Lowering humidity in flowering as it relates to Vapor Pressure Deficit?

[TurboBucket]

I did some more research because i have trust issues.
You are flat out wrong.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-10-14-0264-R

Temperature, RH, & wetness duration are the factors that give rise to bud rot, and in the studies conducted above, albeit grapes, it is made clear just how large a factor temperature plays.

Grey mold doesn't mess with temps exceeding 30C.

So your going to run 30°c+ temps for 24 hours a day? LMFAO. Eat that Crow boy.

Your getting caught up on finer details and missing the bigger picture. Step back breathe and listen to some of these folks that have been growing for decades longer than most have even thought about it.

Mold/rot set in during the transition period from day to night.

Also just to be clear, i have adequate ventilation, fans, i even have a stupid enviroklenze in my tent. My RH is under 60%, but I don't want to fall out of the suggested VPD ranges. (Pretend that I have the correct chart)

How can you worry about the VPD if your not checking LST and using the correct charts? That will cause headaches.


Growing and life in general are all about balance. There is no one size fits all answer but some vpd chart makers stop everything at 75%. If you had a different wrong chart would you still make your case?

 

[The Celt]

@HowToWeed , to better help you understand what we have been trying to explain:

First, VPD affects the plants growth, and has little if anything to do with various infections. Basically, the VPD affects how fast water moves from the roots to the leaves, which affects nutrient uptake, essentially osmosis.

So with a high VPD, moist soil/low humidity, water (and nutes) travel though the plant faster enabling faster growth, this assumes sufficient CO2 and light. Now the chart you supplied, I see little information regarding if it was species specific or just a general all species guideline, it also doesn’t give light or CO2 conditions. These things also affect plant growth.

It all works together in a balancing act, use the chart as a guide, not gospel

As for the discussion on fungal issues, let me see if I can explain that further so you can have a deeper understanding of what I was trying to convey In regards to cannabis.

OK, so during the day, your temps are say 30C and an RH of 50. When the lights go out and temperatures drop, the air becomes denser and can hold more water, your RH goes up. This is were the problem occurs and here is why:

as the buds develop and become denser, the internal RH of the bud climbs and a 2 VPD develops, high in the bud and lower in the air allowing water vapour to exit the bud into the air. If the ambient RH is too close to the buds internal RH, little to no exchange takes place and conditions become ripe for fungal growth.

Although the article you supplied concerning grapes helps gain knowledge, it does not translate well to cannabis as the fungus grows on the surface of grapes with are directly exposed to ambient RH, whereas our favourite plant, the fungus starts inside the bud where the RH is generally higher than ambient.

This is why we shoot for RH in the 40 to 60% zone, creating a high VPD between the bud and the air so we have faster vapour exchange and lower risk of infection.

I hope I explained that in a way that all can understand and will help you on your quest

 

[HowToWeed]

So your going to run 30°c+ temps for 24 hours a day? LMFAO. Eat that Crow boy.

Your getting caught up on finer details and missing the bigger picture. Step back breathe and listen to some of these folks that have been growing for decades longer than most have even thought about it.

Mold/rot set in during the transition period from day to night.



How can you worry about the VPD if your not checking LST and using the correct charts? That will cause headaches.


Growing and life in general are all about balance. There is no one size fits all answer but some vpd chart makers stop everything at 75%. If you had a different wrong chart would you still make your case?

I mean at 2,000 PPM CO2, yeah I was running at 30C, with a few degree drop at night.

The side rant about bud rot began as an example of how the VPD chart, which isn't inherently wrong, just requires I maintain leaf temps 2 degree's lower for it to be accurate, may conflict with some of the very widely accepted practices. I'm not disagreeing with the practices, i'm questioning how they work.

It seems apparent to me that when talking about Temperature or Humidity, you necessarily have to talk about the other in order to give accurate advice. This is the basis in which vastly different micro climates can produce growth on equivalent scales despite drastic differences in temperature and humidity, because these fabled micro climates likely sit in that ideal VPD range.
I've proven to myself that this is even true for bud rot, T & RH are always interconnected. It might seem like a obvious statement at freezing and boiling. I feel like this is going to get misinterpreted and someones going to come at me with "no shit they're interconnected: (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature "

That's not the same as what I said.

 

[HowToWeed]

@HowToWeed , to better help you understand what we have been trying to explain:

First, VPD affects the plants growth, and has little if anything to do with various infections.

If the ambient RH is too close to the buds internal RH, little to no exchange takes place and conditions become ripe for fungal growth.

This is why we shoot for RH in the 40 to 60% zone, creating a high VPD between the bud and the air so we have faster vapour exchange and lower risk of infection.

I hope I explained that in a way that all can understand and will help you on your quest

I'm truly loving this dialogue, each time you're getting closer to where my train of thought is.
Sorry to cherry pick again but this seems contradictory. I feel like you've integrated some of that article into your understanding and now you're in the same boat as me, maybe.

High & Low RH is relative to temp.
low RH at 40C is 70% RH
dangerously low RH at 40C is 65% RH
Running hot at 30C with added CO2, 50% RH is bordering the redzone, 40% is well into, and 30% would probably kill my plants. Following standardized advice, i'd run into trouble. (i'm not running hot because i ran out of CO2).

I'd just like to reiterate that i'm just trying to get a better understanding of all this and you're doing such a great job and bringing together the relevant info @The Celt

I hope you can see the nuance of this, and that i'm not just trying to argue a redundant point.

I'd like to get back to my original question, Is there merit in dropping into the dangerously high transpiration VPD's? Because that's what ~50% rh and below would essentially mean to me.

