VPD: Ideal temperature/relative humidity

[cannaisok]

Yeah, I have to be little more precise next time. I am thinking of Air exchange and air circulation as different things. air exchange is how often the air inside my tent is completely exchanged(max 2times a minute)
Air circulation is air movement inside the tent. eg i shut down my inline exhaust I wont have air exchange but still lot of air movement cuz of my fan inside the tent.
If I exchange the air inside my tent to cool it down I would suck all moisture out and quickdry my plants.
this is the reason I was asking if air circulation/movement(WIND ) inside the tent would help preventing molds.

Finally i think i could answer the question myself.
If we have a room with 68%rh and very few air MOVEMENT/WIND. the RH is NOT equal in all parts of the room. For example inside the bud or between the leaves it can still have 90%.
Same exact room with 68%Rh and a fan that is moving the airflow between all leaves and budsites will make the enviroment more even and the spaces between the canopy are maybe 72% then.
hope you can understand my theory, english is not my native language.


if we want to adjust the overall RH inside the tent, we have to play with air exchange.
peace.

 

[Limeygreen]

That is a lot of air exchange so you are probably alright on this front, are you able to draw cooler dry air into your tent? This will lower humidity as cool air holds less moisture and using it for air exchange will lower humidity in your room through out take of warmer humid air. As you heat the space and plants transpire then they humidity raises of course. How is your canopy, do you trim off fan leaves as well? Do you have space for a de humidifier and would it raise temperatures too much to put one in?

 

[cannaisok]

pretty much has changed over the last week. temps dropped from 90 to 75 outdoors and the humidity has dropped significant!
placing fan and pointing it away from the plants did work very well too, it is still pretty windy in there and all the leaves are moving very slightly all the time, looking much more plant friendy than blowing straight on or overhead the plants!
Pulling Air from outside isnt possible, only passive intake with tilted window in 15m² room.
dehumidifier is ATM overengineering a bit imo, I think my 55-65%Rh right now is exactly where I wanted it to sit in flower.
Summer is time for outdoors, wont grow in summer indoors again unless I use LED but thats stuff for a new thread in future
thanks for nice tipps man, was usefull for me!

 

[buttonpusher]

Would the Vpd calculation change if you are running a sealed room with 1000ppm CO2? I thought one has to keep the humidity below 60% when adding CO2 at higher levels.

 

[Loc Dog]

Would the Vpd calculation change if you are running a sealed room with 1000ppm CO2? I thought one has to keep the humidity below 60% when adding CO2 at higher levels.

I do not know, but would not think so. I grow GG4 and canopy temp ideally is at 80F, feed is 68 - 76F, and lights out not below 65F, seemed to be ideal.

 

[junior_grower]

been running 80-83 rh 70-75, and 1050 ppm co2 for years zero issues and happy plants, i crash my rh and co2 last two weeks.

 

[Limeygreen]

VPD chart shows what it is always sealed room makes no difference.

 

[DemonTrich]

I've ran a spot on VDP run and my recent run with out my ultrasonic fogger with a rh of 50 thru veg. Growth in the 50% veg room suffered drastically. About 1' shorted than my VDP matched room. Went back to running my ultrasonic fogger from now on.

 

[junior_grower]

I've ran a spot on VDP run and my recent run with out my ultrasonic fogger with a rh of 50 thru veg. Growth in the 50% veg room suffered drastically. About 1' shorted than my VDP matched room. Went back to running my ultrasonic fogger from now on.

Time and time again its proven, but bro science wnt seem to die

 

[linky]

I am trying to find some accurate good info on VPD in perpetual flower room. I have everything to keep my room at whatever temp/humidity I need and would like to grow by the VPD chart but am concerned with late flower. I always have plants in the final couple weeks of flower, will low 80's temps, ~75% humidity and co2 at ~900-1000 be a problem? loose buds, mold etc?

 

[mojave green]

Time and time again its proven, but bro science wnt seem to die

yup, it will live on!

 

[Lost in a SOG]

Exactly!

I bet you any money that a significant amount of these calcium/Nitrogen lock out issues people get is the whole "grow at 40% RH to avoid PM problems" it's absolute bro science.

It would make sense to a caveman with all the tools and none of the intuition..

