Can my leaves be hotter than the air temp under LED?

I think I know the answer but I can screw things up pretty good if given the chance…..so I’ll ask you folks.
I have 3 plants in a 2.5’x6’ closet with 8’ ceiling.
My light is an HLG 320 xl r-spec LED, hanging about 22” above the canopy. I bought one of those laser thermometers to check my leaves. I also have a couple other temp/humidity gauges in the closet. No matter what I do the laser always reads about 2F degrees above the ambient temps.
Right now for instance, I have 3 different thermometers in the closet that all read between 75.2 and 75.7F……so I feel like all 3 cannot be wrong. When I shoot the red dot onto one of the top leaves it’s reading 77.0-77.6F.
My understanding was w/ an LED the leaf surface temps will always be a couple degrees cooler than the air…..but mine says otherwise.
I’m guessing and hoping that the laser thermometer is off and needs to be calibrated, which cannot be done with this model.
Am I safe to assume that a 75F air temp will result in leaf temps of approximately 73F?
What’s the deal?
I’ve tried for the ideal VPD throughout the grow and just went off the air temp and subtracted 2 degrees but I figured I would ask about the chance of that reasoning being flawed.
It seems to be working I guess, as I’m in week 12+ of my light flip to 12/12 and the plants seem fine and should be ready in a week or two…..one actually has already been harvested so I’m down to 2….but I digress.
thanks for chiming in.

The thermometer is most likely off. Make sure you’re using it at the right distance.

none the less I would try to increase your temps a few degrees. My leaf temp is 84F with some hlg r spec lights and the plants are pretty happy.

Yes I've seen that but maybe my 3 thermometers were also off when I was getting equal to ambient temps?

pahpah-cee said:

The thermometer is most likely off. Make sure you’re using it at the right distance.

none the less I would try to increase your temps a few degrees. My leaf temp is 84F with some hlg r spec lights and the plants are pretty happy.

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After going round and round trying to figure out why this reverse temp difference was occurring I finally decided to make an effort to calibrate my inexpensive infrared thermometer.
I filled a glass with crushed ice and added enough cold water to reach the top of the rim. I let it sit for a couple min and zapped it with the infrared laser……it is reading between 37.4-38.6F. I know this isn’t 100% full proof but I’m thinking it should read a lot closer to 32F….?
Am I guessing right?
If I am that means my infrared thermometer is reading 5-6F degrees high. This would fit perfectly with my leaf temps being 2-3f degrees higher than the air temp…..adjusting for the inaccurate thermometer puts the LST at around 2-3F below the air temps.
maybe I should end this right now and move on….lol. Although I’m sure I can find something else to obsess over.

Bullmark said:

After going round and round trying to figure out why this reverse temp difference was occurring I finally decided to make an effort to calibrate my inexpensive infrared thermometer.
I filled a glass with crushed ice and added enough cold water to reach the top of the rim. I let it sit for a couple min and zapped it with the infrared laser……it is reading between 37.4-38.6F. I know this isn’t 100% full proof but I’m thinking it should read a lot closer to 32F….?
Am I guessing right?
If I am that means my infrared thermometer is reading 5-6F degrees high. This would fit perfectly with my leaf temps being 2-3f degrees higher than the air temp…..adjusting for the inaccurate thermometer puts the LST at around 2-3F below the air temps.
maybe I should end this right now and move on….lol. Although I’m sure I can find something else to obsess over.

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Point that IR thermometer at the thermometers in the tent.
I think leaf temp 1-2 F under air temp is typical.

I can’t believe these folks giving STUPID advice and OPINIONS.

Leaf temp is hotter than ambient —ALWAYS—with lights on. Go stand in the street barefoot when it’s 90 f outside and wonder why you get second degree burns.

You should aim for 84f ambient in your tent while running LEDs. LST (leaf surface temp) likes to be 86–88f with lights on. Do a little research on cannabis leaf surface temperatures running LEDs.

It very much depends on vpd too, in high humidity the leaf can’t transpire as much so it will be hotter, you need to consider the cooling action of water as it leaves the leaf and how this is beneficial when dealing with high air temps

fskitch said:

I can’t believe these folks giving STUPID advice and OPINIONS.

Leaf temp is hotter than ambient —ALWAYS—with lights on. Go stand in the street barefoot when it’s 90 f outside and wonder why you get second degree burns.

You should aim for 84f ambient in your tent while running LEDs. LST (leaf surface temp) likes to be 86–88f with lights on. Do a little research on cannabis leaf surface temperatures running LEDs.

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1. The sun produces a lot of infrared radiation, LEDs do not.

2. The street is not cooled by transpiration, cannabis leaves are.

1212ham said:

1. The sun produces a lot of infrared radiation, LEDs do not.

2. The street is not cooled by transpiration, cannabis leaves are.

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1. that’s why a heater may be needed—temps can run a little cool with LEDs—too cool for proper LST.

2. not really. Transpiration causes the ambient air to cool from the evaporation of water through the leaves—again get the ambient temps up to 84f when running LEDs. A stand up oil heater works.

fskitch said:

I can’t believe these folks giving STUPID advice and OPINIONS.

Leaf temp is hotter than ambient —ALWAYS—with lights on. Go stand in the street barefoot when it’s 90 f outside and wonder why you get second degree burns.

You should aim for 84f ambient in your tent while running LEDs. LST (leaf surface temp) likes to be 86–88f with lights on. Do a little research on cannabis leaf surface temperatures running LEDs.

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Instead of being an ASS and calling people STUPID, post the research the supporting YOUR OPINION.

1212ham said:

Instead of being an ASS and calling people STUPID, post the research the supporting YOUR OPINION.

