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Lets talk about VPD...it's worth the converstion. Thread Tools Search this Thread
Old 10-30-2015, 06:25 AM #41
kowhite
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Agreed heady...good links. thanks to both led05 and st.phatty.

Phatty - you could just throw some more water at them...but, I think adjusting your temps or RH would prob be a bit better for your plants.

peace and CBD grease
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Old 10-30-2015, 10:33 PM #42
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Originally Posted by kowhite View Post
Plenty of info available about this already and I've done a bit of research but I haven't seen much activity on any recent threads about it but I found it to be an extremely interesting subject so I figured I'd revamp the conversation.

As RH goes down and temps go up VPD increases...the air's ability to absorb water and cause evaporation increases. Too high of a VPD and plants respond to the potential of excess water loss by closing their stomata and curling up their leaves. Which in in effect does significantly slow the rate of transpiration but.....Closed stomata also decreases the plants ability to absorb a sufficient amount of CO2, which is needed for plant growth and curled leaves reduce the amount of surface area exposed to light, both negatively affecting the rate of photosynthesis that occurs within the plant. Essentially air with a high VPD causes the plants to respond to a stressor in a way that only causes further problems....bad news it is.

As RH goes up and temps go down VPD decreases...the air's ability to absorb water and cause evaporation decreases. Too low a VPD and the plants ability to transpire is greatly reduced inhibiting its ability to absorb nutrients up from its roots. When a plant is not able to transpire, and is coupled with a saturated root zone brought about by decreased absorption of water through it's roots, pressure can build up to the point that water is forced out through the leaves in a process called guttation. This in addition to the likely event, in this particular case, that dew has formed as the excess water vapor in the air condenses into liquid, creates an ideal environment suitable for all kinds of nasty little microbial life to invest your plants. Not to mention that despite the fact that a plant responds to elevated levels of humidity by opening its stomata, in turn, increasing CO2 absorption, the plant lacks adequate levels of certain vital nutrients needed to utilize this excess CO2 to it's advantage due to it's low rate of transpiration.

Finding the proper VPD (balance between temp and RH) allows for sufficient transpiration to occur within the plant, thus allowing for adequate nutrient absorption through the roots, and also allowing for sufficient CO2 absorption from the air via the plants stomata. Ideally temps and RH at higher values that are balanced to create a beneficial VPD are most effective in producing optimal levels of these two elements of plant growth.

The greater the temperature the greater the airs ability to hold water and cause evaporation. The greater the RH in the air the wider the plants stomata will open. The reason being, the more water vapor (humidity) in the air the more pressure the air exerts on the plant, so the plant must "push back" and in essence try harder to transpire. Which it does by opening it's stomata wider, in turn increasing it's ability to absorb CO2. So, if humidity is raised and a plant responds by widening the opening to it's stomata and temps are raised to allow for a greater level off water vapor (AKA evaporated water AKA transpiration) to be held by the air......A plant will still, despite the increased pressure being placed upon it (high RH), be able to properly transpire thus absorbing a sufficient amount of nutrients as it pulls up through it's roots water to replace what it has lost...and absorb increased amounts of CO2 from the air by it's widened stomata openings that the plant will be ready to utilize as it has an adequate supply of nutrients at it's disposal.

As long as it is balanced right and a few other key variables are properly controlled....

High temps + High RH = healthy, thriving plants

...just look at the jungle. Very high temps, very high humidity...ABUNDANT and THRIVING plant life.

Most of my research has about VPD has come from this website and these ideas are not by any stretch of the imagination my own, only my understanding of the concept as presented to me through the knowledge of others much more versed in the subject than I.

Please feel free to share any info you feel is beneficial to further one's knowledge about the subject from simple to complex as long as it is applicable it is welcome.
Good stuff. Thanks!
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Old 10-30-2015, 10:36 PM #43
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Hey Ko, hope your adventure is going well.

