You will soon enough.
Vapor Pressure Deficit & What It Means To You!
- Thread starter Archiweedies
- Start date
About 35-40 more days woot!
NorthernCosmos
Well-Known Member
The tent was in the optimal band for flowering for a few hours yesterday. It's doable some evenings here - after it has rained... 
Now considering switching to lights on during the night, although I'm not sure how to do that without stressing and potentially hermying the plants. It would only be 4-6 hours of darkness and lower temps in the daytime, but if timed to coincide with the warmest part of the day it might have an overall positive effect.
Now considering switching to lights on during the night, although I'm not sure how to do that without stressing and potentially hermying the plants. It would only be 4-6 hours of darkness and lower temps in the daytime, but if timed to coincide with the warmest part of the day it might have an overall positive effect.
One hour at a time, lol?
Just change the light schedule, if they’re not in flower they’ll be fine.
Gazoo
Member
I have read timing in veg can be changed by switching them to a 24 light period for a day or so and then change to new scheduled, but have never done it myself
Also have understood if you are changing light timing from a already set scheduled without resetting from 24
its best to only adjust at the lights out end of the day. this is were I adjust from when moving into flower
Also have understood if you are changing light timing from a already set scheduled without resetting from 24
its best to only adjust at the lights out end of the day. this is were I adjust from when moving into flower
I adjust after lights out as well.
I grew some of my best weed in 100F and 80rH .... only cause my AC went down and I said fuckit I'm moving. lol
I don't know why I just read this out loud in a Bill Nye the Science guy voice but hell yeah brother , awesome infoVPD - Overview
Until the last couple years, Vapor Pressure Deficit, was measured mostly by research universities and agricultural laboratories. The tech available to growers these days has enabled us to dial a grow room in very accurately with regards to heat and RH. We all know that cannabis can benefit from an environment between 70F- 85F or 20C - 30C. We also can benefit from slightly colder temps at night and this is encouraged as it promotes maximum growth during lights out and the potential for certain genetic properties of a particular chemovar to manifest such as purple or pink buds, etc. Temperature and Relative Humidity will allow you to grow cannabis, and grow it well, if the parameters are maintained. When we keep our plants in the "sweet spot" of VPD, the stomata of the leaves are manipulated in a way that allows maximum performance from our plant. One analogy is that the VPD happy plant is like an athlete, whereas a plant outside of the ideal VPD zone is just strolling through the park.
I'd like to first state that I was urged to look into this information from fellow growers @Pennywise and @InTheShed . I am sharing what I learned, in my own words, based off the research I did and then began implementing in my own grow. After dialing in the environmental numbers, my plants looked completely different in less than 24 hours. They were perkier, healthier looking, and just plain looked ready to burst with growth. All anecdotal, of course, but if you have the capability to manipulate your temps and RH, you need to try this!
In order to understand what VPD is, we need to define the variables that go into calculating the VPD. Water vapor is the gas form of water. Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature.
The air inside a grow space can hold a given amount of water vapor relative to the grow space temperature before it condenses to the liquid form of water. As temperature increases the SVP rises and as temperature falls, so does the SVP.
This is why we get dew in the morning after a cool night. The SVP is low, because the temps are low, but the Actual Vapor Pressure ( AVP) is higher than the SVP and boom! We have dew all over everything as the water vapor turned to liquid form!
Did you know that Relative Humidity or RH is actually a comparison of the amount of water in the air relative to the maximum amount the air can hold? That's why we measure it in percentages. 50% RH means that at a given temperature, the air is holding 50% of its maximum amount of moisture possible.
So if Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature, and we know Relative Humidity, we can determine the Vapor Pressure Deficit.
Put simply, Vapor Pressure Deficit is the difference, or deficit, between SVP and AVP
VPD= SVP - AVP.
But Archiweedies, I dont want to math! Fear not fellow growers, you only need a couple things to start dialing in that environment so that you can take full advantage of this technique.
What you need:
Laser Temp Gun I got mine at Harbor Freight for quite cheap. We need this to determine our leaf temps at the top of the canopy. We can't assume that ambient is the same as leaf temps. Remember, the point is to get the stomata working so we care about the leaf temps at the top of the canopy.
![IMG_5580[1].JPG](/community/gallery/img_5580-1-jpg.1624739/full)
Hygrometer - You should have one already! It would be nice to have one with a probe that can be placed at canopy level to get accurate RH measurements.
VPD chart This chart is courtesy of Just4Growers.com There are many online and I suggest you find one you like and print it out for your grow room. The example below is a VPD chart that assumes a Leaf Temperature Deficit of -2 degrees Celsius below ambient.The chart you will need is going to be based on how hot your lights are and what the difference is between ambient grow room temps and leaf surface temps. Ive found, with my LED COBS that they run roughly 2 degrees Celsius cooler.
