Confused about VPD

I’m uploading a few pics but I’m confused on a couple of things. I’m in veg and I am trying to reach a VPD of 1.2. But if you scroll down it says to adjust temp lower and Raise RH higher to meet my target. Just confused and if anyone could explain I’d be very grateful. Thanks

btw, it will only allow me to upload one photo at a time

Here’s the other picture

As you see I’m at my target but I’m confused on why it says to lower temp or raise RH to hit my target

Hi Mike - the VPD guidance would suggest for you to adjust your temps or humidity unless you’re dead on the VPD. We pushed an update that if you’re within 5%, it just says that you’re on target!


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@peet, thanks for the reply.

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So these numbers if im dead on vpd are a good environment veg? at the end of week 2 seems like rh and temp are lower then alot of what is suggested online. Maybe it’s my setup but I can’t seem to keep this setting through day( lights on) it especially fluctuates during lights out. Is this an issue?

If you get your temps up, the app will suggest a higher RH to hit your VPD target (since they are related). I’d get your temps up by quite a bit. 80-85 if no CO2, 90-95 if supplementing CO2.

With the higher humidity (which you’ll need at the higher temps to hit the VPD target), you will have to take precautions against mold.

Peet, why bring the temps so high?? Is it because I’m using LED? I thought the sweet spot on chart for veg was 1.1-1.2? The template settings are at 1.2 as well? I’m struggling keeping the RH above 60 for more then an hour at any temp above 76. Running a heater in the lung room and a humidifier in the tent cranked to 90!

The photosynthesis reaction is faster at higher temps, until the temps get too high. CO2 supplementation increases ideal temp for photosynthesis.

See this chart for what I mean.
(note: cannabis may have a different response curve, but the general idea is the same).

If you can’t hit your VPD at those temps, run at a setting where you can hit your VPD.

This is a great chart for visual reference, thanks

Keep in mind that due to all the pollutin’ we’ve been doing, ambient CO2 is now 400 ppm (woo free CO2), so the ideal temp is a little higher than it used to be, even without CO2 supplementation.

If I was to supplement with c02 is my light intense enough for this to work?
2x Hlg 100 v2 3k

224.5 ppdf each

That’s a great question, I’m not sure TBH!

So it’s okay to have high humidity as long as you watch against mold? How do most combat this? I’m new here! Thanks.

Dialing in more with an intermittent timer on my exhaust out every 10 minutes for 30 sec. Running another room fan and humidifier to get rh up. wondering if this is enough airflow for this environment?


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That looks great to me. I have a shutter exhaust fan in my lung room that I keep on a speed of two. This way I always have a fan exhausting out odor, even if it’s not charcoal filtered out that fan. Keeps my inline fan in the tent from working as hard. I have it set at 2. I also have a nearby window. With these theee items and two oscillating tent fans, I feel decent about it. Your numbers look great though.

Hey Peet
I have another follow up question after changing some variables of the environment in my grow space. I am currently at 79-81 temp and rh of 73 I’ve got alot of fans and an exhaust set to turn on every ten minutes for 20 seconds to pull air. Does this seem like plenty of airflow to maintain rh above 70?

A lot of fans is good, just keep a close eye on the plants and maybe start doing some preventative spraying just in case. Keep up with the watering/feeding, give em enough light and your plants will explode in that environment.

I’d try and cram 1-2 more of those light boards into your tent.

Thanks peet, I actually upgraded my light to the hlg 350r over the 2x 100 v2s…which is running another 100 watts then what I’ve got now. I’ll make sure to run it on a progressive.
Thanks again

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