Lets talk Vapor pressure deficit (VPD) in cannabis?

greenspiritz · Sep 18, 2019

As the title says, how close is this chart? And is this what we should all be aiming for. For a perfect environment?

Anyone here specialised on this subject that could give me and others a more in-depth knowledge about the subject.

Thanks

Green :tiphat:

Loc Dog · Sep 18, 2019

It helps tremendously. I prefer this chart. Stay in the gold range.

https://en.wikipedia.org/wiki/Vapour-pressure_deficit

MedFaced · Sep 18, 2019

It’s a goal to shoot for, but not something to stress over. To paraphrase another ICmag member who’s name escapes me, “your environment is your environment, don’t over think it.”

For a commercial operation where time is money and schedules are to be kept, it would be worth the investment to maintain the ideal environment. For the rest of us, the cost and work involved isn’t worth the effort. Cannabis grows just fine outside of those margins with obvious exceptions for extreme variation. But generally, things just slow down a bit.

greenspiritz · Sep 19, 2019

Loc Dog said:
It helps tremendously. I prefer this chart. Stay in the gold range.

https://en.wikipedia.org/wiki/Vapour-pressure_deficit

[URL=https://www.icmag.com/ic/picture.php?albumid=51120&pictureid=1650202&thumb=1]View Image[/url]

Thanks for chiming in Loc Dog, but i don't think that chart is correct for cannabis growing. I could be wrong?

To stay in the gold zone, you would have to have quite a high humidity around 65+%. Which gives the danger zone for botrytis cinerea.

Take another look at the chart i posted, it might make a little more sense, it was posted on Ceres Green House solutions website that aims at the cannabis market.

That's very true Medfaced, sometimes we just like to play around a bit, but with the same K.I.S.S method approach :biggrin:

Shmavis · Sep 19, 2019

greenspiritz said:
Thanks for chiming in Loc Dog, but i don't think that chart is correct for cannabis growing. I could be wrong?

To stay in the gold zone, you would have to have quite a high humidity around 65+%. Which gives the danger zone for botrytis cinerea.

Take another look at the chart i posted, it might make a little more sense, it was posted on Ceres Green House solutions website that aims at the cannabis market.

That's very true Medfaced, sometimes we just like to play around a bit, but with the same K.I.S.S method approach

Well, your chart also shows 65+% humidity in an ideal range - at a specific temp range. Not sure how you're reading the chart(s), but the humidity is relative to temps.

greenspiritz · Sep 19, 2019

Shmavis said:
Well, your chart also shows 65+% humidity in an ideal range - at a specific temp range. Not sure how you're reading the chart(s), but the humidity is relative to temps.

I don't think you're reading the colour legend correctly at the bottom, try zooming in...

greenspiritz · Sep 19, 2019

First part of flowering - 26c & 50% Humidity.

last 2 weeks flowering 24c & 45% Humidity so between 18c & 24c in the last 2 weeks and a humidity between 35% & 45%.

I'll think you'll find the charts right, read the chart correctly....

All temperature information here on RQS https://www.royalqueenseeds.com/blog-indoor-cannabis-growing-relative-humidity-and-temperatures-n243

3snowboards · Sep 19, 2019

It is so dry here
Its basically crank up the humidifier

I usually aim for about 50%rh (no matter what the current room temp is)
Ill turn off the humidifier if i want them to transpire more
i.e. after a heavy watering or later in flower

Tk x sfvog
@5weeks

Ace Rothstein · Sep 19, 2019

Nice..following

greenspiritz · Sep 19, 2019

It's looking snowy in there, they're looking great!

greenspiritz · Sep 19, 2019

Ok, here goes..

In an "Ideal" environment setting, this is what i think we'd be looking at on this chart.

Circled, are the stages of the weeks and the ideal temperature and humidity levels for each stage.

