Led Temps - Air vs Leaf Surface

ComfortCreator said:

The data for what temps to run comes from the old school lighting that simulates the sun in its heating the leaves. Because there are decades of grows the ideal temps were developed very clearly.

LED threw a wrench in that, as most LEDs lack infrared and UV lighting that the sun has, and to some extent was in the HPS/HID spectrums.

To account for the difference between the technologies it was discovered, as you did, that leaf temps are lower with LED due to the missing spectrum.

So obv the longer term solution is to integrate the exact right spectrum into LED...but what that is and how to achieve it with most fixtures is not settled yet.

Most measurements seem to show a roughly 3f to 5f difference in air to leaf temps.

So most use the old standards, which have so many successful grows proven, and increase ambient air temp by the difference.

Exact numbers are less clear than what is too high or too low. Over 84/85f without CO2 supplementation is rarely done, and below 75f, where the leaf temps might only be 70f, is considered OK but not optimal.

Click to expand...

meangreengrowinmachine said:

Hmmm I have been looking at getting some of the supplemental boards from rapid led for flowering... do you think adding one to my veg room, or really any led growing application would be beneficial?

Click to expand...

ComfortCreator said:

The data for what temps to run comes from the old school lighting that simulates the sun in its heating the leaves. Because there are decades of grows the ideal temps were developed very clearly.

LED threw a wrench in that, as most LEDs lack infrared and UV lighting that the sun has, and to some extent was in the HPS/HID spectrums.

To account for the difference between the technologies it was discovered, as you did, that leaf temps are lower with LED due to the missing spectrum.

So obv the longer term solution is to integrate the exact right spectrum into LED...but what that is and how to achieve it with most fixtures is not settled yet.

Most measurements seem to show a roughly 3f to 5f difference in air to leaf temps.

So most use the old standards, which have so many successful grows proven, and increase ambient air temp by the difference.

Exact numbers are less clear than what is too high or too low. Over 84/85f without CO2 supplementation is rarely done, and below 75f, where the leaf temps might only be 70f, is considered OK but not optimal.

Click to expand...

ComfortCreator said:

The data for what temps to run comes from the old school lighting that simulates the sun in its heating the leaves. Because there are decades of grows the ideal temps were developed very clearly.

LED threw a wrench in that, as most LEDs lack infrared and UV lighting that the sun has, and to some extent was in the HPS/HID spectrums.

To account for the difference between the technologies it was discovered, as you did, that leaf temps are lower with LED due to the missing spectrum.

So obv the longer term solution is to integrate the exact right spectrum into LED...but what that is and how to achieve it with most fixtures is not settled yet.

Most measurements seem to show a roughly 3f to 5f difference in air to leaf temps.

So most use the old standards, which have so many successful grows proven, and increase ambient air temp by the difference.

Exact numbers are less clear than what is too high or too low. Over 84/85f without CO2 supplementation is rarely done, and below 75f, where the leaf temps might only be 70f, is considered OK but not optimal.

Click to expand...

meangreengrowinmachine said:

So where does all this data land us as far as a solution for the home grower? And more specifically a home grower like the OP (and myself), that can't get the temps up much higher?

Click to expand...

HippieDudeRon said:

Need to get into the 800+nm to start getting direct surface heating. And there will not be a day ever that those nm are incorporated into a "LED spectrum" as you called it. 730nm...ya, but its not heat just a deeper red.

Click to expand...

ilovereggae said:

I only have my veg tent about half full. was going to throw another light or 2 in there and I think that will get my temps up a little higher. looking at vpd charts again I think my current issue is I need to get my RH up a little more. the dry gas heat sucks the room dry down to 30% so I been putting a dish of water w a rag or sponge in it in the tent. I'm right at 50% rn. I think I need to get up to 60-65%.

Click to expand...

salmonetin said:

VPD Chart

Vapor pressure deficit (VPD) chart & calculator for horticulture in controlled environment. Cannabis, tomatoes, leafy greens, cucumber.

vpdchart.com

Leaf Temp under Dew Point: Danger Zone (Plant Disease)...COLOR GRIS
VPD under 0.4: Danger Zone (Under Transpiration)...COLOR VIOLETA
VPD from 0.4 to 0.8: Early Vegetative Growth / Propagation (Low Transpiration)...COLOR CIAN
VPD from 0.8 to 1.2: Late Vegetative / Early Flower (Healthy Transpiration)...COLOR VERDE
VPD from 1.2 to 1.6: Mid / Late Flower (High Transpiration)...COLOR AMARILLO
VPD over 1.6: Danger Zone (Over Transpiration)...COLOR ROSA

What is VPD?

