Vpd target vs Leaf temp.

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Veteran

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Today i dialed my room in and am curious about your opinion if i did it right or wrong.
To start i will give some input about my setup.
2x 1000 watt Gavita DE hps
watercooled ac
Co2 tank and controller
topfeed coco
first week bloom

before:
wattage lights 600 watt
room temp 30 c/86 F
Rh 65/70
vpd around 1,5
leaf temp 26,5 c /80 F
Ec 1,3
ph 5,8

After:
wattage lights 750 watt
room tem 30 c / 86 F
Rh 50
vpd around 1,9
leaf temp 25 c / 77 F
ec 1,3
ph 5,8

(The vpd and Rh can fluctuate because the ac turns on and of swinging the Rh level)

If we look at both climates i guess most of you will prefer a vpd of 1,5 rather than 1,9 following a vpd chart.

But dont the plants have more evaporation with a 1,9 vpd causing more co2 uptake because of open stomata?

As long as the leaf temp is cooler than ambient room temp it should be ok right? If you dont agree please explain.

Where is the limit we want to push when we want the highest yields and best quality?

The point im trying to make is that leaf temp is a better guide than a vpd chart, i could be wrong and would like to hear your opinions about this.
 
Last edited:
Og_punkgenetics

Og_punkgenetics

180
63
Today i dialed my room in and am curious about your opinion if i did it right or wrong.
To start i will give some input about my setup.
2x 1000 watt Gavita
watercooled ac
Co2 tank and controller
topfeed coco
first week bloom

before:
wattage lights 600 watt
room temp 30 c/86 F
Rh 65/70
vpd around 1,5
leaf temp 26,5 c /80 F
Ec 1,3
ph 5,8

After:
wattage lights 750 watt
room tem 30 c / 86 F
Rh 50
vpd around 1,9
leaf temp 25 c / 77 F
ec 1,3
ph 5,8

(The vpd and Rh can fluctuate because the ac turns on and of swinging the Rh level)

If we look at both climates i guess most of you will prefer a vpd of 1,5 rather than 1,9 following a vpd chart.

But dont the plants have more evaporation with a 1,9 vpd causing more co2 uptake because of open stomata?

As long as the leaf temp is cooler than ambient room temp it should be ok right? If you dont agree please explain.

Where is the limit we want to push when we want the highest yields and best quality?

The point im trying to make is that leaf temp is a better guide than a vpd chart, i could be wrong and would like to hear your opinions about this.
I use the G robot vpd app, it's pretty convenient because it gives you All the calculations based on real time stats. They also have three different variables for leaf temperature versus ambient temperature. I think it's free if I remember right, or if it is and it's only a couple bucks
 
Veteran

Veteran

53
18
I use the G robot vpd app, it's pretty convenient because it gives you All the calculations based on real time stats. They also have three different variables for leaf temperature versus ambient temperature. I think it's free if I remember right, or if it is and it's only a couple bucks
That is exactly what i dont want, and the point i try to make is not to stare blind at charts and calculators. I think for a beginner its ok but when you want to push the limits its not enough.
 
Aqua Man

Aqua Man

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Supporter
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Today i dialed my room in and am curious about your opinion if i did it right or wrong.
To start i will give some input about my setup.
2x 1000 watt Gavita
watercooled ac
Co2 tank and controller
topfeed coco
first week bloom

before:
wattage lights 600 watt
room temp 30 c/86 F
Rh 65/70
vpd around 1,5
leaf temp 26,5 c /80 F
Ec 1,3
ph 5,8

After:
wattage lights 750 watt
room tem 30 c / 86 F
Rh 50
vpd around 1,9
leaf temp 25 c / 77 F
ec 1,3
ph 5,8

(The vpd and Rh can fluctuate because the ac turns on and of swinging the Rh level)

If we look at both climates i guess most of you will prefer a vpd of 1,5 rather than 1,9 following a vpd chart.

But dont the plants have more evaporation with a 1,9 vpd causing more co2 uptake because of open stomata?

As long as the leaf temp is cooler than ambient room temp it should be ok right? If you dont agree please explain.

Where is the limit we want to push when we want the highest yields and best quality?

The point im trying to make is that leaf temp is a better guide than a vpd chart, i could be wrong and would like to hear your opinions about this.
So imo since running co2 makes a plant more efficient at photosynthesis and in turn slows transpiration rates we need to run a slightly lower RH to help compensate a bit and often slightly higher nutrient concentration. Gas exchange is not directly related to the release of water vapor so no more transpiration does not increase the amount of co2 intake but rather the stomatal opening size does.



 
Og_punkgenetics

Og_punkgenetics

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That is exactly what i dont want, and the point i try to make is not to stare blind at charts and calculators. I think for a beginner its ok but when you want to push the limits its not enough.
I'm curious as to why? If there is something that puts you on point in regards to optimum vpd why wouldnt you use it? Just curious
 
Aqua Man

Aqua Man

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Just to add with atmospheric co2 leaf temps of 75-77f are ideal and under co2 enrichment they can be upped a bit.
 
