Why doesn't VPD line up ?

[Jewels]

I have read about VPD.
I can read the chart.
I can see where this falls naturally in veg.
,,,around 24C , near 60-70 humidity.
Most of us veg like that.

Most of the AC QB's recommend lower humidity in flower ,,, say 40-50 %

,,,And where does VPD dictate temperature at that humidity ? ,,,,about 15 Celsius

Show of hands.
Who has flowers @ 15 C , right now ?

Something fishy afoot.

Here is another twist.
Can I apply VPD suggestions to drying harvested cannabis ?
If you follow 'CHtrumps Jiffy-line' past where the VPD chart stops indicating, it would stand to reason that I could successfully dry a harvest @ 21 % and 6C.
The stoma would not recognize the diff.

 

[Boybelue]

6°C what's that around 40°F, with the lower temp you could deal with a drop in humidity. Maybe at those numbers it would dry slow enough to dissipate all that chlorophyll and everything out before your flowers reach around 60%. Timing would be crucial to catch them when they reach that perfect moisture content. That's why everbody sets their drying conditions at 60% cause that's where you want to end up, just get there as slow as possible. Lower temps equals longer drying time and better product imo.

 

[TigerTanks]

6°C what's that around 40°F, with the lower temp you could deal with a drop in humidity. Maybe at those numbers it would dry slow enough to dissipate all that chlorophyll and everything out before your flowers reach around 60%. Timing would be crucial to catch them when they reach that perfect moisture content. That's why everbody sets their drying conditions at 60% cause that's where you want to end up, just get there as slow as possible. Lower temps equals longer drying time and better product imo.

I try to keep my temp and RH both in the low 60’s.... works for me

 

[Old ST1R]

I have read about VPD.
I can read the chart.
I can see where this falls naturally in veg.
,,,around 24C , near 60-70 humidity.
Most of us veg like that.

Most of the AC QB's recommend lower humidity in flower ,,, say 40-50 %

,,,And where does VPD dictate temperature at that humidity ? ,,,,about 15 Celsius

Show of hands.
Who has flowers @ 15 C , right now ?

Something fishy afoot.

Here is another twist.
Can I apply VPD suggestions to drying harvested cannabis ?
If you follow 'CHtrumps Jiffy-line' past where the VPD chart stops indicating, it would stand to reason that I could successfully dry a harvest @ 21 % and 6C.
The stoma would not recognize the diff.

Which VPD chart are you looking at? I’ve found they all aren’t the same.

I like this one: VPD Chart - Just4Growers

 

[Boybelue]

I try to keep my temp and RH both in the low 60’s.... works for me

Yep, 60/60 works great for a 2 wk dry. Drop the temp and extend the time, it just gets better imo.

 

[TigerTanks]

Yep, 60/60 works great for a 2 wk dry. Drop the temp and extend the time, it just gets better imo.

60 is as cold as I can get it. Really nice smoke

 

[yougrowyourway]

I was under assumption the vpd chart is for veg only. Flowering like you said with that high of a humidity would require a lot of fans and airflow.

 

[TigerTanks]

I was under assumption the vpd chart is for veg only. Flowering like you said with that high of a humidity would require a lot of fans and airflow.

Air flow is key, I’ve followed a VPD chart with high humidity in flower and the plants loved it,?ya just gotta move some air. Plump and juicy buds

 

[Old ST1R]

I was under assumption the vpd chart is for veg only. Flowering like you said with that high of a humidity would require a lot of fans and airflow.

According to the VPD chart I provided a link for above, at 45% RH temp should be 20C (72F) and at 50% RH temp should be 22C (76F). I’m certainly no expert, but I think it sounds ideal for flower.

 

[gwheels]

I find its tricky but it does work. My flower room is always 60 to 70% RH because....water systems and its a jungle baby.
And once i exceed 81F half of the girls had trouble with nutes...but it was because the temps did not allow proper uptake of nutes. Or more accurately there was not enough vapour density in the air to allow for healthy flower at that temp.

80F is my magic number. Keep it under that and they LOVE IT. And this is with LED too leaf surface temp is 73 to 75F

 

[damien50]

You have to use an IR thermometer, and a hygrometer. Ambient temp isn't the same as leaf surface and the differential from ambient to leaf has to be factored in.

