Leafs be all like little solar panels. Get like a couple degrees warmer than ambient. It's like science and junk.

Leaves absorb a lot of the light energy that hits them. Air, not so much.

Bullmark69 said:

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.

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I’m sure you read the manual and have the IR gun set up correctly? Like is emmissivity user adjustable. You are measuring from correct distance?
I only ask because you said you just got it and so you may not be aware of those things.

This is the real answer if you want it:


The use of LED technology is commonly assumed to result in significantly cooler leaf temperatures than high pressure sodium technology. To evaluate the magnitude of this effect, we measured radiation incident to and absorbed by a leaf under four radiation sources: clear sky sunlight in the field, sunlight in a glass greenhouse, and indoor plants under either high pressure sodium or light emitting diodes. We then applied a common mechanistic energy-balance model to compare leaf to air temperature difference among the radiation sources and environments. At equal photosynthetic photon flux, our results indicate that the effect of plant water status and leaf evaporative cooling is much larger than the effect of radiation source. If plants are not water stressed, leaves in all four radiation sources were typically within 2°C of air temperature. Under clear sky conditions, cool sky temperatures mean that leaves in the field are always cooler than greenhouse or indoor plants-when photosynthetic photon flux, stomatal conductance, wind speed, vapor pressure deficit, and leaf size are equivalent. As water stress increases and cooling via transpiration decreases, leaf temperatures can increase well above air temperature. In a near-worst case scenario of water stress and low wind, our model indicates that leaves would increase 6°, 8°, 10°, and 12°C above air temperature under field, LED, greenhouse, and HPS scenarios, respectively. Because LED fixtures emit much of their heat through convection rather than radiative cooling, they result in slightly cooler leaf temperatures than leaves in greenhouses and under HPS fixtures, but the effect of LED technology on leaf temperature is smaller than is often assumed. Quantifying the thermodynamic outputs of these lamps, and their physiological consequences, will allow both researchers and the horticulture industry to make informed decisions when employing these technologies.

Link to research: https://www.researchgate.net/public...igh_Pressure_Sodium_and_Light_Emitting_Diodes

Bullmark69 said:

I know the answer but I can screw things up pretty

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Candidly, it sounds like your plants are water stressed; although, not much if you’re only a few degrees over the ambient air temperature.

Here’s the important part for you OP: As water stress increases and cooling via transpiration decreases, leaf temperatures can increase well above air temperature.

Under ideal condition’s your plant will open the leaf stamatas and transpire to cool itself, cooling & transpiration increases and the leaf temp falls below the ambient air temperature. Under less than ideal conditions, the stamatas begin to close, cooling & transpiring decreases, and the leaf temp begins to rise above the ambient air temperature.

Not sure if any mentioned; Get a fan.

Bullmark69 said:

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.

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Depends on the health of the plant and the environment. If my leaf temps under LED was 2 degrees higher-something is wrong; not transpiring normal. Mine are 2-4 degrees below air depending on humitdiy and growth rate.

BehindEnemyLines said:

Candidly, it sounds like your plants are water stressed; although, not much if you’re only a few degrees over the ambient air temperature.

Here’s the important part for you OP: As water stress increases and cooling via transpiration decreases, leaf temperatures can increase well above air temperature.

Under ideal condition’s your plant will open the leaf stamatas and transpire to cool itself, cooling & transpiration increases and the leaf temp falls below the ambient air temperature. Under less than ideal conditions, the stamatas begin to close, cooling & transpiring decreases, and the leaf temp begins to rise above the ambient air temperature.

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^^^THIS^^^

I understand things a little better now…..but IMO I have more than enough air flow w/ 3 fans…..one blowing under the plants canopy, one fixed blowing between the light and the plants tips, and an oscillating fan blowing opposite the fixed fan also blowing on the bottom of the light and tops of the plants. I’ve often thought it was overkill.
I’m not allowing the RH to get above 40%. And my exhaust is rated a good bit higher than what is called for in a space my size.
My space is only 2.5’ X 6’ w/ an 8’ ceiling.
I have a 4” hurricane exhaust that I run 24/7 on full power and a passive intake consisting of 3 different 2” holes cut at the bottom of the wall leading into the lung room.
The way I understand it, my condition (LST greater than air temp) is caused by 1)high humidity, 2)poor airflow, or 3) incorrect light spectrums. None of these things seem like they are applicable. I’m not light expert but that particular HLG light gets very high marks with its users.
If things are truly resulting from one of those 3 things, i would say that maybe my intake and exhaust isn’t performing. I know a 4” exhaust is not the most efficient and i wonder if my 3 round holes are letting enough new air in??? They are actually holes cut in two separate walls w/ a 4” space between them. I cut 3 pieces of pvc pipe to create a direct path from wall to wall…..and I have a filter over the holes in the grow space….it’s called Merv 13, which came in a sheet and looks like white cotton sheet. Anyway, perhaps my passive intake design isn’t efficient enough.
Thoughts anyone?

