VPD (vapor pressure deficit)

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Moe.Red

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It sounds like your dehumidifiers are already programable enough by being able to set them to a desired rh with their built in humidistat. The plug in humidistats that most growers use are only needed if you have analog dehumidifiers and or humidifiers that do not have or display the rh setting.

Try to see how much you can dial things in with your current setup. You have three VPD variables to work with, if your ability to change one is limited you can focus on the other two.

How humid will your grow area get without the dehumidifier? If RH gets into the 60%'s without it and while running it can dehumidify the grow area into the high 40%'s then adjusting temps in the 70's while modifying light intensity (by height and or power) for correct leaf temps should give you all the control you need from veg to flower with what you have.

For example, in veg you could set the dehumidifier to 64% and could probably expect an actual room rh of 60-68%, keep the temps between 70-78f and as long as the leaves are staying a bit cooler than the room temps your vpd should be good.


Yes, this is how I run it now. I have AC which pulls a lot of water too, and it is a closed system with CO2.

BUT, I could do a better job of proper settings (night too which needs to be lower) if I had control over my dehumidifier like I do everything else. No doubt what I have is working, just looking to optimize.
 
FourthCity

FourthCity

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Yes, this is how I run it now. I have AC which pulls a lot of water too, and it is a closed system with CO2.

BUT, I could do a better job of proper settings (night too which needs to be lower) if I had control over my dehumidifier like I do everything else. No doubt what I have is working, just looking to optimize.
Why does your rh need to be lower at night?
 
FourthCity

FourthCity

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Because plants don't transpire so there is no need to have it higher... reduces risk of pathogens... Ideally 10% lower at night.


When reversed its like PM heaven... low day and high night humidity = not good.
I wasnt suggesting having the night time rh be higher, I was asking why they needed it to be lower. Nothing wrong with the rh being consistent throughout both day and night, same goes for temps too.
 
Aqua Man

Aqua Man

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I wasnt suggesting having the night time rh be higher, I was asking why they needed it to be lower. Nothing wrong with the rh being consistent throughout both day and night, same goes for temps too.
Tenls can affect node spacing so I prefer 5-10f lower at lights out.
 
Moe.Red

Moe.Red

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It would make sense to me to try to duplicate nature as best you can. Getting colder with lower rh is a typical nighttime cycle.
 
FourthCity

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Tenls can affect node spacing so I prefer 5-10f lower at lights out.
I always see more stretch when lights out temps drop and less stretch with consistent temperatures.

Also, higher vpd opens the plants stomata which will allow for greater transpiration if the plants can take advantage of it. If you decrease the temperature and the humidity this will increase vpd, doing this at night and then raising the temp and rh back up (and lowering the vpd) during the day means you are opening the stomata wider when the plants are transpiring less then closing them back up when you are saying they will transpire more, why do this?

It would make sense to me to try to duplicate nature as best you can. Getting colder with lower rh is a typical nighttime cycle.
Only to a point, lots of things happen in nature that are not ideal or even good for the plant. Its not like it gets colder at night because the plants asked for it to.
 
Moe.Red

Moe.Red

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Only to a point, lots of things happen in nature that are not ideal or even good for the plant. Its not like it gets colder at night because the plants asked for it to.

yeah, they didn’t ask for it. But they sure had lots of time to adapt to it.
 
Aqua Man

Aqua Man

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I always see more stretch when lights out temps drop and less stretch with consistent temperatures.

Also, higher vpd opens the plants stomata which will allow for greater transpiration if the plants can take advantage of it. If you decrease the temperature and the humidity this will increase vpd, doing this at night and then raising the temp and rh back up (and lowering the vpd) during the day means you are opening the stomata wider when the plants are transpiring less then closing them back up when you are saying they will transpire more, why do this?


Only to a point, lots of things happen in nature that are not ideal or even good for the plant. Its not like it gets colder at night because the plants asked for it to.
VPD doesn't count at night... they don't transpire in the dark or atleast very very little.
 
FourthCity

FourthCity

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yeah, they didn’t ask for it. But they sure had lots of time to adapt to it.
I get what you are saying but its not that simple. Climate has changed dramatically over time, not just over millions of years but even in the last thousands of years. Plants haven't had any more time to adapt to these changes than people have.