Veteran

Veteran

53
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So imo since running co2 makes a plant more efficient at photosynthesis and in turn slows transpiration rates we need to run a slightly lower RH to help compensate a bit and often slightly higher nutrient concentration. Gas exchange is not directly related to the release of water vapor so no more transpiration does not increase the amount of co2 intake but rather the stomatal opening size does.



The stomatal opening size ? So how do we know when its at it best? I thought when transpiring stomata is open and when closed the plant doesnt want to transpire because it will dry out or it cant because of too high rh causing higher leaf temps. About compensating higher nutrients , low ec with high evaporation is the same as high ec with low evaportation. I keep my ec low because vpd is high.
 
Aqua Man

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The stomatal opening size ? So how do we know when its at it best? I thought when transpiring stomata is open and when closed the plant doesnt want to transpire because it will dry out of it cant because of too high rh causing higher leaf temps. About compensating higher nutrients o low ec with high evaporation is the same as high ec with low evaportation. I keep my ec low because vpd is high.
It's all in that thread but if you create to high of a vpd the plants close stomata because the plant is trying to conserve water. This is because it will be losing water faster than the roots can pull it from the media. In fact a higher RH to a point will increase stomatal opening... to high we lose transpiration rates needed for uptake. To low RH the plants close stomata to prevent water loss.

As I said co2 will improve photosynthetic rates reducing transpiration rates which reduces water stress. This is why you can run plants with enriched co2 in much warmer and dryer environments.
 
Veteran

Veteran

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I'm curious as to why? If there is something that puts you on point in regards to optimum vpd why wouldnt you use it? Just curious
Because i believe you can get a higher yield if you are in the red zone of the chart and the vpd chart is not always the best. I mean with a vpd of 2,2 and 2 celcius cooler leaf temps than ambient temp you can still be better of than the green zone 1,2 vpd as long as your ec is not too high to prevent tip burn at your leaves and lockout at the roots. But its pushing the limits like racing a car and its risky of course.
 
Veteran

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It's all in that thread but if you create to high of a vpd the plants close stomata because the plant is trying to conserve water. This is because it will be losing water faster than the roots can pull it from the media. In fact a higher RH to a point will increase stomatal opening... to high we lose transpiration rates needed for uptake. To low RH the plants close stomata to prevent water loss.

As I said co2 will improve photosynthetic rates reducing transpiration rates which reduces water stress. This is why you can run plants with enriched co2 in much warmer and dryer environments.
If the plant closes stomata with high vpd the leaf temp should rise right? My leaf temp is low while my vpd is high so thats contradictory right?
 
Aqua Man

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Because i believe you can get a higher yield if you are in the red zone of the chart and the vpd chart is not always the best. I mean with a vpd of 2,2 and 2 celcius cooler leaf temps than ambient temp you can still be better of than the green zone 1,2 vpd as long as your ec is not too high to prevent tip burn at your leaves and lockout at the roots. But its pushing the limits like racing a car and its risky of course.
Yes you are right kinda.... if you read my first post VPD when using co2 may benefit from the upper lever or as I put it lslightly lower RH.

Leaf temps are highly affected by transpiration rates.. so if your leaf temls are high and you have room to increase the VPD of course your going to see an improvement but only if your leaf temps are an issue.

There is so many variables to this like light source, wind, root temls and a million others... its not simply VPD like you are attributing to.
 
Veteran

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I mean my vpd is too high according to a vpd chart and still is doing good because my leaf temp is lower than ambient temps . The roots are still able to uptake enough water.
What im trying to say is a vpd chart is limited its a safe zone.
 
Aqua Man

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If the plant closes stomata with high vpd the leaf temp should rise right? My leaf temp is low while my vpd is high so thats contradictory right?
You need to read the post then come back. I wrote so I don't have to explain this a million times.

Absolutely not... to high and to low are both bad... like Goldie locks said... one is just right.
 
Aqua Man

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I mean my vpd is too high according to a vpd chart and still is doing good because my leaf temp is lower than ambient temps . The roots are still able to uptake enough water.
What im trying to say is a vpd chart is limited its a safe zone.
You leaf temps being cooler have absolutely no bearing on being ok or not... its the actual leaf temperature that matter not how much cooler in relation to ambient temps.
 
Veteran

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You need to read the post then come back. I wrote so I don't have to explain this a million times.

Absolutely not... to high and to low are both bad... like Goldie locks said... one is just right.
I dont agree, i meant my vpd is too high according to a vpd guide chart but that doesnt mean my vpd is too high for a succesfull grow. As long as my leaf temp is cool enough it means my plant is doing fine.
 
Veteran

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You leaf temps being cooler have absolutely no bearing on being ok or not... its the actual leaf temperature that matter not how much cooler in relation to ambient temps.
In that case we should follow the ideal leaf temp like you suggested earlier. And with co2 suplementation that can be higher.
 
Aqua Man

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I dont agree, i meant my vpd is too high according to a vpd guide chart but that doesnt mean my vpd is too high for a succesfull grow. As long as my leaf temp is cool enough it means my plant is doing fine.
What chart? That's your problem you are using a chart... THEY ARE NOT ACCURATE you MUST calculate your own based on LEAF TEMP.
 

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