Leaf Surface Temps

MIhomegrown · Jan 14, 2019

Ibechillin said:
I follow what your saying MIhomegrown, ideally for optimal growth you would want to maintain the 88F leaf surface temperature during lights on, and there are a few things that influence leaf surface temperature other than room temperature. Like Switcher56 Mentioned distance from your light source has a major influence, as well as the spectrum of the light.

From what youve told us it seems when you A/C is off the environment is optimal during lights on to maintain 88F leaf surface temperatures, maybe try setting the a/c thermostat a few degrees higher so it runs for less time?

From Black Dog Led Leaf Surface Temperature Study:

When a photon of light hits a plant leaf, it can either be reflected or absorbed. Reflected photons will not affect the leaf temperature at all, but physics dictates that all photons absorbed by the leaf will increase the leaf temperature; how much depends on the energy (wavelength) of the photon and whether or not some of that energy was used to trigger other chemical reactions, such as photosynthesis. Photons fully utilized by the plant in chemical reactions will heat the leaf less than photons which are absorbed but not utilized. Therefore, measuring leaf surface temperature indirectly measures the efficiency of the light spectrum for growing plants-- less-efficient spectrums will tend to heat the leaf more, while more-efficient spectrums will heat the leaf less as more of the light energy is being converted to chemical energy.

Thank you for the info ibchilling. My lights are an optimal distance away they're just strong and I don't want to raise them. I don't have signs of leaf stress or burning. Thankfully my a/c runs more than it does when it's off. When's it running the lst are what i like to see. Also why do you prefer to have a 88F leaf temp and what temps do you prefer in your rooms?

Switcher56 · Jan 14, 2019

MIhomegrown said:
Thank you for you time and the insight. I've stood there with the infrared to get those readings. Our rooms are completely different if you don't need a/c to cool so your readings would be different. The lights I use are the correct distance from my canopy but they are double ended hid lights. My environment is controlled that's why I'm confused on how the leaf temps differ so much from my rooms temp. I would think it would stay relatively similar to the room temps. I've come to the conclusion that surfaces under the lights just cool and heat up faster than the air.

I am not disagreeing with you good buddy. You're the one with the problem. Instead of saying my lights are at the optimal distance, why don't you try raising them and see what happens. A 20 deg spread, never heard of that much. Someone who has an 800 sqft room, I would think they would have/could afford a PAR meter to read what is happening at canopy level. Just my 2 cents.

<<[FONT=Arial, Helvetica, sans-serif]When a photon of light hits a plant leaf, it can either be reflected or absorbed. Reflected photons will not affect the leaf temperature at all, but physics dictates that all photons absorbed by the leaf will increase the leaf temperature; how much depends on the energy (wavelength) of the photon and whether or not some of that energy was used to trigger other chemical reactions, such as photosynthesis. Photons fully utilized by the plant in chemical reactions will heat the leaf less than photons which are absorbed but not utilized. Therefore, measuring leaf surface temperature indirectly measures the efficiency of the light spectrum for growing plants-- less-efficient spectrums will tend to heat the leaf more, while more-efficient spectrums will heat the leaf LESS as more of the light energy is being converted to chemical energy (read work)>>

, which is nothing more than wasted energy. Layman's terms... Energy not used for work results in heat. Optimum room temp = 20-25 deg C or 68 to 78 deg F
[/FONT]

Ibechillin · Jan 14, 2019

MIhomegrown said:
Thank you for the info ibchilling. Also why do you prefer to have a 88F leaf temp and what temps do you prefer in your rooms?

No problem, glad to help. Ive only flowered under HPS up till now at the typical ~75F room temp (with some seasonal fluctuation from drawing outside air). I came across the leaf surface temperature information a few months ago researching into LED lighting and spectrum effects more deeply.

Under single ended HPS lighting at the typical recommended 24" height above canopy and 75F room temperature leaf surface temperature averages about 88F. That seems to be the baseline used to compare against.

MIhomegrown · Jan 14, 2019

Ibechillin said:
No problem, glad to help. Ive only flowered under HPS up till now at the typical ~75F room temp (with some seasonal fluctuation from drawing outside air). I came across the leaf surface temperature information a few months ago researching into LED lighting and spectrum effects more deeply.

Under single ended HPS lighting at the typical recommended 24" height above canopy and 75F room temperature leaf surface temperature averages about 88F. That seems to be the baseline used to compare against.

