Leaf Surface Temps

[MIhomegrown]

I've been following my lst recently this grow and need to know if I should be following that more than the rooms temp. I always kept my rooms temps accordingly for healthy plants and to prevent disease or pests. So i am concerned that the leaf temps change more drastically than the room temps. I use central a/c to cool my room. When it's running the temp of the room is about 78 consistently on average and will go down to 76 and then shut down. When the a/c turns off the temp will usually get to about 80 degrees before it reads on the thermostat and kick back on.
However the leaf surface temp changes way more drastically. When the a/c shuts off the leaf temps get to about 68 degrees and then creeps as high as 88 degrees! Is this normal since the air temp don't change that much? I haven't ran into any issues so far but with that temp differential it makes me nervous. Any input is greatly appreciated.

 

[St. Phatty]

Sounds like your leaves are doing good.

I'm always impressed by what the temperature is in direct sunlight.

Something I think about, while I'm standing in the shade.

Would be interesting to see what temp readings you get on an outdoor grow.

 

[Ibechillin]

Ideal leaf surface temperature is 88F for cannabis, The heating to optimal lst and cooling to prevent damage is probably beneficial.

 

[Switcher56]

I've been following my lst recently this grow and need to know if I should be following that more than the rooms temp. I always kept my rooms temps accordingly for healthy plants and to prevent disease or pests. So i am concerned that the leaf temps change more drastically than the room temps. I use central a/c to cool my room. When it's running the temp of the room is about 78 consistently on average and will go down to 76 and then shut down. When the a/c turns off the temp will usually get to about 80 degrees before it reads on the thermostat and kick back on.
However the leaf surface temp changes way more drastically. When the a/c shuts off the leaf temps get to about 68 degrees and then creeps as high as 88 degrees! Is this normal since the air temp don't change that much? I haven't ran into any issues so far but with that temp differential it makes me nervous. Any input is greatly appreciated.

Do you have circulating fans running? A 20 deg spread is enormous. I understand that cold air sinks but where is your thermocouple located? It should be at canopy height. Running circ fans. (they should be running 24/7) jt makes your environment homogeneous.

The other thing is: What are your controller settings set at and, what is the band set at? e.g temp differential

 

[Switcher56]

Ideal leaf surface temperature is 88F for cannabis, The heating to optimal lst and cooling to prevent damage is probably beneficial.

Lights on perhaps but, not during lights out unless his house is at that temp. Lights out in my grow, my tent is at room temp and, that is taken at to canopy. Lights on for me 78 deg, lights out 68-70 deg, same as ambient temp. Depends if I'm cold or not LOL. Plant comes second!

During summer (will no longer grow in summer here, weather fluctuations too drastic. (maritimes)) tent temp was around 79-81 deg @ canopy, whilst the intake (floor level) from the lung room (semi-sealed den) was 68 deg. My tent thermostat in this set up is exhaust fan speed.

 

[Douglas.Curtis]

Ideal leaf surface temperature is 88F for cannabis

Well that sucks for my cannabis. lol I vastly prefer the quality from cannabis grown in 68F and low humidity. Oh well, I'm not growing for them anyway.

 

[MIhomegrown]

Do you have circulating fans running? A 20 deg spread is enormous. I understand that cold air sinks but where is your thermocouple located? It should be at canopy height. Running circ fans. (they should be running 24/7) jt makes your environment homogeneous.

The other thing is: What are your controller settings set at and, what is the band set at? e.g temp differential

I have everything you mentioned. My thermostat is across the room at canopy level, away from the a/c intake and where the cold air blows. I think my thermostat is trash. It read too slow but like I said when the a/c shuts off the the temps kicks on and by the temp in turns on and reaches the desired temp, the lst is around 68F. But the the air temp is what I set it at.

 

[MIhomegrown]

Well that sucks for my cannabis. lol I vastly prefer the quality from cannabis grown in 68F and low humidity. Oh well, I'm not growing for them anyway.

68F is what your day temps are on night temps?

 

[CannaRed]

Well that sucks for my cannabis. lol I vastly prefer the quality from cannabis grown in 68F and low humidity. Oh well, I'm not growing for them anyway.

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

 

[Switcher56]

I have everything you mentioned. My thermostat is across the room at canopy level, away from the a/c intake and where the cold air blows. I think my thermostat is trash. It read too slow but like I said when the a/c shuts off the the temps kicks on and by the temp in turns on and reaches the desired temp, the lst is around 68F. But the the air temp is what I set it at.