PLANT GET ROT.. ME BLOW DRY AIR AT PLANT.. MAKE GO DRIER..

In most grow room temperatures if your RH is below 55% then you are probably stressing them.

 

[jidoka]

Did I overshoot?

 

[jidoka]

Here is the room same day. We shoot for vpd but every once in a while ghs have a mind of their own

 

[Lost in a SOG]

^^ WOWEE jidoka that is beeeeeeautiful!!

I don't know why I am only just clicking on to this whole thing, I feel like a moron lol,, it could have been from some serious concussion but wtf.

This is my theory, based on a lot of reading. I would love someones opinion on my horticultural musings here.

It would make sense that it is actually maintaining too high a VPD (low RH/medium to high temps) would be the thing that causes bud rot towards the end.

I think probably a lot of people run their plants down through low RH/high VPD indoors under intense lights and using dehumidifiers out of paranoia leaving their plants susceptible, potentially quite early on, through mineral deficiencies to fungal spores such as powdery mildew or botrytis which for all we know infect to some extent parts of the plant very early on during stress periods, spores must be constantly landing on plants throughout their entire lives unless HEPA filters are used. However it's only if the RH accidentally raises do you actually see the results of the infection manifest visibly on the leaves or in nugs as the conditions are right for fungal fruiting inside the nugs as they swell and possibly because a lot of spread of spores is by agitation from fruiting bodies by rain and the fungi possibly assume high RH equals rain.

Like with Powdery mildew outbreaks in outdoor plants, IME it is only after periods of VPD stress from very low RH that causes it not after very wet periods because the plant's transpiration is retarded worse by super high VPD, as is the flow of new healthy fresh nutrients throughout the plant which is what protects the plant. Their internal pressure is one of their best defences against things getting inside them.

Which makes me wonder if that extra frost some people describe that they get by dropping the RH loads at the end isn't actually slightly fungal like so people have postulated? I don't know about that one.

But a stressed plant is a stressed plant and the ideal transpiration chart described by VPD has to be ideal so long as you haven't set yourself up for a fall early on by retarding the plant through low RH in which case slowing down an infection in the final weeks might logically take precedence over the health of the plant as a whole and one of our best ways is to drop the RH but it's a bad solution to poor horticulture practice.

Thank god I figured this out iv'e been banging my head against a wall trying to figure out what I was doing wrong. You can't grow AAA bud consistently without understanding VPD.

Does this sound right?

And Cannabis is a C4/C3 (possibly with preference depending on genetics?) plant that can use the Carbon 3 pathway when super happy (In ideal VPD, feed, Co2 and Par) and when super happy the plant grows best and this primary and more ideal carbohydrate pathway works at a higher RH.

So you have to be 100% spot on in the VPD range throughout the entire grow and do everything else pretty spot on, more or less, including having a slightly raised CO2 and you can get away with having what seems like a very high RH throughout without any ill effects because the plant has been stress and infection free throughout it's life, and i'm sure will see pretty insane results, I have in the past but haven't understood.

Outdoor Cannabis lives happily through some ridiculous humidity even getting rained on in flower and without getting any fungal issues which also shows blatantly to me that it must be something we are doing to the plant indoors causing the bud rot, which can be stress of lot's of kinds, but water pressure is one of the worst for infections it would seem to me.

I also read that plants experiencing high VPD/V.low RH are less affected if they have more leaves which makes me think about the defoliation argument being even more flawed.

A complex question I have for someone is:

If cannabis as stated does switch into C4 mode when its stomata are closed during high VPD stress, allowing it to maintain photosynthesis, then Will it be carbon satiated (full of carbon) or not? and therefore will adding CO2 after correction from high VPD be beneficial or inversely drive them too hard? (I explain a paper below that says during high VPD CO2 is a super bad idea to give plants but how long after if at all?)

Because they can't get any co2 in the stomata and the rubisco in the chloroplasts will all be bound up to the o2. So in theory they should want CO2 HOWEVER if they have gone into some sort of drought mode like many species do they will return reluctantly back to full C3 and may also me very low on minerals in the upper parts of the plant from the lack of transpiration and continued photosynthesis.

Cannabis obviously finds anyway to produce something but the difference between a stressy cannabis plant driven into C4 than a happy plant seems drastic.

