# How Vapor Pressure Deficit (VPD) Is Effecting my Grow



## Bdoggg (Apr 28, 2016)

2x 400w hps
Pure coco in small nursery pots
GH nutes drain to waste (hand water)

I built a new flower room recently and decided to give the VPD guidelines a shot to see how my plants fare, their health, amount of water drank etc. I'd imagine that having a high RH% would slow the plants transpire rate, meaning I can feed higher EC, they will need less waterings etc. This seems to be holding true as of now, i'm feeding strong and everything's looking amazing. The growth over night is incredible. Cons obviously is risk of mold, mildew, etc. Which in my opinion, makes this technique not good in my book.

Chart which I followed:
 

My question now is.. Is it really the vpd that's effecting so much? Maybe the fact that I can feed higher EC with higher humidity? Just letting things go natural in the room, (no exhaust, no intake) just a fan blowing around, the RH% climbed to roughly 75%-80% and temps were a stable 80-82F, which line up well with those guidelines. Basically I said fuck it and decided to give it a shot.

Setup
 

Has anyone ever ran this throughout the entire flower period? I feel like the buds would get fucking monster the way they're growing now.. but with my low night temps mold would be disaster. I will duct my room within the next few days to make sure no problems arise. Any input would be appreciated


----------



## GroErr (Apr 28, 2016)

That's a good chart and accurate but not necessarily practical, I've seen it around for a while now and my thought is RH is too high from a practical sense.

That said, I typically run in the yellow or edge of red zone at 80F and 50-60% RH and results are good but I'm constantly on the lookout for PM as my area is covered in it (outside).

No disrespect but I'm an IT guy and deal with this sort of thing all the time. Your experiment (this round) is kind of invalid because you introduced too many variables including a room you've never run plants in before. Only way to properly asses any change is to use clones and the same methods (watering, feed levels, feed brand, lighting type etc.) then change only one variable, in this case temps/humidity. Otherwise, how do you confidently know which change made the difference? In your case, being a new room you could record what you've done this round, clone the plants, then make a single change in your next run to see if there are any differences/effects. Cheers.

*Note*: One way to manage lights-off is something I'm currently trying in my setup called negative diff or trying to keep temps higher during lights off than light on. it's another theory that claims to increase bud production. To do that I fired up a dehumidifier but didn't vent it (normally I would as it generates a ton of heat) that comes on only during lights off. It reduces my RH when it's the highest and keeps the temps up during lights-off.


----------



## RM3 (Apr 28, 2016)

IMO, the VPD chart means more outside or in green houses than in an indoor grow

also it should be noted that VPD guidelines are for other plants not cannabis

I use heat index to set temp & humidity by using a calc

http://www.easysurf.cc/cnver16.htm

adjust for heat index to be 4 points above ambient temp

and watch how plants respond  

also a 10 degree temp swing lights on to lights out is a good thing


----------



## Bdoggg (Apr 28, 2016)

GroErr said:


> That's a good chart and accurate but not necessarily practical, I've seen it around for a while now and my thought is RH is too high from a practical sense.
> 
> That said, I typically run in the yellow or edge of red zone at 80F and 50-60% RH and results are good but I'm constantly on the lookout for PM as my area is covered in it (outside).
> 
> ...


Yeah 50-60% RH is my norm indoors as well, but you are for sure right about the actual experiment. I normally take a decently scientific approach to growing so experiments are nothing new to me, I knew going in I wouldn't get any definite answers I was sort of mental note thing. But I've seen rooms kept 85+ degrees 90+ RH and do nothing but produce enormous yields. I always figured mold, mildew, all that would be a problem but underneath 75F spores can't germinate? Or something like that.  I'm too stoned for this lol. This is an interesting topic though, and I do believe there is something useful behind it. But running high humidity inside is never a good idea.. 55% for me


----------



## Bdoggg (Apr 28, 2016)

RM3 said:


> IMO, the VPD chart means more outside or in green houses than in an indoor grow
> 
> also it should be noted that VPD guidelines are for other plants not cannabis
> 
> ...


So you don't think it has any difference indoor? I may not agree 100% with that.. but interesting calculator thanks for sharing that. Also you are damn right sir temperature fluc has always had a positive effect in my room


----------



## ttystikk (Apr 28, 2016)

Bdoggg said:


> 2x 400w hps
> Pure coco in small nursery pots
> GH nutes drain to waste (hand water)
> 
> ...


I've done this for years and I'm not convinced it's the best. Powdery mildew can get out of control quickly. 

To do it right, one needs to manage overnight temps and RH to ensure they are stable. 

I'm taking delivery Friday (knock wood) of a chiller that takes the rejected heat from the cold side and puts it into a hot water circuit, allowing me to reuse the heat for direct dehuey, room heating, pushing nighttime temperatures up, etc. 

