VPD leaf temp

Hey everyone, so I recently purchased an IR therm so that I can see what my VPD leaf temp is. Come to find out, my leaf temp is significantly lower then my air temp. I took 4 averages of both canopy and non canopy items and I came out with a difference of -6.5. Once updating that to my pulse my vpd dropped half and it’s telling me to either increase temp or significantly drop RH. It seems kind of like a huge drop so I just wanted to post this and see if anyone had any ideas on if I should follow my pulse recordings…? I am also in week 2 of veg Incase this matters…

Question, are you running LEDs?
The microclimate at the leaf surface is what’s important. It’s what the plant feels and thus regulates the opening and closing of its stomata.
LEDs will heat the leaf surface to a lesser degree than HID or even florescent lights.

I believe the sweet spot for plant metabolism iin flower s close to 80f.
In my experience with LED is the ambient air runs about 5-6f higher than the leaf surface.

This will let you run a hotter room without slowing down optimal growth.

I can control the light intensity easier than humidity and room temp. (Cheaper than a/c or dehumidifiers. So I use the a/c to cool my lung room and help dry the air, and when the VPD is close to perfect, I will increase the light intensity to dial VPD perfect.

I only use an exhaust fan to lower humidity in my tent
A/c in the lung room controls the ambient air temp.
Dimmer on the light fine tune tent temp.

I also know my DLI that my lights are cranking out, so I stay above the minimum I want to run for my ladies.

Run as high a temp as possible for your state of growth to maximize plant metabolism.
Keep VPD in the correct range to maximize plant metabolism.
Keep DLI at least the minimum to maximize plant metabolism. Too much is a waste of electricity.

If you run co2, you can increase the temps somewhat.

Light and temp are like the throttle on and engine.
VPD is like the fuel flow to power the engine.
They all should be in balance to each other so the engine doesn’t run too rich or too lean and performance will suffer.


Wow you are so spot on man! Don’t know how you knew, but yes I’m running LEDs, I have 640watt cob led timber grow lights to be specific! I somewhat got a better understanding of how to proceed from your response so thank you very much for the input!! One question, what is DLI? You referred to it above regarding your lights but I’m just a little unsure as to what that mean? Sorry for rookie questions

DLI is Daily Light Integral.
You can take a deep dive into this topic, you should google “cannabis DLI” to get the terms and concepts down.

Basically, all those photons that rain down on your canopy have different intensities depending on distance from the light and distance from the center of the footprint of light.

Plants can only use so much photons in a 24hr period. Any more is wasted electricity, and the photosynthesis will actually stop after the plant is maxed out.

So I want 12 hrs of light in flower.
I want to hit the canopy with the max amount of photons it can use in that 12 hrs.

I can vary the height of the light, or the intensity with a dimmer.

I use a dimmer to let the light help me control the temp, and adjust the height to max out the DLI. I shoot for a DLI of 50 to 60.

Note that under 18hrs of veg lighting, I can have a higher light, or lower intensity for what would be 6 more hours of light compared to flowering stage. End of the day the DLI would be about the same.

I use a par meter and an online calculator to figure my true DLI.

Pulse is a great tool for tweaking and monitoring your environment.

I run a little higher VPD than what you showed. But I am in flower, and I’ve run this strain before so have it dialed in.

I have 3 weeks left, so am going to let temps and humidity drop a little to raise trichs and preserve terps.


Nice calculator for DLI.

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Wow thats super helpful man thank you! I was going to ask how do you measure all of this till i saw you said you use a par meter. I think I’ll
Be needing one of those so that I can dial this grow in better. Do you think this one would do for my 640watt cob led lights?

I’m pretty sure a lux meter and par meter is two different items

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Yes, they are different.
And the meter you want are typically very expensive for lab grade equipment.

Mine is a hydro farm unit that works well enough for relative comparisons.


You could also refer to the par chart that most light manufacturers publish.
I run Timber and Rapid fixtures, and both have charts for their light output at a given distance. Meter helps me dial it in.

After using it and getting a handle on what the intensities are in my tent, I don’t use it often. Maybe consider you and another grower friend or two purchasing and sharing. I’ve heard some grow stores lend them out, but not around me. :frowning:

Also, you dont have to go deep into all this to be successful. I just feel like it’s a hand of poker, if you can stack the cards in your favor, you’ll win every hand.

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Well said! Thank you

Right now I’m trying to dial in my pulse meter but I can’t much figure it out. It seems my leaf temp and random temps around the tent are the same so what is the offset for? Maybe someone can post a vid of fine tuning their environment so others can see how

Maybe someone else will comment or the staff here, but if your ambient air temp and leaf temp are the same, then no offset is needed.