Vapour Pressure Deficit (VPD), is an advanced metric that combines air temperature, leaf temperature, and relative humidity (RH) into a single unit of pressure.


How to calculate VPD?

VPD = VPleaf - VPair
VPleaf = 610.7 * 107.5*Tleaf/(237.3+Tleaf) / 1000
VPair = 610.7 * 107.5*Tair/(237.3+Tair)/1000 * RH/100

VPleaf - vapor pressure of the leaf (canopy) in kPa
VPair - vapor pressure of the air in kPa
Tleaf - leaf temperature in Celsius
Tair - air temperature in Celsius
RH - air relative humidity
VPD - vapor pressure deficit in kPa

Or simply, use the VPD chart and calculator.
You need a thermometer/hygrometer to measure the grow room temperature, RH and ideally an infrared thermometer, so you can measure the canopy temperature.
If you don't have an infrared thermometer, position your thermometer's remote probe at canopy level and take the reading after a few minutes, or assume that leaf temperature is 2°C cooler than air temperature.

Why is VPD important?

VPD expresses how temperature and relative humidity actually affect the plants - growth, stress and diseases.
VPD is a great indicator of plant transpiration rate.
Transpiration is necessary for photosynthesis, nutrient uptake, and plant cooling.
Plant stress is brought about by either excessive transpiration (high VPD values) or the inability to transpire adequately (low VPD values).
Therefore, VPD can help growers make decisions about irrigation, heating / cooling and dehumidification in their grow room / greenhouse.

What is the ideal VPD?

The ideal VPD range varies with the crop species and the stage of growth.
Currently, the ranges in the chart are for cannabis, but soon other plant species will be added.

What if VPD is too low ?

Humidity is high and plants are unable to evapotranspirate enough water to enable the transport of nutrients to cells, even though the stomata may be fully open.
Some plants may even exude water through special leaf tip or edge structures (hydathodes or water glands), forming drops, in a process called guttation.
In cases where the VPD is extremely low, water may condense onto leaves and other plant parts.
This can provide a medium for fungal/mold growth and disease.
Plants are unable to evaporate water and turgor pressure within the cells can cause splitting and cracking of fruits.

What if VPD is too high ?

Humidity is low and the rate of evaporation from the leaves can exceed the supply of water into the roots.
This will cause the stomata to close and photosynthesis to slow or stop.
The leaves might be at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate.
To avoid injury and death from wilting, many plant species will either curl their leaves or orient them downward in an attempt to expose less surface area to the sun.
This can significantly reduce the growth rate and quality of crops.

How to control VPD?

To increase the VPD, moisture must be removed from the air, or the moisture holding capacity of the air must be increased by raising the temperature.
Moisture removal can be accomplished by using dehumidifiers (expensive), or by replacing moist air with drier air (typically through ventilation).
This is the standard practice for avoiding direct condensation onto crop or greenhouse surfaces.

To decrease the VPD, operate fog or misting devices to add water vapor to the air, or cool the air.
Both would reduce VPD level and the evapotranspiration stress in the crop.

Click to expand...

salmonetin said:

VPD Chart

Vapor pressure deficit (VPD) chart & calculator for horticulture in controlled environment. Cannabis, tomatoes, leafy greens, cucumber.

vpdchart.com

Leaf Temp under Dew Point: Danger Zone (Plant Disease)...COLOR GRIS
VPD under 0.4: Danger Zone (Under Transpiration)...COLOR VIOLETA
VPD from 0.4 to 0.8: Early Vegetative Growth / Propagation (Low Transpiration)...COLOR CIAN
VPD from 0.8 to 1.2: Late Vegetative / Early Flower (Healthy Transpiration)...COLOR VERDE
VPD from 1.2 to 1.6: Mid / Late Flower (High Transpiration)...COLOR AMARILLO
VPD over 1.6: Danger Zone (Over Transpiration)...COLOR ROSA

What is VPD?

Vapour Pressure Deficit (VPD), is an advanced metric that combines air temperature, leaf temperature, and relative humidity (RH) into a single unit of pressure.


How to calculate VPD?

VPD = VPleaf - VPair
VPleaf = 610.7 * 107.5*Tleaf/(237.3+Tleaf) / 1000
VPair = 610.7 * 107.5*Tair/(237.3+Tair)/1000 * RH/100

VPleaf - vapor pressure of the leaf (canopy) in kPa
VPair - vapor pressure of the air in kPa
Tleaf - leaf temperature in Celsius
Tair - air temperature in Celsius
RH - air relative humidity
VPD - vapor pressure deficit in kPa

Or simply, use the VPD chart and calculator.
You need a thermometer/hygrometer to measure the grow room temperature, RH and ideally an infrared thermometer, so you can measure the canopy temperature.
If you don't have an infrared thermometer, position your thermometer's remote probe at canopy level and take the reading after a few minutes, or assume that leaf temperature is 2°C cooler than air temperature.