Like i've read through quite a few research papers on C3/C4 stomatal responses to humidity and co2 like the following to piece this together.

https://www.plantphysiol.org/content/plantphysiol/71/4/789.full.pdf

This paper interestingly shows amongst other things that all of these C3 & C4 grasses stomatal conductance (essentially transpiration rate or rate of guard cell closure/opening) decreased worse from 400ul CO2 to 800ul CO2 as the VPD also raised (drier air).. It shows clearly that (these) plants all love CO2 when the humidity is really high (low VPD) and hate it the drier things get (High VPD) which can be seen by them slowing transpiration (decreasing stomatal conductance)..

For all you guys, just like myself, that have found that you lose taste in your weed in the last few weeks unless you cut CO2 then this is undoubtedly part of that reason. The Co2 makes the plants close their stomata even worse than they already want to because they just don't want to drink that fast as it hurts their osmotic balance.

A final note is that I discovered that the blue section of the light spectrum almost forces stomata to stay open more through special sensors on guard cells,, So say if you were to put a different light that had more blue in it above a plant that was looking low on Calcium and in a highish VPD environment then I bet you it will look more unhappy quickly with thicker crispier leaves and more micro nutrient/immobile nutrient deficiency.

A green star to anyone that makes it all the way through this post LMAO

 

[jidoka]

Coco

 

[Lost in a SOG]

I should chime in with my recent discovery

I tried 10gl pots of promix for the first time and have noticed they do much better in 45-55%

The soil takes longer to dry out when it's at 60+ and I have gotten fusarium twice now. A buddy strongly recommend I lower my rh and keep temps between 80-82. They fuckin love it.

Had I been in coco, it'd be 60-70% because coco loves to be wet,All the damn time. So it thrives in a high vpd.

So I guess my theory is every medium has a different vpd that it's more comfortable with. Also the stage of the plants growth makes a difference too, almost always a higher rh from clone down to a lower by chop.

To be honest I reckon the ideal VPD for a grow room to get the best growth and least infections probably depends on the entirety of factors that affect your grow room and plant transpiration.. RH fluctuations throughout the day and night, Nutrient ratios, watering frequency, light spectrum etc etc..

Not to mention from what I have read various genetics have different tolerances to VPD stress, as i'm sure you can imagine, depending on whether they came originally from a subtropical monsoon like region such as Thailand/India or somewhere very different like Afghanistan where a lot of our Indica genes are coming in from that I would imagine sees much much lower humidity, or Africans that must come from quite a wide range of climates and altitudes.. And some genes come from places that experience large swings as well. of course these are hybrids now but I think the landraces and genetics that made them will give a clue to whether you can get away with different kinds of VPD variation.

A lot of sources say stay at a blanket 50-55% RH through flower which probably is not bad assuming temps don't go above 82f or 26-27C. But I've seen some plants stress slightly at that range.

I only grow from seeds so I see a lot of variety in how genetics respond, I think a lot of the elite cuts are probably pretty tough.

 

[Ras79]

Ciao a tutti
Una domanda:la tabella del v.p.d. mostra un umidità molto elevata...rispetto le solite dicerie di internet..é da rispettare sia in veg .che fior.x avere una sana crescita?

 

[Douglas.Curtis]

You're all upside down from how I grow.

I grow high mountain cannabis. 68F and 20-25% RH. VPD is 18+ which keeps my transpiration high. Vital for proper hydro. Terpene retention is ridiculous, excess 'plant' growth is minimal, trichome density is stellar. (Edit: Word of warning: This is trichome dense cannabis, so there's no huge explosion of flower growth at flip. You have to veg a bit longer to achieve the same yield as high temp/RH.)

I also haven't seen mold or PM in my flower rooms in 15+ years, only in non-optimal veg with low airflow conditions.

 

[Lost in a SOG]

Hydro is obviously a way different kettle of fish to soil in a lot of ways.. imo the roots dont get stressed the same way as they can in soil from sudden drying due to the way ph swings salt availability and osmosis/transpiration stress essentially effects the roots.. in hydro the roots are spoiled and friggin huge so vpd is less of an issue the plants can easily drink what they need and have the energy there to do it.