Sacrificing temperature for VPD?

Hey guys,

Atm my tent is at about 25 C and humidity at about 40% RH

Here are some graphs of recent RH and temperature:


The dips and spikes are when the lights are off and the exhaust fan is cycling every 15 minutes.

When I run my humidifier the humidity gets too high when lights are off and only increases the RH in the tent by 5 - 10% when lights are on. I cant set it up to turn of a night. So I've decided to get rid of the humidifier.

I was wondering if it would now be a good idea to lower the temperature in my tent during lights on by a couple of degrees to get a better VPD... atm my VPD is at about 1.54. If I lower the temperature to about 21 degrees it should be at around 1.20. This is assuming that leaf temperature is about 2 degrees cooler than air temperature.

So what do you think, is it better to have a higher temperature and a worse VPD or vice versa.

Any other advice re: climate control would be much appreciated also.

For me, the worst are big swings in temp and humidity - increased risk of powdery mildew.

Techno info.

Fan and humidity timers/controllers. Inkbird has a wifi unit.

Cannabis grows great at high temps low humidity, kind of how the wild conditions can be.

Wouldnt realky base too much on those vpd charts or reset them for a more realistic approach.

Id sacrafice humidity and keep the temps, low temps are just slowing photosynthesis, low humidity is not.

plebschmo said:

I cant set it up to turn of a night. So I've decided to get rid of the humidifier.

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Can’t you do what I do?

I plug my humidifier into my lighting controller so that it only activates when lights come on.

are you measuring your levels amongst the canopy? From the perimeter Of your grow tent? On the floor of your grow tent? At pot level?


personally I stopped worrying about VPD as soon as I read about it around 8 years ago lol.

it’s just yet another variable that you can get hung up on trying to keep control of.

rule of thumb.
temps 21C-28C
RH 35-55%

of you fall within this section you’ll have successfull grows.

now if you’re a production orientated grower with multiple lights +5 etc or a breeder looking for max numbers or flowering times etc then full climate control is beneficial and looking into how VPD plays a part in other aspects of the cycle is beneficial.

For most average single tent growers etc VPD isn’t a massive issue.

Thanks for the advice guys. I think ill just ditch the humidifier and not worry about VPD.

jondamon said:

Can’t you do what I do?

I plug my humidifier into my lighting controller so that it only activates when lights come on.

are you measuring your levels amongst the canopy? From the perimeter Of your grow tent? On the floor of your grow tent? At pot level?

Click to expand...

Buttons on the humidifier need to be pressed every time the humidifier is powered on so putting it on the light timer wouldn't work. The temperature and humidity are being measured by an external probe which sits next to the canopy.

plebschmo said:

Thanks for the advice guys. I think ill just ditch the humidifier and not worry about VPD.



Buttons on the humidifier need to be pressed every time the humidifier is powered on so putting it on the light timer wouldn't work. The temperature and humidity are being measured by an external probe which sits next to the canopy.

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Ah ok. Mine is a dial. And when the dial is set it auto starts without pressing a button.

is your probe amongst the canopy?

Could be an idea to get some readings amongst your canopy out of direct light. Your Rh amongst your plants will be higher than any other place In your grow.

Okay so for most of the years I've been growing I haven't heard of vpd. Most books written on growing herb were written before vpd was being talked about so much. I don't doubt that it helps yield but it's not essential to growing. If your not following vpd I recommend 75-79 degrees F and 45-55% humidity. It's the way people grew for years and it produces fire. Now on a large scale grow when u get a little bit more per light that weight really adds up if you have a lot of lights and on that scale I'd modify the vpd chart a bit but try and stick to it somewhat.

I usually vent my rooms so In that case it's really hard to hit exact temps and humidity. I have ordered an AC but shipping taking forever. I do my best to keep temps at 79-83F with 60% humidity for the first 3 weeks of flower, I also try to run night temps within a couple degrees of day temps during this period. I lower temps to around 77-79 for weeks 4-6 with humidity around 50-55. For weeks 7-8 I try to lower to 75 and 45-50 percent humidty. For the last week I lower humidty as low as I can, pull it down to 35% if possible and temps pretty low also down to 66-70 if I can. That being said none of that is nesscessry you can just keep it at 75 F and 50% humidty. I've just started tweaking here and there over the years and this is where I've ended up.

I personally ditch any and all thoughts of VPD during bud set. Some strains have a higher resistance to botrytis, PM and the like. Those strains may be just fine with higher RH% in flower to maintain VPD. Other strains might just shit the bed with bud rot with the same numbers. I don't like to see my RH% go up past 50 in flower (when there are decent sized nugs to mold). I generally maintain flowering RH% in the 35 - 45% range with temps up in the low 80's (co2 enriched). If it wasn't CO2 enriched I would wanna be around 79F.

See my humidity cant build because i have extraction air flow, it can be 80% mid flower for all i care and often is when its wetter outside like in a week of rain. If water isnt dropping out of the air at dew point i aint going to get mold.

Now there seems to be some fucking weird understanding of humidity and what seems to be NO concept of air flow and why we use extraction in the first place.

So instead of exspensive humidity control i just use established proffesional methods and avoid the crap. If your running a warehouse that deals with a ton of air moisture every hour then VPD is going to be needed so bad stuff dosent happen, in a small grow we are more than doing it by just applying extraction and maintaining the right temps without any humudity control as its a few grams of water an hour and wont spell disaster in a matter of mins if not managed properly.

This humidity mis understanding has now spread to drying buds i now see, again frickin no understanding of the relationship of air flow and humidity.