Weeks 1 - 3 (21c - 28c) (50%Humidity) 1.89 kpa
Weeks 4 - 6 (20c - 26c) (45%Humidity) 1.85 kpa
Weeks 7 - 8 (18c - 24c) (40%Humidity) 1.79 kpa

Quote: RoyalQueenSeeds

Flowering Period

Humidity levels need to be lowered to 40-50% (extremely important)
You can get away with 55% (anything over 60% is real bad)
It’s best to slightly lower temperatures in flowering
Temperatures with lights on: 20-26 C° (avoid high temperatures)

Late flowering (1-2 weeks before harvest)

The following steps are no necessity, but can improve yield, flavour and appearance
Bring down humidity levels as much as you can: 30-40%
Lower daytime temperatures, and also increase the temperature difference (day/night)
Temperatures with lights on: 18-24 °C (lights off: minus 5-10 C°)

:tiphat:

Shmavis · Sep 20, 2019

greenspiritz said:
I don't think you're reading the colour legend correctly at the bottom, try zooming in...

It’s definitely possible.

But, for example, does your chart not show that 70% relative humidity is fine at 89.6º, in flower? (As one example of an even higher percentage that you spoke of.)

I see no indication on your chart of a timeline regarding levels at beginning and end of flower. Am I missing that?

greenspiritz · Sep 20, 2019

89.6f and 70% humidity would be a disaster waiting to happen, those values are way to high.

The only way this would work is, if CO2 was added and you was in the vegetative cycle.

In flower... You'd be expecting bud rot around week 6.

greenspiritz · Sep 20, 2019

It's well known fact, that slowly reducing temperature and humidity over the course, lets say an 8 week strain, improves yield, flavor and appearance. Not to forgot (botrytis) a.k.a dreaded bud rot near the end weeks. Or am i missing the point and that the breeders (RoyalQueenSeeds) have it all wrong...

Can any breeders here on ICMAG confirm this. My discussion on VPD is aimed at near perfect values, like a previous poster said, basically don't sweat it. But what i'm speaking of is the perfect environment and with regards to VPD.

Shmavis · Sep 20, 2019

Yeah, not an issue with CO2.

But does the chart not show those numbers as fine for flower?

Varietal dependent, I suspect.

greenspiritz · Sep 20, 2019

It sure does, but would you recommend having a grow room in flower at 32c/89.6f and a humidity of 70%. It's possible to get away with possibly during half the cycle, but that environment would not be ideal.

greenspiritz · Sep 20, 2019

I think that would aim more towards landrace/sativa varieties with added CO2, is my only guess for being so far up the chart?

bsgospel · Sep 20, 2019

That combination would not bug me terribly. That's the whole point of matching these %'s to these degrees. At that temperature, the air will hold more moisture and thereby draw more moisture away from the plant through transpiration. It would be reductive to say that no matter what, high humidity causes rot. Mildew and rot are resultant of fluctuations, not one static measurement. But a reduction in that humidity at the same temperature causes too much humidity to reside on one side of the balance- it's no longer being removed from the plant because the air can no longer draw moisture from it. That moisture would then become logged inside the flower and temperature fluctuations from day-to-night-to-day inform how rot may begin to present itself.

80 degrees and 60%-~65 has done well for me right up to harvest. And if you are careful to not introduce any elements which may sporulate and keep your VPD constant, you should not invite anything unwanted.

moose eater · Sep 20, 2019

Obviously it's past time to put a rheostat on the exhaust air from my shop. Winter RH varies considerably from summer here, and the only functional way I have to increase RH in the winter, in the shop, without putting frost on the triple-pane windows in the remainder of the house during any extreme cold spells, is for there to be less exhaust in the shop. but that also means that the shop sucks less heat in through the intake from the basement.

Thanks for the thread.

PurplePotion · Sep 21, 2019

I believe CO2 level has a profound effect on VPD that these charts do not account for. Ever run out of CO2 and notice your plants look entirely different after being under light for 8+ hrs without it? I have, but I’ve yet to find any mathematical/scientific formula that addresses the correlation between CO2 and VPD.. there definitely is one but no one seems to be able to quantify it

Lets talk Vapor pressure deficit (VPD) in cannabis?

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