I noticed I did not know what kPa was and did some research I thought I'd share with you guys..

https://cronklab.wikidot.com/calculat...essure-deficit

"

For example take 80% RH at 25 C. From the list above, SVP = 3167 Pa

100-RH = 20
20/100 = 0.2
0.2 * 3167 = 633.4 pascals (Pa)

Note: for convenience VPD may be given in kilopascals (kPa). For this example 0.6334 kPa."

....


In regards to Led05, do you have any links to these study's from MSU? I am very curious as to what they found...thanks

It was taught to me as 'differential' as opposed to deficit. I'm asking, not saying; do you think deficit is the proper word? As explained to me, differential, is more accurate as whatever value your metering is giving you isn't a deficit per se. It's just the value and is only a 'deficit' IF it's not in line with your targets.

Or am I over thinking it? Thanks
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Old 10-30-2015, 10:58 PM #44
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thanks led, got some reading to do...

this or another thread with valuable vpd info should be a sticky xray, I agree.

Jackstrat- glad to see you enjoying this data too, not sure to be honest amigo.. I'm new to VPD(almost been a year now) and very very few people I know have the slightest clue about it.
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Old 10-30-2015, 11:16 PM #45
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Originally Posted by WaxyTaxi View Post
thanks led, got some reading to do...

this or another thread with valuable vpd info should be a sticky xray, I agree.

Jackstrat- glad to see you enjoying this data too, not sure to be honest amigo.. I'm new to VPD(almost been a year now) and very very few people I know have the slightest clue about it.
Good stuff. The explanation, in my view, is excellent; just enough geek to sound as impressive as it is and just enough regular folk to be understandable with a little thinking about it. I mean to say that is really good growing knowledge stuff. Stuff we may intuitively know or sense but couldn't readily explain.

Good stuff!
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Old 10-31-2015, 02:09 AM #46
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Jackstrat - I would have to say, in my opinion, while I definitely hear where you're coming from and fitting differential in there made me have to think for a minute, that deficit is the correct word. While the word differential most definitely has its place within this conversation about VPD, I'm going to venture out and say, it would refer to the difference between the temp and RH (as represented by the values on your metering) and the value that is presented to us as a result of this differential between these two factors is what would be referred to as a deficit. Labeled as such in leu of differential because it is what the ambient air LACKS (is deficient) before it becomes fully saturated with water vapor (100% RH). So finding out what the differential of temp vs RH is, is a step on the way to finding out what the deficit is. Just my opinion...anyone feel free to oppose it. I welcome healthy debates it is how true knowledge is attained and, ALAS, we learn shit....hope my ramblings helped.


peace and CBD grease
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Old 10-31-2015, 05:52 AM #47
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^ i was thinking similar. Interesting and very thoughtful explanations guys... happy halloween

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Old 11-21-2015, 12:25 AM #48
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What is better with VPD??


A) canopy RH 65-72 canopy temp 28.5-29.5C
bush RH 70-75 bush temp 25.8-26.5C



B) canopy RH 45-55 canopy temp 26.5-27.5C
bush RH 55-60 bush temp 24.8-25.5C
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Old 11-23-2015, 05:03 AM #49
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Thanks alot for this info, it gave me a better understanding about plant growth in different environments.
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Old 12-02-2015, 05:07 PM #50
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Excellent thread

Hi folks, just thought i would throw in my 2 cents worth. As a commercial greenhouse grower of 30 odd years, collecting, controlling and logging vpd and T.diff values is standard daily fare for me. The real value of this data is not in just setting and controlling it, but its ability to be used for what is known as plant steering. I have not seen this term used for cannabis cultivation but it is essentially what all production horticultural based environmental, rhizosphereic and plant cultural activities seek to achieve. VPD and Temp diff manipulation are simply two of many tools in the growers steering arsenal. Steering is undertaken either in a vegetative or generative direction dependent on cultivar, time of year, position in plant life cycle and many other variables. All designed to achieve maximal yield based on the genetic potential of the particular cultivar. Poor steering may significantly reduce yields and quality. Learning why and how to manipulate VPD and T.diff for yield and quality maximization is what we as growers are all seeking to achieve. I do it at work on a large scale and at home on a much much smaller scale. Hope this blurb is of value. Cheers and happy growing.
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