Where on those VPD charts are the airflow factors? Them charts only give you half the story and mis guide you down a rabbit hole.

2Hearts said:

See my humidity cant build because i have extraction air flow, it can be 80% mid flower for all i care and often is when its wetter outside like in a week of rain. If water isnt dropping out of the air at dew point i aint going to get mold.

Now there seems to be some fucking weird understanding of humidity and what seems to be NO concept of air flow and why we use extraction in the first place.

So instead of exspensive humidity control i just use established proffesional methods and avoid the crap. If your running a warehouse that deals with a ton of air moisture every hour then VPD is going to be needed so bad stuff dosent happen, in a small grow we are more than doing it by just applying extraction and maintaining the right temps without any humudity control as its a few grams of water an hour and wont spell disaster in a matter of mins if not managed properly.

This humidity mis understanding has now spread to drying buds i now see, again frickin no understanding of the relationship of air flow and humidity.

Where on those VPD charts are the airflow factors? Them charts only give you half the story and mis guide you down a rabbit hole.

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My sentiments exactly.

I’ve had VPD shoved down my throat by someOne telling me my room doesn’t conform to VPD.

based on the fact I was running 41% RH at 27C lol.

my gauge was set to the external areas of my room.

so to prove a point I put my gauge inside my canopy. And reset all readings.

it’s low was 41% ( where I Reset the gauge) it’s high was 68%. lo 21C hi 25C

So correct VPD numbers don’t often show all over your grow Areas but amongst canopy is the most crucial reading that you can track.

I’m with @Renfro as long as I keep a decent higher RH during Veg and a lower <50% during flowering then I’m happy.

I only get mold from water not humidity, the air moisture has to leave the air and form a liquid for mold to use, it cannot pull it from the air.

I do get the advice for lower in flower but it is not the full story only a precaution and there are many places in the world where it is high humidity and running sealed low humidity tents totally impractical so never going to happen.

If i go outside on a hot humid no wind day i can really feel the effect of the humidity, add some wind and thats all but gone and i dont feel it.

Every grower who starts finds more often their humidity is out of the canna range and then fusses without understanding all the variables - its become such a massive confusion in growing and drying.

Its hard to not read the light and wind in the canopy - such a big market for an actual canopy thermometer in the canna world suprised no ones pushed one so far.

jondamon said:

My sentiments exactly.

I’ve had VPD shoved down my throat by someOne telling me my room doesn’t conform to VPD.

based on the fact I was running 41% RH at 27C lol.

my gauge was set to the external areas of my room.

so to prove a point I put my gauge inside my canopy. And reset all readings.

it’s low was 41% ( where I Reset the gauge) it’s high was 68%. lo 21C hi 25C

So correct VPD numbers don’t often show all over your grow Areas but amongst canopy is the most crucial reading that you can track.

I’m with @Renfro as long as I keep a decent higher RH during Veg and a lower <50% during flowering then I’m happy.

Click to expand...

2Hearts said:

I only get mold from water not humidity, the air moisture has to leave the air and form a liquid for mold to use, it cannot pull it from the air.

I do get the advice for lower in flower but it is not the full story only a precaution and there are many places in the world where it is high humidity and running sealed low humidity tents totally impractical so never going to happen.

If i go outside on a hot humid no wind day i can really feel the effect of the humidity, add some wind and thats all but gone and i dont feel it.

Every grower who starts finds more often their humidity is out of the canna range and then fusses without understanding all the variables - its become such a massive confusion in growing and drying.

Its hard to not read the light and wind in the canopy - such a big market for an actual canopy thermometer in the canna world suprised no ones pushed one so far.

Click to expand...

I get totally what your saying.

this is evident with the lack of wind movement in some grows and you see the drops of water forming between leaf surfaces that are touching (another reason I tell people to make their plants dance a little with air movement) and when they touch they are creating the environment for mould spores to populate.

ultimately you can grow with +80% humidity it’s all about your control of that humidity with air movement/exchange/air purification.

My exhaust has just enough cfm to keep my tent fresh, the plants even leaves never move. A thin strand of cotton moves when held in the tent.

But thats where every grow is different, i have a lot of space round each plant and they never form a canopy or touch much. If i packed it out then as you say i would need a fan to boost things or would totally get water like described.

It does serve for new growers to be more cautious, i can skate closer to the edges though and push more. Always noticed with all its pointy leaves the bud does a good job of keeping leaves from touching and creating water, mainly the leaves with stems and fans.

The talk on VPD and the variables is a lot more relevant than any chart or trying to hit certain numbers.

jondamon said:

I get totally what your saying.

this is evident with the lack of wind movement in some grows and you see the drops of water forming between leaf surfaces that are touching (another reason I tell people to make their plants dance a little with air movement) and when they touch they are creating the environment for mould spores to populate.

ultimately you can grow with +80% humidity it’s all about your control of that humidity with air movement/exchange/air purification.

Click to expand...

Renfro said:

I personally ditch any and all thoughts of VPD during bud set. Some strains have a higher resistance to botrytis, PM and the like. Those strains may be just fine with higher RH% in flower to maintain VPD. Other strains might just shit the bed with bud rot with the same numbers. I don't like to see my RH% go up past 50 in flower (when there are decent sized nugs to mold). I generally maintain flowering RH% in the 35 - 45% range with temps up in the low 80's (co2 enriched). If it wasn't CO2 enriched I would wanna be around 79F.

Click to expand...

Do you check leaf temperature as well?