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Not opinion. Fact. Anyone who reads my posts can do there own research. Everything I post is fact based—never opinion. Opinions don’t help folks looking for solid advice. Fact is you have no idea what you’re talking about regarding the topic at hand. Just relax and do a little reading. You’ll get learned.

fskitch said:

Not opinion. Fact. Anyone who reads my posts can do there own research. Everything I post is fact based—never opinion. Opinions don’t help folks looking for solid advice. Fact is you have no idea what you’re talking about regarding the topic at hand. Just relax and do a little reading. You’ll get learned.

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I don’t mean to pick sides as all I’m trying to do is grow buds that are better than my brother in law’s…..but I’ve did quite a bit of reading over the past 24 hours and every published source that I read indicated that in an ideal environment, the LST should be slightly lower than air temps. There’s a lot of highly technical things going on that cause this relationship and since I’m in sales, I skipped over it. But basically my leaves aren’t transpiring properly I believe.
Also, I’ve tried to calibrate my infrared thermometer and it’s been a puzzling activity.
i made an ice bath in a small glass bowl and took several readings. They were very inconsistent to say the least. They ranged anywhere from 32.9F (which I think is what it should be) to 41.5….so I either need to forget about LST or buy a more accurate device.
Anyway, I’m forgetting about it for now…I have another week to 10 days on this grow and I have my temp set at 74-76F and Rh at 46-48%….that gets things pretty close to an ideal VPD.
thanks to everyone for their input. I need all the help I can get.

Can it?...sure..You could light it on fire with a blow torch...Should it?...no...a simple google search will tell you this.

https://www.blackdogled.com/pages/lst
Link above for easy read

Below was taken from ‘cannabis business times’ article regarding LST.

The actual temperature is very cultivar-dependent, but the ambient air temperature with LED lighting runs generally about 10 degrees cooler than other systems, says Ryall. Under HPS, growers aim for a 75-degree air temperature, which puts the leaf surface temperature at about 85 to 88 degrees. Since LED lighting runs cooler, he’s seen the best results by raising the ambient air temperature by about 10 degrees. Ryall likes to see that number at about 82-85 degrees, which will make a lot of growers “fall out of their chair,” he says. It might actually mean bringing the temperature of the grow area up a little to the best range for the plant.

“Once [the plants] get too cool, it affects transpiration, growth and morphology, nutrient uptake. It can even affect CO2 uptake,” says Ryall. When growers report to him that the plants are feeding less, it usually means that the room has gotten too cool. “It all has to do with keeping the leaf surface temperature the exact same on that plant under HPS as it is under LED, regulated for ambient air temperature.”

fskitch said:

I can’t believe these folks giving STUPID advice and OPINIONS.

Leaf temp is hotter than ambient —ALWAYS—with lights on. Go stand in the street barefoot when it’s 90 f outside and wonder why you get second degree burns.

You should aim for 84f ambient in your tent while running LEDs. LST (leaf surface temp) likes to be 86–88f with lights on. Do a little research on cannabis leaf surface temperatures running LEDs.

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Science say no

Under LED lighting, LST is generally 2-3 °F LOWER than the ambient air temp. This is due to the transpiration of water which cools the leaf the same way sweating cools the skin.

Billy the Mountain said:

Science say no

Under LED lighting, LST is generally 2-3 °F LOWER than the ambient air temp. This is due to the transpiration of water which cools the leaf the same way sweating cools the skin.

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Sounds like the op is going your way. Way to spread bullshit there turbo.

fskitch said:

https://www.blackdogled.com/pages/lst
Link above for easy read

Below was taken from ‘cannabis business times’ article regarding LST.

The actual temperature is very cultivar-dependent, but the ambient air temperature with LED lighting runs generally about 10 degrees cooler than other systems, says Ryall. Under HPS, growers aim for a 75-degree air temperature, which puts the leaf surface temperature at about 85 to 88 degrees. Since LED lighting runs cooler, he’s seen the best results by raising the ambient air temperature by about 10 degrees. Ryall likes to see that number at about 82-85 degrees, which will make a lot of growers “fall out of their chair,” he says. It might actually mean bringing the temperature of the grow area up a little to the best range for the plant.

“Once [the plants] get too cool, it affects transpiration, growth and morphology, nutrient uptake. It can even affect CO2 uptake,” says Ryall. When growers report to him that the plants are feeding less, it usually means that the room has gotten too cool. “It all has to do with keeping the leaf surface temperature the exact same on that plant under HPS as it is under LED, regulated for ambient air temperature.”

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The example you’ve just given is obviously someone who isn’t controlling vpd very well, like I’ve allready said, in high humidity low vpd environment the leaf will heat up due to no transpiration of water through the leaf but in an ideal environment when light temp and humidity are matched well then the leaf should be abit cooler than the air

fskitch said:

Sounds like the op is going your way. Way to spread bullshit there turbo.

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Science is real

Blue brother said:

The example you’ve just given is obviously someone who isn’t controlling vpd very well, like I’ve allready said, in high humidity low vpd environment the leaf will heat up due to no transpiration of water through the leaf but in an ideal environment when light temp and humidity are matched well then the leaf should be abit cooler than the air

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You people have the op not believing his own eyes. He’s ready to toss his thermometer cuz it won’t calibrate.
LST is hotter than ambient under any light—Hps, LED the sun —ANY LIGHT. Not sure if you people are ignorant or stubborn.
The water evaporates from the leaves and cools the ambient air.

fskitch said:

You people have the op not believing his own eyes. He’s ready to toss his thermometer cuz it won’t calibrate.
LST is hotter than ambient under any light—Hps, LED the sun —ANY LIGHT. Not sure if you people are ignorant or stubborn.
The water evaporates from the leaves and cools the ambient air.

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No the plant draws cold water through the roots and out through the leaves to cool itself, the same as when we sweat