I am not following you hear do you have a htr coming on during lights out? You are not very clr good buddy.

If your ambient room (read not tent) is @ 70 deg, why is there a htr coming on. Your tent will be at ambient temp when lights are out and, the entire temp of your tent will be 70 deg, to include @ the canopy. Not getting too technical, you want your "lights out" temps to be 5-8 deg lower than when lights are on, not 8 degrees higher.

How many air exchanges are you getting from your exhaust fan per hour? The other thing I am thinking about is a 2 deg band is quite narrow and causes short cycling (read a LARGE waste of electricity) I like my settings to be 3 deg low = kicks in. 1 deg high = shuts off.

What controllers are you running. A pic is worth 1K words.

 

[MIhomegrown]

I am not following you hear do you have a htr coming on during lights out? You are not very clr good buddy.

If your ambient room (read not tent) is @ 70 deg, why is there a htr coming on. Your tent will be at ambient temp when lights are out and, the entire temp of your tent will be 70 deg, to include @ the canopy. Not getting too technical, you want your "lights out" temps to be 5-8 deg lower than when lights are on, not 8 degrees higher.

How many air exchanges are you getting from your exhaust fan per hour? The other thing I am thinking about is a 2 deg band is quite narrow and causes short cycling (read a LARGE waste of electricity) I like my settings to be 3 deg low = kicks in. 1 deg high = shuts off.

What controllers are you running. A pic is worth 1K words.

Sorry about the confusion. Nope no heater. When my a/c shut off when it reaches the temp i have it set at, the a/c shuts off. During that time when the a/c shuts off and then kicks back on, is when the lst reaches 88F. My room is a closed room, I use a central a/c unit exclusively for my room. My concern is that the leaf surface temp are way different than the ambient room temps. On average my room stays 78-80 when lights are on.

 

[MIhomegrown]

So when my lights are on and the a/c reaches the temp I set at 77F the lst is at 68F. At 77F my a/c will shut off. During that time my room heats up a bit obviously and at the time my a/c will kick back on the lst is around 88F. When I stated I believe my thermostat is trash is because it reads slow. So I have multiple digital thermometers palced around my room, same height as my thermostat. When my a/c shuts off the thermostat reads 77F but as the room temps creep up the thermostat is slow to read them. I'll get readings of 80-81F on my other thermometers while my thermostat is still reading 77F. The a/c usually kicks on shortly after that.

 

[Switcher56]

This is getting mighty confusing

My lighting schedule is 10pm to 10am for my location.
I have 2 Inkbird controllers: one for RH; and the other for htg/cooling.

Portable thermometers and hygrometers are for ref only. Mine are within 1 deg and a couple % points from my controllers.

My tent is in my den. The den is fitted with a window AC. It runs Jul and Aug. The htg controller doesn't turn the AC on or off but, during winter, a small portable heater is connected (to the htg leg) for supplemental htg during the day or night if required. The house thermostat, that controls the furnace is set @ 62 deg from 10pm to 4am.

The 2nd controller controls RH. One side a humidifier, the other leg a de-humidifier.

My settings are as follows:

• humidity controller
  • HS 60% (end of flower right now)
  • HD 3%
  • DD 2%
  • AH 80%
  • AL 45%
• Htg controller
  • TS 65 deg (I control lights out temp) "lights on temps" 77-78 deg
  • Hd 5
  • CD 5
  • AH 85 deg
  • AL 60 deg

Legend:

• HS Humidity set point
• TS temp set point
• HD humidity differential
• DD de-humidify differential
• AH alaem high
• AL alarm low
• CD cooling differential
• HD Htg differential

I had my heating controller HD setting @ 2 deg. Even at 3 deg the htr was short cycling. Not allowing ambient temp (from house htg) to catch up (if required). My temps are kept on set point or 1-2 deg off.

I have a 190cfm 4" exhaust fan that is capable of exchanging 9700 cfh (60 complete exchanges (fan capacity 11400cfh)) Since my fan is adjustable speed, this is where I fine tune my tent. Tent is 3x3x6'

It's been a long time since I lazered a measurement right off the top leaves but, we are only talking a couple of degrees here. Your exhaust fan is the fine tweaking knob for both RH and Temp. You will not get both of them in harmony with this method but you will get them close over a couple of days. e.g you might be dialed in with lights out, only to find your temp/or humidity too high during lights on. In the case of RH, RH lowers when lights are on and increases when lights are off. Finding the sweet spot takes trial and error. Hope this helps...

