Vpd, Temp, and RH

If I keep vpd in the sweet spot for my temp and humidity, does the temp and RH really matter? Will a change in temperature to x affect my yield? I’m currently at 75ish temp and 65%RH with a vpd in the, .8 range.

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Vpd has a relationship with temp and humidity. If one changes they all do to some extent


Agreed. Is there an ideal temperature to shoot for, obviously adjusting the RH to stay in the sweet spot, or is there no difference assuming the vpd is correct?

The plant prefers temps from 78 to 85F in general. Terpenes tend to evaporate at a higher rate at 80+F. For me this puts my sweet spot at the low end of that range. Then adjust humidity at the rate that puts you in the ideal VPD range: about 55% in flower.

Hope that answers your question.


I agree with @Myfriendis410. Use the chart to learn how the temperature humidity effects the transpiration rates. You want to drive the transpiration rates higher during flowering, and this will cause the plant to draw more water/nutrients up through their roots. This will insure maximum growth.

It does thank you. So all things being equal and taking into account I’m in veg I should be shooting for 81ish and 65-70rh?

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I use that chart, but follow it somewhat loosely. For example, after my plants are too big for the domes. I keep the humidity in the lung room at 50-60% no matter the temperature. It is just not worth trying to run the lung room up to 70% humidity for the plants in veg.

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You’ve got it. It’s a target but I wouldn’t lose my mind if I can’t hit it.


Thanks. It’s definitely elusive and I have empathy for anyone trying to manage it without some type of control system.

Dr Bruce Bugbee has answers for many topics on YT

I run my tent from 70-75 degrees, RH 40-50, and VPD 1.25. CLONES, SEEDLINGS, and flowering ladies all share the love and tent, for continous grow.
Except the dry/cure box & tent.
Good growing to you.

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I’m guessing that you are asking about air temperature, but instead think about what an ideal leaf temperature/ range might supposed to be, & try to shoot for that as your primary temp target, because photosynthesis is going to slow down when the leaf temperatures aren’t in ideal range.

You want to know where your leaf temps are once you get your space stabilized & running some baseline numbers, & your light is set up for proper intensity for whatever stage the plants are are in.
Then record your leaf temps, air temps, & RH across the top of the canopy, & then you can start to work towards your leaf temp targets, & your Leaf VPD targets, by adjusting air temp, RH, &/or leaf temps. Just don’t go to extremes trying to hit a number. I use a handheld IR temp gun to get my leaf temps, & here is the Leaf VPD calculator that I use:

Cloudline Controller Software math for VPD is defined as (Leaf Temp) minus (Air Temp)
and then determines a number. The hand-held IR gun tells leaf temp for calibration input.
Ever changing readings, most fun. Possibly leaf temp = air temp is an indicator for stress approaching.
I prefer -3 degrees for safe margin

Looks like a -2f offset chart & not a -3f offset chart. Not sure if that is what you meant.

If you’ve been checking your leaves with an IR gun, what leaf temps & what offsets do you actually see?

Tent 74 degrees
leaves 68 degrees

formula VPD (leaf minus ambient)
68 minus 74 = -6 degree correction utilized by software and VPD number results.

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I use an IR gun to test leaf Temps. I plug that offset into the controller software and it adjusts the vpd. I’m around -2, - 3* pretty consistently across the canopy.

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Yup, that’s my sweet spot.
i mught presume, lowering lights until Zero offset, might be possible.
However, I practice safety and love the -3degree margin.
Yup, that’s my sweet spot
71-74 degrees 45%RH and @1.25 VPD and 1000 watts @ 20-25 inches
Unheated garage.

I think if you guys can get to optimal Leaf temperatures & still maintain your Leaf VPD targets, then that is what you want to go for. If your leaves are at 72f & Daves are at 68f, then my thought is that there is room for higher leaf temp & they are in the slower range of photosynthesis. That’s kind of where I am now, my leaves across the canopy are 68f to 71f now & probably going to get a little colder until I turn up my light. I feel that they have been moving towards slower photosynthetic performance because the leaves have been getting too cold. I don’t know what the highest leaf temps should be, but from searching around the internet it seems that 80f to 82f for the leaf temps is probably a good upper limit, & approaching leaf temps of 70f & below would probably be when photosynthesis would be slowing down. Over the summer once I started flowering I was able to push my leaf temps, & they seemed to be good until about 84f leaf temps. But leaf temps are my target in terms of temps, & over the summer the air temp ended up wherever it ended up & the leaves were usually running at around 81f to 83f at the warmest spots across the canopy. That being said, tuning towards ideal Leaf VPD targets for me was not always possible, mainly because I don’t want to run a dehumidifier.

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More to learn, I am sure.
I am familiar with burning up the flowers, so I avoid the 80’s F (remarkably, that was when I burned the grows)

Yes, this issues persist
Lights, create heat if on and reduce RH, when on.
Fans, reduce RH, minimally but cool somewhat.
Oscillating helps.
De-humidifier adds heat and reduces RH.
Windows open, free heat rediction but no RH control.
Tent Intake/Exhaust control some temp and RH.
Move or adjust one equipment and all numbers change, I know.
TBS, VPD might be used to determine light distance optimum.
Or just let it grow.

That would be doing it backwards/ the hard way, I think.
Target DLI using the least amount of energy while keeping the light far enough away to not do damage. The light should end up where it should be, & you can continue to rough in air temp & RH. Then you can start recording meaningful Leaf Temps, & then use a VPD calculator to target Leaf VPD. The VPD calculator will tell you where you could move Air Temp, RH, &/or Leaf Temp to hit target Leaf VPDs.