We will be putting 24 hour VPD control and higher overnight temperature schemes to the test shortly!


----------



## WeedFreak78 (Apr 29, 2016)

Use one of these on your room intake, should eliminate 99% of contaminates getting in. Fries anything that goes through it, I think it'll help with mold. That ones [email protected] 2000cfm.

https://www.theacoutlet.com/product.php?id=2984&gclid=Cj0KEQjwjIy5BRClh8m_9Zu64d8BEiQAtZsQf4dZEGeLd0bmThi6r3enuM4SLrhtU4unwBCINvoLdEUaAjOW8P8HAQ


----------



## Bdoggg (Apr 29, 2016)

ttystikk said:


> I've done this for years and I'm not convinced it's the best. Powdery mildew can get out of control quickly.
> 
> To do it right, one needs to manage overnight temps and RH to ensure they are stable.
> 
> ...


Thank you for your reply and that sounds excellent. As of now what i'm understanding is if I can keep my night temperatures up and RH steady this could work all the way threw without mold. Awesome  The real question.. Is VPD really a big enough varible to invest in equipment that could make running the specs possible? Or playing it safe and running a normal room with 55% RH the best.. I'm interested in your tests with VPD, keep us posted. Opinions?


----------



## Bdoggg (Apr 29, 2016)

WeedFreak78 said:


> Use one of these on your room intake, should eliminate 99% of contaminates getting in. Fries anything that goes through it, I think it'll help with mold. That ones [email protected] 2000cfm.
> 
> https://www.theacoutlet.com/product.php?id=2984&gclid=Cj0KEQjwjIy5BRClh8m_9Zu64d8BEiQAtZsQf4dZEGeLd0bmThi6r3enuM4SLrhtU4unwBCINvoLdEUaAjOW8P8HAQ


You know it didn't even occur to me that without the spores in the room I guess mold couldn't form anyway. I would still have to watch my feet and clothes, but i'll have to look into those filters. Thank you!


----------



## GroErr (Apr 29, 2016)

Bdoggg said:


> You know it didn't even occur to me that without the spores in the room I guess mold couldn't form anyway. I would still have to watch my feet and clothes, but i'll have to look into those filters. Thank you!


Those are a good idea if you're bringing in outside air. But you are correct, mold. particularly PM spores will get in regardless. Unless you strip/shower every time you go into your room, while in theory is good practice, it's not very practical in real life.


----------



## Bdoggg (Apr 29, 2016)

GroErr said:


> Those are a good idea if you're bringing in outside air. But you are correct, mold. particularly PM spores will get in regardless. Unless you strip/shower every time you go into your room, while in theory is good practice, it's not very practical in real life.


Agreed. Well today I installed the venting so no more perfect VPD. I'm curious to see how everything's holding up tomorrow with the new conditions though! If it all starts to look rough then i'll read up more, but if everything looks the same (I'd imagine it will) then my "experiment" will be concluded. Either way I can be happy my new flower room is almost complete


----------



## Thorhax (May 4, 2016)

medicropper claimed his biggest yield with high temps and high humidity and had no problems with mold....im still skeptical


----------



## Bdoggg (May 5, 2016)

Thorhax said:


> medicropper claimed his biggest yield with high temps and high humidity and had no problems with mold....im still skeptical


Me too. I've seen those videos and they're pretty convincing.. but there's so many more variables that are giving him those huge yields


----------



## The Nine (May 5, 2016)

Bdoggg said:


> 2x 400w hps
> Pure coco in small nursery pots
> GH nutes drain to waste (hand water)
> 
> ...


Your most vulnerable time is the first hour after lights off, wh n the air is rapidly cooling and dumping the moisture. Plants absorb some of this and the rest condensates on the plants surfaces. 
If you get huge colas, their insides will trap this brief condensation jump, and will not be unable to get rid of it as the outside surfaces do, due to evaporation and air movement. 
The longer they are subject to this effect, the greater the chances of bud rot killing your monster colas.


----------



## WeedFreak78 (May 6, 2016)

The Nine said:


> Your most vulnerable time is the first hour after lights off, wh n the air is rapidly cooling and dumping the moisture. Plants absorb some of this and the rest condensates on the plants surfaces.
> If you get huge colas, their insides will trap this brief condensation jump, and will not be unable to get rid of it as the outside surfaces do, due to evaporation and air movement.
> The longer they are subject to this effect, the greater the chances of bud rot killing your monster colas.


How would one go about mitigating the moisture issue without sophisticated environmental controls? I could see ramping temps down, slowly, if using AC, just before lights of so there isn't as much passive cooling. Conversely, heating the incoming air just after lights off to temper the cool down. I don't have either of these options. I don't see a dehuey making much of a difference in that hr time frame.