growsince79 said:

^^^THIS^^^

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BehindEnemyLines said:

Candidly, it sounds like your plants are water stressed; although, not much if you’re only a few degrees over the ambient air temperature.

Here’s the important part for you OP: As water stress increases and cooling via transpiration decreases, leaf temperatures can increase well above air temperature.

Under ideal condition’s your plant will open the leaf stamatas and transpire to cool itself, cooling & transpiration increases and the leaf temp falls below the ambient air temperature. Under less than ideal conditions, the stamatas begin to close, cooling & transpiring decreases, and the leaf temp begins to rise above the ambient air temperature.

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Thank you for trying to help educate an old fart like myself. Can you explain exactly what “water stressed” means?
Does it have anything to do with my watering habits? Or does it refer more to the low rate of transpiration, thus causing the leaves to stay warmer?
See my comments below b/c the causes for this just don’t seem to be present….but I could be sadly mistaken somewhere along the way.
Really, I do appreciate everyone that chimes in and tries to help a fella. Thanks again.

The VPD or vapor pressure differential is a chart you can use to optimize your humidity and temperatures. Making the plants grow their best. The leaf temps are calculated for in some of the charts. It's a bit to chew on. But it helps you with this topic. Science and junk.

Bullmark69 said:

Thank you for trying to help educate an old fart like myself. Can you explain exactly what “water stressed” means?
Does it have anything to do with my watering habits? Or does it refer more to the low rate of transpiration, thus causing the leaves to stay warmer?
See my comments below b/c the causes for this just don’t seem to be present….but I could be sadly mistaken somewhere along the way.
Really, I do appreciate everyone that chimes in and tries to help a fella. Thanks again.

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Do they look ok?- pics would help

growsince79 said:

Do they look ok?- pics would help

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I’m growing in Fox Farms 70/30 buffered coco that I amended with different dry organics….my first attempt in coco.
I grew 3 different strains (like a fool) and each one had a different appetite.
Anyway, if I grew the same 3 again, I would top dress less amounts but more often.
I just harvested the Jack Herer but the other 2 (a gelato og and an amnesia) were much later kicking into flower, so they’ve got another week or two.
Both have started to fade a good bit. My last top dress was about 3 weeks ago and I’ve given a tea w/ EWC, molasses, kelp meal paste, and crushed langebeinite a couple times since the last top dress.
Other than the obvious fade and a few yellow tipped sugar leaves they look pretty decent (my standards are fairly middle of the road).
More importantly the buds are frosty and reek….both strains smell like lemon pledge and pinesol.
The amnesia doesn’t have the biggest buds but I swear there isn’t a popcorn or larfy bud on it. Every single bud is a perfectly shaped little egg and is hard as a rock.
The Jack was ambered up big time by the time I chopped it and was just dry enough to go into the jars this morn. I got a feeling it’s gonna be an outstanding smoke with an amazing high….but most Jacks are.
Sorry for the long winded message. Here are some recent pics from the last week to 10 days.

They look good and almost done. I wouldn't worry about it.

growsince79 said:

They look good and almost done. I wouldn't worry about it.

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I thought I’d update on my situation……I tried to calibrate my infrared thermometer and was left scratching my head.
I made an ice bath in a small glass bowl, mostly ice with just enough water to stir it. I took several readings over a 5-10 min period and they ranged from 32.8F (which is about what it should be) to 41.5F…..so completely unreliable and inconsistent.
I’ve also taken a bunch of leaf temp readings and I haven’t gotten wild swings but 2-3 degrees difference from one reading to the next…..but it’s given some readings where the LST and air temps are the same. So who knows…?
I have a max of 2 weeks left on this grow and have my system set to 75F and 48%rh. If the leaf temps are within 2 degrees on either side, my VPD should be good or very close to it.
Thanks for the input. Cheers.