Vpd At 18+?? I Don't Believe It

[Douglas.C]

Vapor Pressure Deficit
Buzzword that was just hitting the forums a few years ago. Used to prevent moisture buildup on plant surfaces in greenhouses?

I remember the common chart being shown around. I checked my room environment against it back then and shrugged. Years later I come back to the forums and what do I see? 70% humidity and 80F+ room temps? Wow, talk about growing upside down for cannabis.

I'm sure VPD has it's place in cultivation, even for cannabis, but the chart folks are using is definitely wrong. At 72F it says 35% humidity produces 17.something VPD. I flower at 25-30% humidity, 35% lowers quality in my garden. So somewhere around 18+ VPD in my plants? Poppycock! Or, if it is 18+, cannabis loves it.

Here it is, years later, and my cannabis is even more trichome dense than ever. Cannabis around me has gone downhill in quality (or just stayed the same??). This sucks for people using cannabis, and this VPD stuff has a lot to do with it. Every time I hear about a grower complaining about how hot it is in their flower room, I cringe. Every time someone hands me a bowl of "their best," I gag or feel the burn.

Cannabis LOVES 72F and 25% humidity during flower. Or it could be that it hates it, either way it responds by increasing the number of trichomes per square inch. Terpene and cannabinoid production are amazing, I can't wait to get my next harvest tested.

Keep in mind that these conditions need to be present BEFORE you initiate flowering, the density of the trichome coverage is pretty much set at the beginning of flower. At least as far as I've noticed.

Anyone else found the joys of low temp, low humidity quality?

Douglas

 

[GT21]

Vapor Pressure Deficit
Buzzword that was just hitting the forums a few years ago. Used to prevent moisture buildup on plant surfaces in greenhouses?

I remember the common chart being shown around. I checked my room environment against it back then and shrugged. Years later I come back to the forums and what do I see? 70% humidity and 80F+ room temps? Wow, talk about growing upside down for cannabis.

I'm sure VPD has it's place in cultivation, even for cannabis, but the chart folks are using is definitely wrong. At 72F it says 35% humidity produces 17.something VPD. I flower at 25-30% humidity, 35% lowers quality in my garden. So somewhere around 18+ VPD in my plants? Poppycock! Or, if it is 18+, cannabis loves it.

Here it is, years later, and my cannabis is even more trichome dense than ever. Cannabis around me has gone downhill in quality (or just stayed the same??). This sucks for people using cannabis, and this VPD stuff has a lot to do with it. Every time I hear about a grower complaining about how hot it is in their flower room, I cringe. Every time someone hands me a bowl of "their best," I gag or feel the burn.

Cannabis LOVES 72F and 25% humidity during flower. Or it could be that it hates it, either way it responds by increasing the number of trichomes per square inch. Terpene and cannabinoid production are amazing, I can't wait to get my next harvest tested.

Keep in mind that these conditions need to be present BEFORE you initiate flowering, the density of the trichome coverage is pretty much set at the beginning of flower. At least as far as I've noticed.

Anyone else found the joys of low temp, low humidity quality?

Douglas

Yup i prefer under 30% humidity for veg and flower. These plants sat at about 95 degrees and 0% humidity today. STOMA ADAPTS to harsh climate

 

[rmoltis]

The main factors of trichome production from what I've read. Is uv protection & to help the plant from drying out in low humidity environments.

So in flower a low humidity environment will help with potency. But if you were looking for low stress plant health you would want a higher humidity.

I prefer a dry environment in flower. The dryer the better. It helps stimulate trichome production. Under %30 is good in flower. I'm at %44 today (veg) but it fluctuates With the temp & seasons. I let it ride until flower then the dehumidifier comes out.

Plants don't grow for us to smoke. They grow to survive and reproduce.
We just manipulate them for our needs.

 

[Douglas.C]

The main factors of trichome production from what I've read. Is uv protection & to help the plant from drying out in low humidity environments.

Checks out with my experience, yes.

So in flower a low humidity environment will help with potency. But if you were looking for low stress plant health you would want a higher humidity.

I actually have found the plants are unstressed in super low humidity, as long as they don't have to use transpiration for cooling, as well as hydration. The use of silica/silicon products during veg and stretch help immensely with building very strong vascular systems. They handle much higher transpiration rates with great health.