 

[The Celt]

Lol, glad you pulled that it about RH at 10C out lol

Actually, I didn’t need the article to help me understand RH lol Being in my position, RH plays a big role in the work I oversee as does VPD.

Let me try again as I guess what I was trying explain didn’t translate, sorry.

Lets try a thought experiment:

At 30C, 1 cubic meter of air can hold 30.4g of water...100% humidity.
so at the same temperature 70% rh means there is 21.28g of water in 1m3 of air.
Now let’s assume that same air is in a big ballon and we drop the temperature 10C, the ballon will shrink in volume but still contains 21.28g of water.

Your turn, what happens inside the ballon?

 

[TurboBucket]

I mean at 2,000 PPM CO2, yeah I was running at 30C, with a few degree drop at night.

You need a larger drop at night for the flowering stage. A lower overnight differential helps trigger the plants natural flowering responses. I aim for a 10-15°f overnight differential in flower and 5-7°f in veg. This is where the rot problems typically start. As @The Celt mentioned.

There are different charts that need to be applied depending on what your measuring for leaf surface temperature versus ambient.
Here's the link for the online calculator of that chart. VPD Calculator by Everest Fernandez

Depending on your light source (s) and environment you could have multiple charts that apply within the same room.

 

[HowToWeed]

As temperature drops the RH increases to 100% and then condenses into a liquid.
I heat it back up and the volume increases again.

Let's pretend the balloon has a membrane that can release humidity if the outside of the balloon has less humidity than the inside. And let's say that there's a fungus spore inside my balloon. I might be able to push that cock sucker out of my balloon through the membrane if there's a good deficit outside the balloon to encourage it to transpire out. I however certainly will never evacuate myself of the spore if i'm not transpiring due to a higher ambient humidity than inside the balloon.

I mean, reading what you said VPD is more important than RH, since it's the level of transpiration that has the highest effect on whether rot can develop, and that means lowering your RH during flowering for a higher rate of transpiration, but I'm having trouble understanding how VPD isn't relevant to fungal growth. The experiments I linked, are all clearly in the red zone for VPD. Except the 30C 65% experiment one that had 0 incidents of rot, nvm i guess that doesn't explain why the 30C higher humidity ones didn't have rot. Disregard this >.>

 

[The Celt]

Just keep this in mind, keeping you rh in the 50 +/-5% means a good VPD enabling higher transpiration and faster growth which generally means a healthy plant and healthy plants are less prone to disease

 

[The Celt]

Damn near forgot, you assessment of what happens when the temps dropped in the ballon are spot on, but here’s the kicker, in real world situations, because you past the dew point, that excess water drops out of suspension and guess where it lands, on our bud driving up the ”wetness” in the bud.

 

[HowToWeed]

Damn near forgot, you assessment of what happens when the temps dropped in the ballon are spot on, but here’s the kicker, in real world situations, because you past the dew point, that excess water drops out of suspension and guess where it lands, on our bud driving up the ”wetness” in the bud.

And this is just a bullet I have to bite in order to comply with the recommended night time drop in temperature?
I asked earlier in the thread about VPD as it relates to lights off, and i get that it's not relevant because stomata are mostly closed. This leads to a follow up question on that matter;

Is night time humidity something i should be accounting for in my grow? There's a benefit in a temp drop at night, is there any advantage of a humidity drop at night to counteract said dew point?

 

[The Celt]

For the most part, so long as your Rh doesn’t climb too high for extended periods, you should be ok.

For example a 50% rh at 30C becomes 87% at 20C, if you keep a fan running during lights out to extract the humid air at 20C and replace with a lower Rh air also at 20C , you will gain equilibrium in short order as the air exchanges.

 

[TurboBucket]

Is night time humidity something i should be accounting for in my grow? There's a benefit in a temp drop at night, is there any advantage of a humidity drop at night to counteract said dew point?

Yes absolutely. That's the point we've been trying to make.

Damn near forgot, you assessment of what happens when the temps dropped in the ballon are spot on, but here’s the kicker, in real world situations, because you past the dew point, that excess water drops out of suspension and guess where it lands, on our bud driving up the ”wetness” in the bud.

If the interior balloon surface drops below the dew point of the air it will condense on that prior to reaching the saturation point though.

 

[HowToWeed]

How much of what I've said is just blatant misinterpretation of science?
I don't know if there might be an element of us saying the same things, me convoluting things more than it should be or just a fundamental lack of understanding, but if you can point out some of the straight up bad science I've managed to keyboard warrior out today i'd appreciate it.

edit: I think the missing piece just finally clicked, feeling kinda silly now.
VPD doesn't equate to total dictation of the rate of transpiration. It's just how that rate of transpiration is effected by the vapor pressure deficit between the leaf and ambient RH. I feel all too silly now

 

[HowToWeed]

Don't know how or why I came to think that VPD was the sole dictating factor on the rate of transpiration, but this misconception fucked up how i was interpreting everything that was being said to me.

 

[TurboBucket]

Don't know how or why I came to think that VPD was the sole dictating factor on the rate of transpiration, but this misconception fucked up how i was interpreting everything that was being said to me.

Root zone pressure also comes into the equation. That's why hydro can get away with higher temperatures than soil to a certain degree. Higher root zone pressure allows higher rates of transpiration.

 

[Calvados]

Don't know how or why I came to think that VPD was the sole dictating factor on the rate of transpiration, but this misconception fucked up how i was interpreting everything that was being said to me.

I thinks its a good chart. There is an inverse relationship between RH and temperature. When your lights go out the temp drops and the RH goes up. Try and stay in the zone. PM and rot are also a function of airflow and spacing. Without those you are looking for trouble.