Vapor Pressure Deficit VPD Calculator - Dimlux Lighting - The Best Grow Lights

Vapor Pressure DeficitCalculator Calculate both Room VPD and Leaf VPD automatically Just enter the value below to generate the VPD values for your room and your plants. You’ll need to know the room’s temperature, relative humidity, and the plant’s temperature. We recommend calibrating your...

www.dimluxlighting.com

That has a calculator that helps calculate everything with charts for ambient to leaf differentials. Plus I don't think. VPD is the end all be all unless you're using c02 injection and even then controlling environment is part of crop steering that helps dictate regenerative and generative growing. Creating artificial drought through irrigation and environmental manipulation or lush tropical environment with plenty of water creates different plant responses I think it's just about understanding and managing them.

 

[NoWaistedSpace]

6°C what's that around 40°F, with the lower temp you could deal with a drop in humidity. Maybe at those numbers it would dry slow enough to dissipate all that chlorophyll and everything out before your flowers reach around 60%. Timing would be crucial to catch them when they reach that perfect moisture content. That's why everbody sets their drying conditions at 60% cause that's where you want to end up, just get there as slow as possible. Lower temps equals longer drying time and better product imo.

I stick a lot of my flower in the frig for a few days. Hardly wilts in those days. I think it helps , but maybe it's just me.

 

[Boybelue]

I stick a lot of my flower in the frig for a few days. Hardly wilts in those days. I think it helps , but maybe it's just me.

That's another thing I love about winter time. I even notice a difference in my outdoor because the drying conditions differ from my earlier harvested plants vs those that go into oct.

 

[gwheels]

That VPD calculator is great. I measured leaf temp and ran it.
12 hours of lights on keeps me at 80F i find above that temp i have problems and they suffer.

It makes sense according to the chart. And with it being a flower room 80F and 55% RH is just about right.

And it taught me that you can run your lights softer and if you get the temp right you actually gain yield compared to turning everything to 11.

I think this is the best place for someone to start setting up and understanding

What should my temp be ? What should my RH be ?

Leaf temp makes a difference in where it all ends up. And that is related to plant respiration so this is probably right on the money.


I got there because the plants kind of told me. .8 - 1.2 KPA is the sweet spot for my bloom room but 1.0 is.......Nirvana

 

[TigerTanks]

That VPD calculator is great. I measured leaf temp and ran it.
12 hours of lights on keeps me at 80F i find above that temp i have problems and they suffer.

It makes sense according to the chart. And with it being a flower room 80F and 55% RH is just about right.

And it taught me that you can run your lights softer and if you get the temp right you actually gain yield compared to turning everything to 11.

I think this is the best place for someone to start setting up and understanding

What should my temp be ? What should my RH be ?

Leaf temp makes a difference in where it all ends up. And that is related to plant respiration so this is probably right on the money.

View attachment 29134
I got there because the plants kind of told me. .8 - 1.2 KPA is the sweet spot for my bloom room but 1.0 is.......Nirvana

Yelp, I’d like to have 55%rh but I’m close. I agree VPD is a good place to start for beginners or even seasoned growers.

 

[iShatterBladderz]

Which VPD chart are you looking at? I’ve found they all aren’t the same.

I like this one: VPD Chart - Just4Growers

the biggest reason you see variables in VPD charts is that they require custom data, that being your leaf temp offset. My leaf temp offset is -2 while someone who’s growing with an HID light mat have a +5 to +7 degree leaf temperature offset. I like using pulses site because you plug in all your info and then it shows different VPD charts for clone, veg and flower, based on your data. I’m sure there are other sites that allow for inputting this data, but I don’t trust generic VPD charts

 

[PokerJay1983]

Is this chart strain dependent?

 

[Turpman]

No

 

[iShatterBladderz]

I was under assumption the vpd chart is for veg only. Flowering like you said with that high of a humidity would require a lot of fans and airflow.

You can use VPD for all of growth, you just aim for different values during flower. I usually start around 0.8 when they’re seedlings and gradually work my way up to 1.5 during late-flower.

 

[iShatterBladderz]

Is this chart strain dependent?

No, it isn’t, although charts aren’t just one size fits all, there are certain variables that need to be taken into the equation. Pulse has a VPD chart generator on their website where you can input all of your info and it will generate a chart for you. Most charts are made with a 0 degree leaf-temp offset, while LED growers typically have an offset of around -2, while DE HPS can have an offset of +7 or +8.

that being said, the difference is likely going to be small, so even using a generic chart is probably better than not using one at all.