I've never seen RH over 55%, but it's something I want to be educated on, just incase.


----------



## ttystikk (May 6, 2016)

WeedFreak78 said:


> How would one go about mitigating the moisture issue without sophisticated environmental controls? I could see ramping temps down, slowly, if using AC, just before lights of so there isn't as much passive cooling. Conversely, heating the incoming air just after lights off to temper the cool down. I don't have either of these options. I don't see a dehuey making much of a difference in that hr time frame.
> 
> I've never seen RH over 55%, but it's something I want to be educated on, just incase.


Why don't you think your dehuey unit would work? It would remove moisture and add heat to the air, both of which are desirable in that situation.


----------



## WeedFreak78 (May 6, 2016)

ttystikk said:


> Why don't you think your dehuey unit would work? It would remove moisture and add heat to the air, both of which are desirable in that situation.


I'm in a vented room with higher air exchange rates, so that's where my heads usually at, sealed rooms don't cross my mind often. I don't see any any reasonable way to control it without sealing the room and ac/dehuey.

I could probably mount a heated grid on my intake to temper the incoming air and gradually cool down over a set period of time to minimize condensation, but I don't feel like designing a control board to do it.


----------



## Bdoggg (May 7, 2016)

The Nine said:


> Your most vulnerable time is the first hour after lights off, wh n the air is rapidly cooling and dumping the moisture. Plants absorb some of this and the rest condensates on the plants surfaces.
> If you get huge colas, their insides will trap this brief condensation jump, and will not be unable to get rid of it as the outside surfaces do, due to evaporation and air movement.
> The longer they are subject to this effect, the greater the chances of bud rot killing your monster colas.


Well said, thanks


----------



## Bdoggg (May 7, 2016)

WeedFreak78 said:


> How would one go about mitigating the moisture issue without sophisticated environmental controls? I could see ramping temps down, slowly, if using AC, just before lights of so there isn't as much passive cooling. Conversely, heating the incoming air just after lights off to temper the cool down. I don't have either of these options. I don't see a dehuey making much of a difference in that hr time frame.
> 
> I've never seen RH over 55%, but it's something I want to be educated on, just incase.


After lights off, adjust your outtake speed to a lower setting and almost cut the intake entirely. No cold air in and the temps would cool down much slower so there'd be less condensation. What i'd do anyway


----------



## ttystikk (May 7, 2016)

Or just set the dehuey to handle the spike and then shut off the rest of the time.


----------



## Bdoggg (May 7, 2016)

^^ what he said


----------



## The Nine (May 8, 2016)

WeedFreak78 said:


> How would one go about mitigating the moisture issue without sophisticated environmental controls? I could see ramping temps down, slowly, if using AC, just before lights of so there isn't as much passive cooling. Conversely, heating the incoming air just after lights off to temper the cool down. I don't have either of these options. I don't see a dehuey making much of a difference in that hr time frame.
> 
> I've never seen RH over 55%, but it's something I want to be educated on, just incase.


This is the issue my friend. 
The outside temps and RH are also big factors especially when drawing in fresh air via intake fans.

I've never seen this being an issue until the colas become 'monsters'. 
I've seen grow after grow in the same set up produce a1 bud, then In the same set up but with monster colas, bud rot sets in from when they reach a certain size/density. 

I don't have the answers right now, but my friends are trying heating the air just on lights out then maintaining a gradual drop of 10 degrees throughout the 12 hour cycle before allowing the air to heat up naturally from the lights on again. 

I will keep you all updated as the trials are ongoing.


----------



## The Nine (May 8, 2016)

Bdoggg said:


> After lights off, adjust your outtake speed to a lower setting and almost cut the intake entirely. No cold air in and the temps would cool down much slower so there'd be less condensation. What i'd do anyway


Thanks. 
But the warm air within the room would be very moist and looking to dump the moisture as the air cools due to lights off. 

There really are many variables to consider.


----------



## ttystikk (May 8, 2016)

Dry heat = dehumidification. It's that simple.


----------



## Bdoggg (May 8, 2016)

The Nine said:


> Thanks.
> But the warm air within the room would be very moist and looking to dump the moisture as the air cools due to lights off.
> 
> There really are many variables to consider.


Yep the hardest part of growing.. listen to ttystikk he knows whats up. Get yourself a good ol' dehumidifier you'll be good


----------



## The Nine (May 9, 2016)

Bdoggg said:


> Yep the hardest part of growing.. listen to ttystikk he knows whats up. Get yourself a good ol' dehumidifier you'll be good


I agree mate
Thanks


----------