Now that we have our leaf temperature and our canopy RH, we simply cross reference the chart of your choice. If we are in the red zones above we are outside of the sweet spot for VPD. Once outside of the sweet spot and your plants are not transpiring as efficiently as they could be.
If you are outside the acceptable range, simply determine if it would be easier to manipulate the RH or temperatures to get back in line.
Example: I check my hygrometer and it says 25C. I check the leaf temps they are at 23C. RH is at 37%.
This places us just outside the VPD and it is up to you to determine if adjusting your RH up or your temps down makes more sense for you. For me, and this depends on your unique environment at the time which of course changes daily, I would bump RH up to 45% if I was in late veg/preflower. In flower I would likely choose to maintain the RH and lower temps a couple degrees Celsius to bring it back into line.
Summary
Dialing in the VPD allows us to turbocharge plant growth by allowing the plant to transpire efficiently. When the plant is transpiring more efficiently, it is drinking more nutrients from the roots. I think that is the goal of growing cannabis. Grow roots in veg and use those roots during bloom to produce flowers. By giving the plant a happy environment with the perfect VPD balance, we accelerate the plants growth across all phases of growth.
Thank you to the wonderful community here at. I hope that by presenting this information, I have in some way given back to the community that has given me so much over the last few years.
Everyone at 420 needs to save this tutorial if you want to up your game.
NorthernCosmos
Well-Known Member
And if you can't measure temps precisely or don't plan to "follow it up", the VPD charts still gives you a good ballpark figure for the optimal temp/humidity combo to aim for. I never hear much (any, except for this thread...) talk about that there is a band of temps AND humidity where your plants breathe optimally and that there are charts for that.
So even if I don't have an IR thermometer and only limited control over the environment, at least I know quite a bit more about what I should strive for to make the best growing conditions
Good candidate for a sticky!
Yes it is, people obsess over nutrients when VPD is far more important.
JollyRodger
Well-Known Member
This is mine, its simple but effective.
Any chance you’d mind explaining how your controller works?
I’ve been researching for one and the info on the web is pretty limited.
I have and inkbird temp/humidity controller but it doesn’t allow me to set day and night conditions.
also, I want to be able to run a dehumidifier and a humidifier depending on conditions. Does yours allow that?
All I use for temp control is an inline fan. It does the job well enough to control day temps, and the house a/c and heat take care of night temps.
So basically, I need a controller that will run a humidifier, dehumidifier and a fan and allow me to set day and night conditions.
Are you using the "1 deg C (1.8 F) cooler than Air Temp Chart" ?
OR THIS one for zero differance
Are you using the "1 deg C (1.8 F) cooler than Air Temp Chart" ?
OR THIS one for zero differance
Hi my friend. Im new here, i dont know if i have to post in another place to say hello. If is like this i say, sorry!!!
Very interesting information in this post about VPD, i didnt heard before and it open my eyes. Thanks to @Archiweedies
I dont understand one think about the zero differance table and make me be confused. Why the 21,22,23 degrees are in yellow and then 24,25,26,27 in green, and then 28,29,30 in yellow???? If all of them have their green and yellow zone in the table.
Thanks a lot my friend.
Rheaun
Well-Known Member
The temperature range for optimal transpiration is in green for that column. Temps in yellow and red are not effective for vpd.
Hi my friend, thanks a lot to answer. I dont understand you. What im asking is: why the teperature column have colors??? (if the temperature alone is neutral, just with the combination of humidity goes to a color zone). All of degress have yellow and green zone, green is good for growing and yellow for flowering.
For example: in te temperature column the degree 26 is in color green, but if you check the chart 26°c - 52% humidity, have VPD 1,5kPa and that is yellow zone and that is perfect for flower. Then why the number 26 are in green in the column degrees. That is what i dont understand. In the column of the number degrees, the numbers degrees 21,22,23 are in yellow. The numbers degrees 24,25,26,27 in green (but they have very good VPD in yellow zone like the numbers degrees in yellow). Thats what make me confuse. Why the numbers degrees aren't all in color white like the chart 1degree diference???
Attachments
Gazoo
Member
I have never used that first chart but suspect its the same theory as the RH Side of the chart
the temp side is also showing the best temp range in green, Yellow is good but not best, Red is out of range
Keep both sides of the reading rh & temp in the green zones for best zone
Green is best zone only for vegetative. For flowering the best is yellow, in all the charts they tell you that. And here you can read the same Understanding VPD and Transpiration Rates for Cannabis Cultivation Operations the best for vegetative is from 0,8pKa to 1,1pKa (green zone), and the best for flowering is from 1,1pKa to 1,5pKa (yellow zone).