ETA: Fan running between 65-70% power. Current temp: 65 deg F and 61.4% RH. De-huy kicks in at 62%

 

[MIhomegrown]

I appreciate your help but I don't think I can explain it any better but I'll try.

My room is in a giant room about 800sq ft and my flowering room takes up about 150 sq ft of that. My Flowering room has it's own air conditioner. My thermostat that is connected to that a/c and is what controls it. I too have portable digital thermometers placed in parts of my room for reference. My a/c runs until the thermostat hits 77F and then shuts off. So I use an infrared thermometer to to read the temp of the leafs(lst). So when my air conditioner shuts off and I use my infrared thermometer to read the temps of my leafs they are around 68F. However as the a/c stays off and the temps creep back up the leaf surface temp raises dramatically to avout 88F. During this time my room temps stay consistent opposed to the rise and drop of the leaf surface temps.
I want to know should I focus more on the leaf surface temps and could I run into problems? When my lights are the leaf surface temps are fine. This is happening only when lights are on.

 

[MIhomegrown]

Ideal leaf surface temperature is 88F for cannabis, The heating to optimal lst and cooling to prevent damage is probably beneficial.

Even if it drops to about 68F. It's just weird how much the lst changes oppose to my room temps

 

[Switcher56]

What type of light are you using? Light is focused energy but... 10 deg delta between leaf surface temperature as LST is commonly known as "low stress training". From what I have read, your AC is running from its own "onboard" thermostat vice the room thermostat/controller. It should get its signal from the "room temperature" controller. That is why the controller has 2 outputs: heat and cooling.

Are the 2 rooms separated or is everything happening in the same room e.g veg anf flower. If so, I would separate the two as VPD (vapour pressure deficit) for seedlings, veging and flowering. 3 totally different climates, and each need to be dialed in, dependent on what stage you are at.

This is about all I have based on the thinnest of the info provided. I am retired engineer not a dentist

 

[MIhomegrown]

What type of light are you using? Light is focused energy but... 10 deg delta between leaf surface temperature as LST is commonly known as "low stress training". From what I have read, your AC is running from its own "onboard" thermostat vice the room thermostat/controller. It should get its signal from the "room temperature" controller. That is why the controller has 2 outputs: heat and cooling.

Are the 2 rooms separated or is everything happening in the same room e.g veg anf flower. If so, I would separate the two as VPD (vapour pressure deficit) for seedlings, veging and flowering. 3 totally different climates, and each need to be dialed in, dependent on what stage you are at.

This is about all I have based on the thinnest of the info provided. I am retired engineer not a dentist

I use double ended hps light fixtures. I keep writing lst based on the topic which what we have been talking about. I have separate veg and bloom rooms. The discussion is about my bloom room. I don't follow vpd because I like it a bit dryer in my rooms. The a/c gets turned on and off from the thermostat. The thermostat is in the flower room and there is no on board thermostat in my a/c unit. There is really no more info I can give so if you can't understand, you just can't. My main goal of this post was to determine the importance of leaf surface temps compared to the room's temperature because I noticed a drastic rise and drop of temp to the lst compare to my room temp which stays consistent.

 

[Switcher56]

Well if you want to stand there with a lazer thermometer. please feel free to do so. As I am sitting here writing, 20 some odd ft from my tent, tent temps are 63.6 deg F @ 61.2 RH. Lights are out. IMHO you control your environment. If you are getting that large of a delta, your lights may be too close. A 20 deg spread IMHO is excessive! I don't get anywhere near that. As Abbey would say on NCIS. you got something "hinky" going on there Hope you figure it out

 

[MIhomegrown]

Well if you want to stand there with a lazer thermometer. please feel free to do so. As I am sitting here writing, 20 some odd ft from my tent, tent temps are 63.6 deg F @ 61.2 RH. Lights are out. IMHO you control your environment. If you are getting that large of a delta, your lights may be too close. A 20 deg spread IMHO is excessive! I don't get anywhere near that. As Abbey would say on NCIS. you got something "hinky" going on there Hope you figure it out

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.

 

[Ibechillin]

At 77F my a/c will shut off. During that time my room heats up a bit obviously and at the time my a/c will kick back on the lst is around 88F. When I stated I believe my thermostat is trash is because it reads slow. So I have multiple digital thermometers palced around my room, same height as my thermostat. When my a/c shuts off the thermostat reads 77F but as the room temps creep up the thermostat is slow to read them. I'll get readings of 80-81F on my other thermometers while my thermostat is still reading 77F. The a/c usually kicks on shortly after that.

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.