Marijuana can grow in a lot of places but it does better in some more than others. I think if you are going to use nature as your guide you can't just pick anywhere as your comparison. I would look at not just where marijuana is grown but where it thrives then try to mimic that environment. This would likely not be representative of what most marijuana experiences in nature, just what a small subset has the fortune to.

VPD doesn't count at night... they don't transpire in the dark or atleast very very little.
Right, which is why I don't understand raising it when it doesn't count then lowering it back down when it does.
 
Aqua Man

Aqua Man

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I get what you are saying but its not that simple. Climate has changed dramatically over time, not just over millions of years but even in the last thousands of years. Plants haven't had any more time to adapt to these changes than people have.

Marijuana can grow in a lot of places but it does better in some more than others. I think if you are going to use nature as your guide you can't just pick anywhere as your comparison. I would look at not just where marijuana is grown but where it thrives then try to mimic that environment. This would likely not be representative of what most marijuana experiences in nature, just what a small subset has the fortune to.


Right, which is why I don't understand raising it when it doesn't count then lowering it back down when it does.
You have it backwards... it should be higher with lights on.... or am I missing something. Ideally 10% lower with lights off.
 
Moe.Red

Moe.Red

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Marijuana can grow in a lot of places but it does better in some more than others.

I think if you are going to use nature as your guide you can't just pick anywhere as your comparison.

Better at what?

Sure I can. Why do we have landraces?

I understand where you are coming from but I look at it differently than you. That’s cool
 
FourthCity

FourthCity

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Better at what?

Sure I can. Why do we have landraces?

I understand where you are coming from but I look at it differently than you. That’s cool
Some climates may produce better yields than others, some may have more potent than plants others, some may result in different cannabinoid profiles. Not sure what you are asking about on the landraces.
 
Aqua Man

Aqua Man

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I always see more stretch when lights out temps drop and less stretch with consistent temperatures.

Also, higher vpd opens the plants stomata which will allow for greater transpiration if the plants can take advantage of it. If you decrease the temperature and the humidity this will increase vpd, doing this at night and then raising the temp and rh back up (and lowering the vpd) during the day means you are opening the stomata wider when the plants are transpiring less then closing them back up when you are saying they will transpire more, why do this?


Only to a point, lots of things happen in nature that are not ideal or even good for the plant. Its not like it gets colder at night because the plants asked for it to.
I'm not sure if you get whats being said so let me try to explain. A lower VPD opens stomata not a higher VPD.

If you reduce both temp and humidity the VPD is not likely to change much eg. Leaf diff of 5f for all examples

80F 60%RH KPA 0 87

75F 55%RH KPA 0.87

70F 50%RH KPA 0.86

And as I said plant don't transpire at night they close the stomata in the dark so the VPD makes 0 difference.

Think you are looking at the whole thing backwards.
 
Kanzeon

Kanzeon

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I've had zero PM problems since growing in proper VPD, day and night. Of course, small sample size, etc, etc. But I think that having some humidity at night is still beneficial to the coating of wax on top of the leaf that helps protect the plant from things like PM.
 
FourthCity

FourthCity

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I'm not sure if you get whats being said so let me try to explain. A lower VPD opens stomata not a higher VPD.

If you reduce both temp and humidity the VPD is not likely to change much eg. Leaf diff of 5f for all examples

80F 60%RH KPA 0 87

75F 55%RH KPA 0.87

70F 50%RH KPA 0.86

And as I said plant don't transpire at night they close the stomata in the dark so the VPD makes 0 difference.

Think you are looking at the whole thing backwards.
I just had the stomata opening part backwards, my mistake. My question still stands that if the plant doesnt transpire as much at night and is less effected by vpd at that time then what is the point of fluctuating your temps and rh between night and day?

https://www.cannabissciencetech.com/view/understanding-vpd-and-transpiration-rates-cannabis-cultivation-operations said:
The key to healthy VPD and transpiration rates is to provide controlled environmental conditions, which will come into play when you are selecting equipment for your operation.

To put it simply: Consistent temperature and relative humidity in the space --> consistent vapor pressure deficit --> plant transpiration --> plant growth.
 
Aqua Man

Aqua Man

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I just had the stomata opening part backwards, my mistake. My question still stands that if the plant doesnt transpire as much at night and is less effected by vpd at that time then what is the point of fluctuating your temps and rh between night and day?
Temp differential affect node spacing. And generally speaking if you lower RH a bit before lights out you reduce the spike... its just to reduce the chances of pathogens. I wouldn't say it a must by any means.
 

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