My current crop is the first time I was interested in leaf temp. I had extra cash I got a infrared thermometer and I love the thing. Spend a lot of time playing with the thing :biggrin:

. What led me to learn more about leaf surface temps was actually triggering the plants response in order to fight of disease and pests. Obviously healthy plants will help fight them off but my main concern over the years has been mold and mildew. I was always cognizant of the temp swing when my lights were on to being off and to make sure that it wasn't a big difference. With that being said do you believe the main culprit for temp swings is powdery mildew. This is my concern now, does pm need the air temp or the leaf temp to be in it's range to germinate?

Ibechillin · Jan 14, 2019

Powdery mildew needs stagnant moisture or somewhat large swings in room humidity/temperature to thrive. At lights off there is a spike in humidity, so its good to dehumidify/exhaust that moisture after lights off. If your plants have high levels of calcium and decent amounts of silica in their tissue pests/disease/fungi will have a hard time bothering the plants.

Here is a good article to read through that explains it well:

https://www.maximumyield.com/what-to-do-about-powdery-mildew/2/1400

MIhomegrown · Jan 14, 2019

Ibechillin said:
Powdery mildew needs stagnant moisture or somewhat large swings in room humidity/temperature to thrive. At lights off the plants release all the moisture they didnt use into the air causing a spike in humidity, so its good to dehumidify/exhaust that moisture after lights off. If your plants have high levels of calcium and decent amounts of silica in their tissue pests/disease/fungi will have a hard time bothering the plants.

Here is a good article to read through that explains it well:

https://www.maximumyield.com/what-to-do-about-powdery-mildew/2/1400

Harley is a smart dude, his videos got me into brix gardening. Still trying to raise my brix numbers. Highest I got was 15

siftedunity · Jan 14, 2019

Ibechillin said:
No problem, glad to help. Ive only flowered under HPS up till now at the typical ~75F room temp (with some seasonal fluctuation from drawing outside air). I came across the leaf surface temperature information a few months ago researching into LED lighting and spectrum effects more deeply.

Under single ended HPS lighting at the typical recommended 24" height above canopy and 75F room temperature leaf surface temperature averages about 88F. That seems to be the baseline used to compare against.

if you have 88f leaf temps you must be frying your plants(unless using co2 with higher temps and even then that's on the high end) if your leaves are healthy they wont be the same temp as your ambient room temp.
doesn't really make sense how the room temp can be 75f but the leaves are 13f higher unless you have the lights too close and your temp probe isn't where your plants are.

to the op.. is your temp probe next to the tops of your plants? if the leaf temps fluctuate that's normal.

Switcher56 · Jan 14, 2019

siftedunity said:
if you have 88f leaf temps you must be frying your plants(unless using co2 with higher temps and even then that's on the high end) if your leaves are healthy they wont be the same temp as your ambient room temp.
doesn't really make sense how the room temp can be 75f but the leaves are 13f higher unless you have the lights too close and your temp probe isn't where your plants are.

to the op.. is your temp probe next to the tops of your plants? if the leaf temps fluctuate that's normal.

... point to add. The sensor should not be coming into direct light. HPS is hot stuff!

Douglas.Curtis · Jan 14, 2019

Sorry, missed these originally.

MIhomegrown said:
68F is what your day temps are on night temps?

Night temps are generally 65F. I don't shut my a/c off at all. It runs 24/7.

CannaRed said:
Hey D.C.,. 68F room air temp or 68f leaf surface temp? Wondering about your temps and temp swings for lights on and off as well

Room temp. Leaf temp I'm not sure, actually. I do know someone with a laser thermo, perhaps I can get them to take some readings.

siftedunity said:
doesn't really make sense how the room temp can be 75f but the leaves are 13f higher unless you have the lights too close and your temp probe isn't where your plants are.

Infra-red from HID lighting can easily heat a plant above room temps. Have to keep this in mind.

Fascinating discussion by the way. Thank you.

MIhomegrown · Jan 14, 2019

siftedunity said:
if you have 88f leaf temps you must be frying your plants(unless using co2 with higher temps and even then that's on the high end) if your leaves are healthy they wont be the same temp as your ambient room temp.
doesn't really make sense how the room temp can be 75f but the leaves are 13f higher unless you have the lights too close and your temp probe isn't where your plants are.

to the op.. is your temp probe next to the tops of your plants? if the leaf temps fluctuate that's normal.

I do use co2 and 88 is the highest it reaches before my a/c kicks back on. It doesn't stay at 88 consistently it fluctuates when the a/c turns on and off. Thermostat is at canopy level.

MIhomegrown · Jan 14, 2019

Douglas.Curtis said:
Sorry, missed these originally.

