Vapour Pressure Deficit ( VDP ) and its surprises...

Bomber

Vegetating
User ID
117
Been noticing that as my plants develop further into flowering, my tents temps and humidity have been getting higher.
Today I decided to research it, given the potential for powdery mildew and bud rot etc.

Heres the chart you've all no doubt studied long and hard with which to dial in the perfect grow room...

Screen Shot 2021-02-10 at 3.22.57 pm.png
So my tent was clocking in at around 28C this morning with 80% humidity. Far too high, right, with us all desiring 21-24C, with RH no higher than 50% in flower?

WRONG.

Either I am reading this incorrectly, and for that I'm sorry, OR, what I thought I knew about grow room requirements has now been completely flipped on its head...

Turns out my RH and temps are pretty good according to the chart, which shocked me to the point I am sharing my shock with the handful of you who can be bothered reading this.

I must be reading the wrong shit, as everything I 'd read thus far had me believing high RH was a killer. Turns out provided your temps are proportionate, everything's gonna be fine.

Anybody ??
 

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Kloud9

Baked
Community Member
User ID
46
Have a look at the pulse template online VPD chart, you change adjust it , u can adjust the leaf temp also. As I cannot see if the upbove VPD chart is -1, or -3 c leaf temp , compared to room temp. Handy to know how hot uour plants are.
 

Bomber

Vegetating
User ID
117
mate its a guide, you need to be focusing on ventilation and air movement
And genetics
And training of the plant
And the medium
And the light source
etc etc etc...

All that's important girlfriend.

I presumed you and others knew this, so didn't bother explaining that side of growing...my point was more along the lines of if you have a hot, humid tent, its not all doom and gloom as I thought it was earlier today.

Thats all.
 

frankreynolds

Curing
User ID
40
Leaf temp comes into play with VPD as well afaik as mentioned above.

I don't think there is a specific set VPD mark as you can see on the chart, there might be for some very well researched phenoes.

If you are running chunkier indica strains then you probably wanna be on the lower side of the humidity range whereas, stuff coming from tropics closer to the equator probably do better with the higher RH.
 

R3za92

Baked
User ID
261
Are you taking lights out rh into account as I think light out time rh is much higher due to transpiration .✌
It’s also more important because you generally see a decent spike after lights out for 1-2 hours. Couple this with temps lowering because lights out and it’s a recipe for some mould and rot.
 
D

Deleted member cpr

Guest
You've got the right idea @Bomber, hot is OK if the humidity is high but near the end I try to decrease temps because flavours are beaten up at high temps, and i try to keep the humidity as low as possible to avoid bud rot.

I reckon buds spaz out when they run hot at the end, fox tails, less terpenes and a weaker high.

Like kloud says, the VPD calculator by pulse labs encourages you to run drier at the end, it kind of addresses the bud rot concerns.
 

Old fox

Customs Avoidance
Community Member
User ID
28
I run my veg tent very differently to my flower tent. Veg growth indoor seems much faster with temps into low 30s, and humidity around 90% - so I let it climb into those ranges where possible. Flowering is totally different, and need to maintain temps into mid-high 20s and humidity 50to 60, which is a struggle during summer in my environment. I regularly see temps of mid 40s in outdoor garden during veg, and outdoor plants (during veg) thrive in those temps as long as they have enough water.
 

HGO

Plant of the Month - Sep,2020.
Community Member
User ID
18
My Tents currently on average (during lights on)
30-32 celcius and 10-20% RH Buds seem to be growing Fine
apart from finding a few open nanners 😆
 

Bomber

Vegetating
User ID
117
Thanks for everyone chiming in.

This morning my tent was 27.4C and 70%RH naturally.

Im a bit worried about the last leg of the flowering, if temps and potency drop off a bit with high temps, but I am not in any position to tweak things, so time will tell...

Nice and warm here in QLD at the mo.
 
D

Deleted member cpr

Guest
Thanks for everyone chiming in.

This morning my tent was 27.4C and 70%RH naturally.

Im a bit worried about the last leg of the flowering, if temps and potency drop off a bit with high temps, but I am not in any position to tweak things, so time will tell...

Nice and warm here in QLD at the mo.
They'll be right, plenty of great buds get made with less than ideal VPD numbers.
 

Old fox

Customs Avoidance
Community Member
User ID
28
Thanks for everyone chiming in.

This morning my tent was 27.4C and 70%RH naturally.

Im a bit worried about the last leg of the flowering, if temps and potency drop off a bit with high temps, but I am not in any position to tweak things, so time will tell...

Nice and warm here in QLD at the mo.
That temp is ok for flowering. Get some Damp Rid containers x4 from Bunnings, and place them in the tent. They should reduce your humidity nearly 10% which lessens( not eliminates) chance of mould. Damprid containers help reduce humidity a bit. I usually keep a few in flower tent, and add a few more in there during last 3 weeks of flowering.
 

itchybro

Sultan Of Soil
User ID
31
as frank said you need to check leaf surface temps over room temps , which is hard cos you'll find different leaf temps round the canopy
although room temps might effect soil temps

i noticed when i first had a play with VPD , hitting the chart numbers didn't make my plants look happy , don't just let the numbers rule your grow , always relate it back to how the plant is preforming / looking

VPD comes from commercial greenhouse growers of plants other than cannabis , although there might be more info on canna & VPD these days , so you have to take the plant your growing & environment into account when setting VPD targets , like growing on the coast or in central oz = very diff temps & humidity in both those places , forcing you to work with what you got

i would also say outdoors moulds winterize in soil & become active in the warmer months so having soil biology right should help to out compete them b4 becoming active , i said this to someone else who said that was BS because he only grows in coco indoors but has had mould b4

easy answer , where is your air that carries these moulds coming from , outdoors where da soil is

having intake filters helps but never going to stop moulds getting in unless you have some super sealed grow room , but moulds don't just live in the soil , otherwise based on what i just said you'd never have mould in winter which would be untrue

all the things mentioned like air movement still apply , although to much air movement can be a negative as well

use those numbers as a guide not the rule
 
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