Why is VPD important?

VPD expresses how temperature and relative humidity actually affect the plants - growth, stress and diseases.
VPD is a great indicator of plant transpiration rate.
Transpiration is necessary for photosynthesis, nutrient uptake, and plant cooling.
Plant stress is brought about by either excessive transpiration (high VPD values) or the inability to transpire adequately (low VPD values).
Therefore, VPD can help growers make decisions about irrigation, heating / cooling and dehumidification in their grow room / greenhouse.

What is the ideal VPD?

The ideal VPD range varies with the crop species and the stage of growth.
Currently, the ranges in the chart are for cannabis, but soon other plant species will be added.

What if VPD is too low ?

Humidity is high and plants are unable to evapotranspirate enough water to enable the transport of nutrients to cells, even though the stomata may be fully open.
Some plants may even exude water through special leaf tip or edge structures (hydathodes or water glands), forming drops, in a process called guttation.
In cases where the VPD is extremely low, water may condense onto leaves and other plant parts.
This can provide a medium for fungal/mold growth and disease.
Plants are unable to evaporate water and turgor pressure within the cells can cause splitting and cracking of fruits.

What if VPD is too high ?

Humidity is low and the rate of evaporation from the leaves can exceed the supply of water into the roots.
This will cause the stomata to close and photosynthesis to slow or stop.
The leaves might be at risk of high temperature injury since evaporative cooling is reduced due to the lack of water to evaporate.
To avoid injury and death from wilting, many plant species will either curl their leaves or orient them downward in an attempt to expose less surface area to the sun.
This can significantly reduce the growth rate and quality of crops.

How to control VPD?

To increase the VPD, moisture must be removed from the air, or the moisture holding capacity of the air must be increased by raising the temperature.
Moisture removal can be accomplished by using dehumidifiers (expensive), or by replacing moist air with drier air (typically through ventilation).
This is the standard practice for avoiding direct condensation onto crop or greenhouse surfaces.

To decrease the VPD, operate fog or misting devices to add water vapor to the air, or cool the air.
Both would reduce VPD level and the evapotranspiration stress in the crop.

Click to expand...

ilovereggae said:

thank you. I was still half asleep when I made my first post. I do get the overall concept and realize that most likely the 81-84 is air temps needed under led to get leaf temps to their proper range. let me ask a more direct question:

When calculating VPD range should I be looking at air temp or leaf temp?

Click to expand...

meangreengrowinmachine said:

Looking at this chart again am I to understand that the correct "flow" for this throughout a plants life cycle would be to flow from the blue zone through the green to end in the orange are for flowering? So theoretically could we set a number to be at each day and change it every day? Every week or two maybe would be more realistic? So for example I'm on day 31 from seed. I would think I would want to be just getting into or in the middle of the green section yes?

Click to expand...

DukeFluke said:

None of this debate accounts for root temps. The measurements should go hand in hand IMO and I think we're missing a big piece of the debate whenever we look at the two things separately.

It's possible you change parameters at canopy level when you change them at root level. That hasn't yet been properly studied in an indoor environment with Cannabis. I've noticed a big difference in plant health and response to LED lighting when I've turned on my underfloor heating. All other conditions being the same or similar.

It's possible the bottom heat and root warmth mitigates some of the effects of the cooler air temps by boosting plant metabolism to enable better transpiration, or some other functions.

If you're struggling to keep air temps stable or warm, you'll definitely be struggling to keep the temps in a soaking wet rootball at the point where they're not significantly affecting plant growth.

Click to expand...

ComfortCreator said:

Think of several stages.

Germination or clone start
Early veg
Late veg / Early flower
Mid flower
Late flower
There is a target for each. The transition is seemless and not important as to the specifics and exact time of changes.

70% ish to start, down to 40% or less at the end. How and when it changes is up to you. The critical change is after the flowers form, say week 4 after flip, you must get the humidity down under 60% and more likely 50%. Rot can begin around then. That is why the chart may say run 60%, but we dont in mid flower.

Exceptions and successes occur outside these guidelines all the time. People can live on bread and water...just not ideally. Plants respond well to proper care like people do.

Click to expand...

ilovereggae said:

I only have my veg tent about half full. was going to throw another light or 2 in there and I think that will get my temps up a little higher. looking at vpd charts again I think my current issue is I need to get my RH up a little more. the dry gas heat sucks the room dry down to 30% so I been putting a dish of water w a rag or sponge in it in the tent. I'm right at 50% rn. I think I need to get up to 60-65%.

Click to expand...