Gazoo
Member
Again I said I never used that chart and was only giving my best understanding of what I was looking at
trying to answer the question
I will leave it up to you to figure it out
Very well explained and written thank you for sharing this important information. CLVPD - Overview
Until the last couple years, Vapor Pressure Deficit, was measured mostly by research universities and agricultural laboratories. The tech available to growers these days has enabled us to dial a grow room in very accurately with regards to heat and RH. We all know that cannabis can benefit from an environment between 70F- 85F or 20C - 30C. We also can benefit from slightly colder temps at night and this is encouraged as it promotes maximum growth during lights out and the potential for certain genetic properties of a particular chemovar to manifest such as purple or pink buds, etc. Temperature and Relative Humidity will allow you to grow cannabis, and grow it well, if the parameters are maintained. When we keep our plants in the "sweet spot" of VPD, the stomata of the leaves are manipulated in a way that allows maximum performance from our plant. One analogy is that the VPD happy plant is like an athlete, whereas a plant outside of the ideal VPD zone is just strolling through the park.
I'd like to first state that I was urged to look into this information from fellow growers @Pennywise and @InTheShed . I am sharing what I learned, in my own words, based off the research I did and then began implementing in my own grow. After dialing in the environmental numbers, my plants looked completely different in less than 24 hours. They were perkier, healthier looking, and just plain looked ready to burst with growth. All anecdotal, of course, but if you have the capability to manipulate your temps and RH, you need to try this!
In order to understand what VPD is, we need to define the variables that go into calculating the VPD. Water vapor is the gas form of water. Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature.
The air inside a grow space can hold a given amount of water vapor relative to the grow space temperature before it condenses to the liquid form of water. As temperature increases the SVP rises and as temperature falls, so does the SVP.
This is why we get dew in the morning after a cool night. The SVP is low, because the temps are low, but the Actual Vapor Pressure ( AVP) is higher than the SVP and boom! We have dew all over everything as the water vapor turned to liquid form!
Did you know that Relative Humidity or RH is actually a comparison of the amount of water in the air relative to the maximum amount the air can hold? That's why we measure it in percentages. 50% RH means that at a given temperature, the air is holding 50% of its maximum amount of moisture possible.
So if Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature, and we know Relative Humidity, we can determine the Vapor Pressure Deficit.
Put simply, Vapor Pressure Deficit is the difference, or deficit, between SVP and AVP
VPD= SVP - AVP.
But Archiweedies, I dont want to math! Fear not fellow growers, you only need a couple things to start dialing in that environment so that you can take full advantage of this technique.
What you need:
Laser Temp Gun I got mine at Harbor Freight for quite cheap. We need this to determine our leaf temps at the top of the canopy. We can't assume that ambient is the same as leaf temps. Remember, the point is to get the stomata working so we care about the leaf temps at the top of the canopy.
Hygrometer - You should have one already! It would be nice to have one with a probe that can be placed at canopy level to get accurate RH measurements.
VPD chart This chart is courtesy of Just4Growers.com There are many online and I suggest you find one you like and print it out for your grow room. The example below is a VPD chart that assumes a Leaf Temperature Deficit of -2 degrees Celsius below ambient.The chart you will need is going to be based on how hot your lights are and what the difference is between ambient grow room temps and leaf surface temps. Ive found, with my LED COBS that they run roughly 2 degrees Celsius cooler.
Now that we have our leaf temperature and our canopy RH, we simply cross reference the chart of your choice. If we are in the red zones above we are outside of the sweet spot for VPD. Once outside of the sweet spot and your plants are not transpiring as efficiently as they could be.
If you are outside the acceptable range, simply determine if it would be easier to manipulate the RH or temperatures to get back in line.
Example: I check my hygrometer and it says 25C. I check the leaf temps they are at 23C. RH is at 37%.
This places us just outside the VPD and it is up to you to determine if adjusting your RH up or your temps down makes more sense for you. For me, and this depends on your unique environment at the time which of course changes daily, I would bump RH up to 45% if I was in late veg/preflower. In flower I would likely choose to maintain the RH and lower temps a couple degrees Celsius to bring it back into line.
Summary
Dialing in the VPD allows us to turbocharge plant growth by allowing the plant to transpire efficiently. When the plant is transpiring more efficiently, it is drinking more nutrients from the roots. I think that is the goal of growing cannabis. Grow roots in veg and use those roots during bloom to produce flowers. By giving the plant a happy environment with the perfect VPD balance, we accelerate the plants growth across all phases of growth.
Thank you to the wonderful community here at
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