Night temps are generally 65F. I don't shut my a/c off at all. It runs 24/7.

Room temp. Leaf temp I'm not sure, actually. I do know someone with a laser thermo, perhaps I can get them to take some readings.

Infra-red from HID lighting can easily heat a plant above room temps. Have to keep this in mind.

Fascinating discussion by the way. Thank you.

Thanks dc, what do you like about keeping the temps that cold?

MIhomegrown · Jan 14, 2019

Just to reiterate my room temps fluctuate between 76-80F when lights are on. Leaf temps during that time are 68-88F. Also I had mentioned that my thermostat is a little slow at reading the temp so that why the temp creeps up before the a/c kicks back on.

Douglas.Curtis · Jan 14, 2019

MIhomegrown said:
Thanks dc, what do you like about keeping the temps that cold?

Terpene retention, resin production, but most importantly, limited plant production. Like the regions of the world where the best hash is produced, I like it cool and dry. You have to veg longer for the same harvest weight, there's no explosion of growth when you flip to flower.

Maximum resin/terpene content, on a minimal flower structure. Even the exhale, from the last toke in the bowl, is so tasty you want to eat it.

I'm definitely interested in leaf temps during my day cycle now.

St. Phatty · Jan 14, 2019

I believe one of the purposes of trichomes is to help cool the buds when they are in super-hot locations.

Based partially from designing heat sinks for air-cooling, where our "secret weapon" - the geometry that worked best - was pin fins.

Where all the fins are round rods.

The other possible purpose the resin serves is to collect pollen, because the trich's are sticky.

CannaRed · Jan 14, 2019

St. Phatty said:
I believe one of the purposes of trichomes is to help cool the buds when they are in super-hot locations.

Based partially from designing heat sinks for air-cooling, where our "secret weapon" - the geometry that worked best - was pin fins.

Where all the fins are round rods.

The other possible purpose the resin serves is to collect pollen, because the trich's are sticky.

Ive heard the pollen theory before, but to me that doesn't make sense. If pollen sticks to the trichome it's not going to pollenate the pistil.

I've heard other theories too-. They act as a sunscreen, anti uv, they act as antipests to keep animals from eating. They trap insects.

We may never know

Douglas.Curtis · Jan 14, 2019

St. Phatty said:
I believe one of the purposes of trichomes is to help cool the buds when they are in super-hot locations.

Interesting. It does provide a much greater surface area for the plant to radiate heat from. I never thought about this function before, but it makes perfect sense. How thermally conductive are cannabinoids/terpenes? lol

siftedunity · Jan 14, 2019

Switcher56 said:
... point to add. The sensor should not be coming into direct light. HPS is hot stuff!

a sensor should be under the hps at plant tip level. if you don't have a sensor that can take 80f then its probably not fit for purpose. whats the sense in having a temp probe reading the temp where it isn't relative to what the plants are growing under?

siftedunity · Jan 15, 2019

Douglas.Curtis said:
Interesting. It does provide a much greater surface area for the plant to radiate heat from. I never thought about this function before, but it makes perfect sense. How thermally conductive are cannabinoids/terpenes? lol

if you had a thermal camera that would be an interesting experiment.
i think it might be multifunctional, probably stops the seeds drying out prematurely too.

siftedunity · Jan 15, 2019

Douglas.Curtis said:
Sorry, missed these originally.

Night temps are generally 65F. I don't shut my a/c off at all. It runs 24/7.

Room temp. Leaf temp I'm not sure, actually. I do know someone with a laser thermo, perhaps I can get them to take some readings.

Infra-red from HID lighting can easily heat a plant above room temps. Have to keep this in mind.

Fascinating discussion by the way. Thank you.

yeah agreed but temp is temp no matter what the source is.. just curious, have you tried different temps and humidity with the same clones and had them tested? I would've thought yield and quality would come hand in hand?

MIhomegrown said:
I do use co2 and 88 is the highest it reaches before my a/c kicks back on. It doesn't stay at 88 consistently it fluctuates when the a/c turns on and off. Thermostat is at canopy level.

id personally work with the differential and try to tweak it so that its coming on and off a few degrees either side of my desired temp.
i guess it could be that the ac is slightly undersized or something. i think most split acs are not really designed for grow rooms so you kind of have to work with them.

Douglas.Curtis · Jan 15, 2019

siftedunity said:
a sensor should be under the hps at plant tip level. if you don't have a sensor that can take 80f then its probably not fit for purpose.

With HID lighting, the issue is infra absorption being different on the sensor and leaf. A laser is most efficient, no?

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