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Old 08-23-2015, 08:51 AM #1
kowhite
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Lets talk about VPD...it's worth the converstion.

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.
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Old 08-23-2015, 09:28 AM #2
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the only question for me is, how the hell to I get my RH back up into the sweet spot now that I'm running a minisplit. fucking thing devours moisture. more humidifiers I guess. man I miss growing at 70%. had to cut my feed down 30% now that I'm in the high 40% range. :(
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Old 08-23-2015, 09:33 AM #3
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the only question for me is, how the hell to I get my RH back up into the sweet spot now that I'm running a minisplit. fucking thing devours moisture. more humidifiers I guess. man I miss growing at 70%. had to cut my feed down 30% now that I'm in the high 40% range. :(
Greenhouse wet wall
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Old 08-23-2015, 09:04 PM #4
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bump
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Old 08-23-2015, 09:39 PM #5
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bump indeed. I've had this shit indoors and out. It looks like neon yellow / orange racing stripes on the leaves, . . . . i always mistook it for some kind of deficiency and continued mucking up my plants to no avail . . . . until somebody finally enlightened me to the VPD.

So yes, let's talk about it, and it IS definitely worth the conversation, as the title suggests.

For me, VPD only happened outside during the driest and hottest part of the year, and I don't use a greenhouse they're just out in the open, . . . . . so there was no way to control it or prevent it. It still happens to me INSIDE much more, and I just try and control it as much as possible now without causing more problems.

My most recent attempt at halting VPD indoors included a humidifier (water treated with mild H202 solution) and completely stopped all the new "racing stripes" (VPD) from appearing, but then caused a fungus from too much humidity at night (duhhh).

I always used to mistake this condition for a ZINC deficiency or an IRON deficiency. I think many other people mistakenly do this too so this thread will hopefully help any of those people out.

FWIW, the leaves that got VPD are still alive and just look ugly. it's not "contagious," it doesn't affect other leaves, it's not a systemic disease or anything. But the fungus that I got from trying to wipe out VPD is all of those things. I just wish I had never even tried to fuck with it.

I've also found that different strains respond differently to VPD, some are more resilient to it and barely get affected at all, while a plant next to it (different strain) at the same height could be totally screwed.
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Old 08-23-2015, 10:18 PM #6
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for those who didn't feel like doing a google image search (the only good pics of it on the net are from ICMag member albums actually), here's what it looks like..... this is my own unfortunate picture...



those yellow stripes on the blades are the tell tale.
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Old 08-24-2015, 12:41 AM #7
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Quote:
Originally Posted by dissolute View Post
the only question for me is, how the hell to I get my RH back up into the sweet spot now that I'm running a minisplit. fucking thing devours moisture. more humidifiers I guess. man I miss growing at 70%. had to cut my feed down 30% now that I'm in the high 40% range. :(
How do you deal with mold running at 70% humidity?
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Old 08-24-2015, 12:47 AM #8
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I like to run rooms at 78f and 70%rh and drop down to 60% rh the final week. I haven't seen any rot in a couple years.. with significant airflow rh at 70% shouldn't be a problem. A room with stagnant air and >70%rh could be a problem
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Old 08-24-2015, 06:45 AM #9
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Thanks guys for the feedback...

dissolute - I am by no means an expert on the situation but I've heard of people using pond foggers to increase humidity. No actual experience with them myself (I actually have 0 zero indoor growing experience) but in theory a fogger sounds like a solution to your problem, a bit more effective at increasing humidity than a humidifier.

RockinRobot - Again, no actual experience with this, all of this info is my interpretation of my research into the subject...experimentation starts soon as I get my setup together. According to the theory tho, as long as the temp is properly controlled in relation with elevated levels of humidity mold should not be a problem. It is my assumption that mold is allowed to enter into the equation from either dew as water condenses out of the air or the process of guttation as explained above or a combination of both of these factors. Coupled with improper air filtration and circulation mold can become a major problem. Both dew and guttation can be prevented by bumping up the temp while keeping the RH the same. Dew forms as the ambient air temp is equal to or slightly above the percentage of water vapor, or humidity, in the air (someone please correct if I am wrong). As a result the air becomes saturated and water, once in the form of gas, begins to turn into liquid. Raising temps increases the amount of water vapor able to be held by the air thus halting the occurrence of condensation as the water remains in gas form and you do not get the formation of dew. Guttation occurs when the air is close to or already saturated and remains so for a period of time. As it is saturated the air is not able to accept any more water molecules or cause any water in liquid form to evaporate. Thus, a plant in this environment cannot transpire (allow the water in liquid form contained within the plant to evaporate into the surrounding air space). When this happens you could say the plant itself becomes saturated and out of necessity pushes excess water out through it leaves. Again, increasing the temp and increasing the air's capacity to hold water to allow for the plant to transpire can solve this problem.

So (again as the theory suggests) in order to prevent mold at 70% RH one would have to raise temps to a degree level above that to eliminate some of the major contributing factors to said problem. Also, as mowood3479 mentions proper air quality control and circulation are key to mold prevention as well.

P.S. If anyone finds fault with the reasoning and logic, or sees that some of the information I share is flat out wrong, I urge you to please point it out. We are all, including myself, here to learn and one cannot learn without making mistakes, having those mistakes brought to light (either from an internal or external source) and correcting those mistake so as not to make them in the future.

Thanks again to all who have contributed to the conversation.

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Old 08-24-2015, 07:32 AM #10
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An article posted in a previous thread about VPD. Don't know enough about the subject to say how legit the charts are but good info none the less.


https://www.just4growers.com/stream/t...ur-plants.aspx


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