room. How fast is your exhaust running? What is ambient humidity outside typically?

Follow the VPD chart, it doesnt lie. Airflow is key, as @HydroKid239 mentioned. With proper airflow, we should be able to run most strains without mold issues at 55%+ through flower. I rarely go below 50%, no problems.

There is a separate VPD chart for flower and bloom. Are you using the bloom chart. Mine bloom chart shows 1.5-1.6 VPD for 78F and 44%rh. Just at the edge of the acceptable green zone.

fYI my chart is set for 1.5 degrees F for leaf temp differential. LEDs.

I do have the advantage of an H203 generator, but my days are 80/60rh at lights on, and by the time 12 hours go by, Im at 85/65rh. But yeah, as mentioned, airflow is a great factor. Like near wind burn airflow.

I can’t get my VPD in my tent bang on. But getting it closer made a big difference. 20-25% up to 50-55% seen big gains. But acorting to the charts I should be 65-70%

visajoe1 said:

Need a stronger humidifier and/or less exhaust cfm leaving the grow room. How fast is your exhaust running? What is ambient humidity outside typically?

Follow the VPD chart, it doesnt lie. Airflow is key, as @HydroKid239 mentioned. With proper airflow, we should be able to run most strains without mold issues at 55%+ through flower. I rarely go below 50%, no problems.

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I’m kinda figuring all this out. My exhaust is a 6” and really hums. It doesn’t adjust but I was able to find a little switch and thought I had my problem solved. But the lowest adjustment might be 10% lower than max, so it hasn’t helped a ton.
Where I live, in Va, the humidity is very low in the winter, so it’s very difficult to raise both the temp and RH. Just today the temps dropped outside and are well below freezing. I can get the room so warm you can’t stand it but the closet is always a good bit cooler. Then when the exhaust is running it just sucks everything out. I have open containers of water, wet towels and a humidifier in the space and it’s a struggle to reach much above 45%.
If I turn the exhaust off, within 5 min it jumps as high as you’d ever need....but then there’s no exchange of air.
I guess the good news is this grow is almost over and the girls don’t look half bad. And I don’t foresee as big a problem come spring and summer.
I think my buds need about 3 weeks and I’d like them to finish up as strong as possible....maybe fatten up a bit more.
Thanks for the input....you can really learn a lot hanging around this forum.

VPD is a good baseline to follow but is not a rule or law in my experience. My best crops have been at weird VPDs.

These are my current rooms goldy zones.
80 °F / 50% VEG (LED & FLUORESCENT)
75 °F / 45% BLOOM (HPS)

I'm assuming you have a 6" exhaust like below? If so, will need a 3 amp variac to adjust the speed properly, $45 on Amazon. The cheap $25 speed controllers will make the motor whine and damage it. Running at full speed will remove the environment in your room faster than it can be maintained, which is what you're experiencing now. A variac or speed controlled exhaust will allow you to find the balancing point for temps/humidity in your grow.

Some newer inline exhaust models, such as Cloudline, come with speed controllers designed for the unit. Something to consider when the time is right.

Is this what I need?? I think I’d have a better chance of understanding brain surgery before understanding basic electrical applications.

Bullmark said:

Is this what I need?? I think I’d have a better chance of understanding brain surgery before understanding basic electrical applications.
View attachment 5061038

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Yep, this is the one.

Plug the variac into 110v outlet, then plug exhaust fan into the variac. Use the dial on top to adjust the speed up/down, the meter will show you how many volts the fan is running at

Keeping optimal leaf temps under led is more important than VPD. Deficiencies show up fast with LEDs during high rates of growth in a cool tent. There is no way I'm going to go over 50% RH in flower just because I'm growing in my living space, and high RH feels gross. It would be a different story in a dedicated grow room. But yes, I would agree it can be a double edged sword-high RH definitely results in more bud rot, but that can be mitigated with strain choices and lots of air circulation.

I was wondering if I have to make a choice and have one or the other, what route to take. Currently I can’t hold more than 42-45% RH…..at least until I can get a controller that slows down my exhaust. Based on VPD charts, the proper temps would be much lower than 80degrees…..probably somewhere close to 70F….maybe less.
So I’d have a better VPD with my temp/RH at 70f/44%rh….Or raise my temp to 80-82f, which is ideal under LEDs, and my RH falls to 38-40%.
So which scenario is best? 70f/44rh OR 82f/38rh….????
I will say that my air flow and ventilation is pretty good….the plants are on a grill type rack raised about 2 feet off the ground. I have a small fan blowing a gentle breeze under them and an oscillating fan blowing between my canopy and the light…..so the top colas are just barely swaying back and forth.

I try to follow vpd with a humidifier in tent, 79-84 deg 55-60%.
Once buds have formed to sugar cube size I drop the humidity to 45/50 w a dehumidifier in tent-goodbye humidifier.
And temps I try to keep 80-82 untill they get close to done, then drop temps further to 74-78 at 45%
I had been using burple led and cmh
Together.
I have airflow below and above canopy.
Tent in basement, house temps 67
And heaters off in that insulated room.
Summertime I used ac;
now I stop in summer.
I have enough to last.

Having a controller to monitor humidity and temps and turn on/off humidifier/dehumidifier/ fans/ac/co2 at set points is the turn key solution.
Sized right, plug it all in, set your min and max, and your done.

A good start is the ac infinity setups.
Before dropping coin, see if a speed controlled exhaust fan can handle your needs.
Overall I’d say try to match bod in veg to get fast growth then fuck it for flower and drop humidity 55 maximum.

“Rabbit holes”
I have to warn myself often.
This hobby has a lot of rabbit holes to go down. I only have so much predictable free time.
Say no to rabbit holes.