My grow style, system and equipment wouldn’t be relevant to your setup. Not sure if a video would help someone else unless they wanted to duplicate my “way”

I grow in a 2×2×6 tent in a 20g fabric pot with living organic soil, and do not till or disturb the soil from one run to the next.
When one run is complete I pull the whole bag out and it goes in my veg room (closet) and gets seeded with a cover crop and left fallow while a second “refreshed” and top dressed bag gets moved into the tent. Root ball stays in the soil.
I’ll chop the cover crop down and let lie on top for the worms and microbes to eat. It’s a great green mulch.

I also have the growbag setting in a big saucer with about 3" of perlite in it. I maintain a water level just below the bottom of the growbag. The water wicks up and keeps the soil moist and the life alive.

I have an Iponic 624 environmental controller. This has a sensor similar to the Pulse unit and operates lights, and exhaust fan (4") that is set to exhaust excess humidity.
It will operate up to 8 devices that you can customize. I will be adding co2 soon. It will also run two tents.
I can access the iponic from my phone and change anything I care to.

No salts or synthetic nutes. Just water. I capture the condensate from my a/c unit to add back to the tent to water soil and add to the saucer as needed.

If a deficiency develops, I may foliar feed with the appropriate product that I get from builasoil.com
I also note the deficiency and amend the soil during its fallow period for the next run.

I want to add I run a worm bin too. Worms are important for my system.

My a/c cools the bedroom all this is in. During summer it always runs, I just set the thermostat and forget it.

I use a small cheap temp/humidity gauge to calibrate my iponic, so its reading are actual.
Then I calibrate the pulse to match those readings.
Then I do an offset for leaf temp.

I run 1.2vpd in veg and 1.5 in flower.

I dont concern myself with high humidity, if the plant is healthy and vibrantly growing, it will be very resistant to molds, mildew, and pests.

I dont clean or disinfect anything. There is no such thing as a sterile environment, and I think efforts are better spent and growing healthy ladies.

Very low maintenance, very low inputs, very low costs, (other than initial investment in hardware)

With new genetics, I would pop seeds and run a strain and observe. Then run clones for a second run making adjustment to optimize the results. Second run of a strain is usually way better than the first naturally.

Read a lot. Stay away from bro science. Make small changes so you can observe results.

It all starts in the soil. Keep its energy up.
Keep the energy of the plant up
Strive not to stall the growth of the plant.

It took me awhile to come to where I’m at now. Good clean, dank flower, concentrate, keif and edibles.


Well said my friend

You have been beyond helpful man, really appreciate the great information you have been providing for a new grower like me. I certainly have a lot to read up on, but with a community like this it’s great having helpful people like you to lend some insight! Thanks again :call_me_hand:

Most definitely have been helpful. Thanks again

I always got problems!!! Lol
ATM: fungus gnat larvae/pupa in my run off. I think…

Magnified x200
Praying :pray:t2: that it’s not root aphids but I’m pretty sure they have live birth n not larvae

I’ve suffered root aphids once. Brought outdoor aloe plant into my veg closet to overwinter.

Not sure what those could be. At 200x magnification, they must be tiny.

What’s your medium?

Fungus gnats are easy enough to deal with.

I was going to create a new thread for this new issue that ive been trying to deal with, but Having seen the help you provided along with wheels, whom has been super helpful to me on some of my other posts, I thought I would just post it here…

I’ve been letting my daytime temps get a little hotter since I know that my leaf temp offset is -6.5, but Now I can’t control my night time temperatures, which has never been an issue.

I grow in a spare bedroom in the upstairs of my house. My house is a old brick/mortar built in 1942 that does not have central air. We get by in the summer months with window air conditioning units to cool certain parts of the home(mainly the first floor since it’s just my parents and I) the bedroom that My tent sits in has a 6,000btu window a/c unit Used to cool the bedroom and my tent. Since it’s only my parents and I that live here(all my other siblings have moved out) we don’t use air conditioners in this upstairs(besides my grow bedroom) since there’s no reason to cause no one is currently living up there. As a result of not having central air or using an air Conditioner, the upstairs hall gets extremely hot since it’s summer right now in Chicago.

I don’t know if the upstairs climate has anything to do with my high temps, but I thought I would just give some info… I do have this bedroom that I grow in sealed off with an oak wood door and I even foam weather stripped the bottom of the door so no draft would get in or out. Is my window ac not strong enough or is the upstairs just too hot?

Root drench with Dr. Zymes!

I would assume that that your brick home is taking a beating by the hot sun and those bricks are soaking that heat up and creating a insulated effect therefore super heating any room that doesn’t have constant A/C. I had this problem with the evening sun. To remedy this I tacked moving blankets on the outside walls. Kept the room 8 degrees cooler than without.

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Tommy, thats a great idea! Just order some moving blankets on Amazon! Appreciate the help man

Cheers for describing your setup i like that its sustainable… ive been thinking Of going that way… i love that they lie fallow :+1: