Lets talk about VPD...it's worth the converstion.

[kowhite]

I live in Florida and mold is an issue here(not just in plants). Every article I ever read said 60-70% during veg but RH should be below 60% for flower and most prefer 50% for flower. According to the VPD chart, for proper VPD, I would need to keep my room at 64 degrees at 50% RH and 66 at 55%. I already have to run a dehumidifier to keep RH low because AC can't keep up with our 90+% humidity.

Temperature control is an extremely important variable of the equation in regards to avoiding problems such as mold when running your rooms at a high rH. Especially when the lights go off. From what I understand from researching and listening to many peoples opinion's on climate control in your rooms, flower in specific, is that the biggest concern is keeping temps down when the lights are on, and while this is definitely an issue that must be addressed. I do not hear many people talking about the the benefit of keeping the temps up (or relatively the same) when the lights go off. Maybe they do, like I said I just do not hear much about it. This, maintaining temps during lights out, becomes an important factor when running a high rH. While the lights are on and the temp is up to say 80F running a rH of 75% shouldn't pose a problem to your plants because the air is at a high enough temp to "hold" the amount of water vapor in the air and thus not condense and cause dew. But.....when the lights go off the temps will, if allowed to, significantly drop. At this point maintaining rH at 75% poses a significant problem. If the temp drops air losses it's ability to hold the amount of water vapor that 75% rH can present it with. So, as a result the water vapor condenses into dew and in very poorly controlled environments guttation may occur as well...more than likely result of this unfortunate (irresponsible) chain of events....MOLD. Given time...LOTS OF MOLD. This problem can be solved by properly maintaining temps to coincide with rH when the lights go off.

I would suggest rockinrobin, in an effort to be more economically efficient, is that you do a bit of research into which strains you grow and choose one(s) that grow best in hot and humid environments, they are definitely out there. Then settle on a nice hot and humid VPD in the beneficial ranges and use that hot and humid Florida air to your advantage. Pull in outside air and use it to maintain the high temps and rH, the closer the conditions you choose to maintain are to the outside conditions the less your environmental control equipment will have to work to get the air where it needs to be. Look into lung rooms if you already haven't...they are quite effective setups for utilizing this particular methodology.

https://www.icmag.com/ic/showthread.php?t=311061

Pretty cool thread thats building right now that has some solid info about what they are and how to design them.....

I say all of this with a grain of salt. Your yields using what ever methods you have been using may very well be quite impressive, I have no clue, but...it just seems to me from the little bit of info you've shared that you're fighting against the current swimming upstream in an effort to get those yields (whatever they may be), learn to go with the flow man. Lung rooms are fucking awesome.

Peace and CBD grease.

 

[kowhite]

How does VPD change in a sealed environment? What I'd really like to see is the ideal VPD for cannabis in an indoor environment, not a greenhouse. According to all of the charts I've seen, in a sealed room with CO2 and 84 degrees f temp. requires 75-80% RH (at least for veg). Maybe indoor and greenhouse requirements are the same?

I'm pretty sure VPD is VPD weather indoors in a sealed environment or outdoors in the open air. As long as the temp and rH are the same whether it be created and controlled by Mother Nature herself or controlled by your personal doing I'm pretty sure VPD effects plants in the same way. The advantage of doing it in a strictly controlled environment is that there is never a day in which conditions aren't optimal, the environment in which your plants grow should remain conducive to healthy growth throughout the plants entire lifetime. Unless, of course, you fuck things up. But, in nature the fluctuations of these conditions from day to day can bring about pockets of time in which those conditions may not be so conducive to growth and the plant does not reach it full potential within it's given season. So, I would think, the optimal VPD to grow plants outdoors/greenhouse would be the same as in a sealed environment.

 

[waveguide]

apparently i saved this from a thread or link last november, soz i can't link the discussion..

micro in the desert.. even during a monsoon, with all the tricks, i never go above ~30% rh good luck to you!

 

[Fly by Night]

Worthy converstion

Worthy converstion

Or, intellectual discussiom

 

[Robrites]

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.

The letters VPD stand for.....?

 

[Granger2]

OK Waxy and Wave,
Those Vapor Pressure Deficit charts are handy, nifty and all that, but I can't read them wiff confidence. I saved a copy months ago for all the good that does me.

WTF is the color I should work to have room conditions fall into?!?! I guess I'm too dense. Prease exprain. Thanks. -granger

 

[kowhite]

Robrites - Like ganger said above Vapor Pressure Deficit.

Granger - hadn't seen that first chart from wave but I've heard about the chart that you want to maintain your temp and rH at levels that coincide with VPD values in the orange squares...

 

[kowhite]

I've posted this before in another thread not of my own creation but figured posting it here as well can only help things. I am currently designing the first growroom (of many I'm hoping) that I've ever built. It comes from the knowledge and know-how of many different sources, the majority from members of this site. Any feedback on potential problems or outright flaws in the design are welcome.....

I have an attic space, lung room (8x7), flower (6x12) and veg (8x5) rooms all within a well insulated 12 x 15 shed....each room is separated by an insulated wall. I will intially be separating the flower and veg rooms in half with temporary but "airtight" walls.

- active/filtered intake (240 cfm) into the attic space to pre-condition the air a bit (warm it up so your climate control equipment will have an easier time with getting it to where it needs to be).

- active exhaust (1600 cfm), filtered again, (just because) from the attic to the lung room

- passive intake setup for both the flower and veg rooms from the lung

- active exhaust (722 cfm form flower/483 cfm veg) from both grow rooms back up into the attic being scrubbed in between

- the exhaust will also be in the attic...three separate passive exhausts with backdraft dampers and a fan (1300 cfm) set to a thermostat to kick on for quick extra exhaust when temps start to rise.

But then I wonder if the backdraft dampers would even be needed...lot of air being actively moved into that attic space so there should plenty of positive pressure to push the air out, right? But then again I think, fuck that, better to be safe than sorry and let unfiltered air seep into your system...put a backdraft damper AND a filter on that shit and have no worries.

In my humble opinion adequate air circulation is a variable that plays a key role in the healthy growth of your plants and while some may say this setup goes a bit far I'm pretty confident that in the end my girls will love me for it. Let me know what you think.


peace and CBD grease

 

[kowhite]

Damn Waxy - I went so deep into the rabbit hole I forgot it was in your thread that I had first come across VPD. I've had been skipping right to the most recent post whenever I go to it...it's right there in your initial post. Thanks for putting that out there for my eyes to see. I think being aware of this whole VPD thing will prove to be very useful in the future.


peace and CBD grease

 

[led05]

VPD Guidelines

VPD Guidelines

I'd follow the Tomato guidelines and follow-up on these studies if wanting more details, solid and relevant, indoors allows for probably slightly more deficeincies but when VPD is right, your RH will be surprisingly high indoors and in GH, always, for near all plant species baring cactai / succulent family & similar .....

"Growers should aim to have fairly low VPD, for example 0.3 kPa, when rooting cuttings in greenhouses. This will reduce the drying of young plants, thereby reducing the frequency of misting and watering required to keep plants hydrated. However, Michigan State University Extension recommends maintaining a greater VPD (greater than 0.5 kPa) in greenhouses while finishing plants, especially when there is a dense plant canopy. Plants will be able to transpire, cool themselves and be less stressed while the environment is less conducive to disease.

Greenhouse vegetable growers harvesting fruits should be aware that one study, “Vapor Pressure Deficit (VPD) Effects on the Physiology and Yield of Greenhouse Tomato,” reported that a VPD of 0.8 kPa during the day and night increased photosynthetic rates and tomato fruit yields compared to plants grown with a VPD of 0.5 kPa. Too dry of an environment can also cause problems. For example, another study, “High Vapor Pressure Deficit Influences Growth, Transpiration and Quality of Tomato Fruits,” showed that a very high VPD of 2.2 kPa could cause plant stress and fruit cracking in tomato.

 

[heady blunts]

i was just gonna bump this thread! how's everyone's projects going?

i just split my flower room into flip flops thanks to this thread and WT's thread.

 

[St. Phatty]

http://www.growell.co.uk/blog/2014/06/temperature-relative-humidity-and-vapour-pressure-deficit

had to look it up, VPD = vapour-pressure-deficit.


When the air is dry, the plants lose more moisture through their leaves and buds, so they have to be watered more.

I think.

 

[led05]

Waxy...

http://msue.anr.msu.edu/news/why_sh...pay_attention_to_vapor_pressure_deficit_and_n

Decent links in there, I shoot for .75-1.2kpa, not running more than 75%RH in winte, inside

 

[led05]

http://www.google.com/url?q=http://wsp.arizona.edu/sites/wsp.arizona.edu/files/uawate

http://www.google.com/url?q=http://wsp.arizona.edu/sites/wsp.arizona.edu/files/uawate

Here's a good one too...

 

[led05]

Sorry about that link, just Google "VPD Tomatoes Arizona", Dry climates like AZ naturally have significant VPD issues so a good place to start, homes, buildings in general are arid due to HVAC both winter & summer & natural effects of having a room(s) much warmer than surrounding creates even greater VPD issues unless you want mold to potentially seep into the rest of your home I'd target Kpa of .75-1.25 which are reasonable but in 80's this still equates to surprisingly high RH numbers... For years what I thought was mag, molybdenum, iron etc deficiency were clearly VPD issues, LED added to this confusion naturally BC the underlying opinion is they cause excessive cal/mag usage, my guess unless you're living in the SE USA, you're feeling some level of VPD, especially indoors. To previous poster about ideal levels indoors vs. A GH, who cares, both are controlled environment, same effect on plants but structures do function differently, this doesn't change the VPD / Kpa required numbers..... Anyone ever grow orchids, inside ....?

 

[heady blunts]

good links thanks led05

 

[Xray Kimono]

There needs to be a VPD sticky... why no this thread as it expands?
Xray

 

[kowhite]

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

 

[JackStrat]

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!

 

[JackStrat]

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..

http://cronklab.wikidot.com/calculation-of-vapour-pressure-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