Plants don't grow for us to smoke. They grow to survive and reproduce.
We just manipulate them for our needs.

Indeed, and this manipulates cannabis into producing extreme levels of terpenes and cannabinoids. Great stuff. :)

Douglas

 

[Douglas.C]

Yup i prefer under 30% humidity for veg and flower. These plants sat at about 95 degrees and 0% humidity today. STOMA ADAPTS to harsh climate

Glad to see someone else with experience in this area. :)
That looks super frosty, good job!

Douglas

 

[Savage Henry]

STOMA ADAPTS to harsh climate

Didn't know that, you've piqued my curiosity, I'll have to research it a bit.

I'm no vpd expert by any stretch but have seen 70%rh and low 80s canopy temp in conjunction with supplemental co2 lead to some incredible growth rates. Pretty handy when they're filling in the light footprint during stretch.

To each their own, to be sure. Also noteworthy are the environmental preferences of different strains.

 

[GT21]

Didn't know that, you've piqued my curiosity, I'll have to research it a bit.

I'm no vpd expert by any stretch but have seen 70%rh and low 80s canopy temp in conjunction with supplemental co2 lead to some incredible growth rates. Pretty handy when they're filling in the light footprint during stretch.

To each their own, to be sure. Also noteworthy are the environmental preferences of different strains.

The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening. The term is also used collectively to refer to an entire stomatal complex, both the pore itself and its accompanying guard cells. Air enters the plant through these openings, and contains carbon dioxide and oxygen, which are used in photosynthesis and respiration, respectively. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration

 

[Ignignokt]

I am quite interested in understanding this topic better. I've done some reading but was also having some trouble relating that to the more successful growing advice and my own experience. The first thing I noticed with CO2 supplementation was a hockey-stick curve in humidity. I used 1200-1500PPM as the controlled level in a totally sealed environment. For the first weeks, it seemed almost a closed circuit environment - I extracted all the water I put in as dehumidifier condensate.

This continued until about the second week of flower - after that the hockey-stick was either not present or subdued.

The point being - my general observation from that is VPD is a more important measure in the vegative growth stage than flowering stage to allow optimal transpiration during that growth stage. Flowering is growth, but is markedly different in desired inputs compared to earlier stages.

 

[Douglas.C]

Didn't know that, you've piqued my curiosity, I'll have to research it a bit.

Please do, I would love verification of what I've been seeing for years. I basically lived in dry environments for all of my growing experience, never sealed up a room and added humidity, just kept everything cool. Every strain has come out unbelievably frosty, aromatic and tasty. As I've gotten better at feeding correctly the quality has gone up significantly.
Just amazing.

I'm no vpd expert by any stretch but have seen 70%rh and low 80s canopy temp in conjunction with supplemental co2 lead to some incredible growth rates. Pretty handy when they're filling in the light footprint during stretch.

I believe it. I wonder what would happen if you dropped humidity and temps at the end of stretch? I would imagine it would be a pretty strong shock to the plant.

To each their own, to be sure. Also noteworthy are the environmental preferences of different strains.

For sure. I haven't noticed any strains or phenos that performed poorly, even strains people didn't like for lack of performance. Actually been quite interesting the last few years, with exposure to other growers and a lot more strains. Looking forward to the future. ;)

Douglas

 

[Douglas.C]

The point being - my general observation from that is VPD is a more important measure in the vegative growth stage than flowering stage to allow optimal transpiration during that growth stage. Flowering is growth, but is markedly different in desired inputs compared to earlier stages.

 

[Savage Henry]

The pore is bordered by a pair of specialized parenchyma cells known as guard cells that are responsible for regulating the size of the opening. The term is also used collectively to refer to an entire stomatal complex, both the pore itself and its accompanying guard cells. Air enters the plant through these openings, and contains carbon dioxide and oxygen, which are used in photosynthesis and respiration, respectively. Oxygen produced as a by-product of photosynthesis diffuses out to the atmosphere through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration

So the stomatal density is based on the heat and amount of co2 present and rh? That's cool, the new growth adapts.

I believe it. I wonder what would happen if you dropped humidity and temps at the end of stretch? I would imagine it would be a pretty strong shock to the plant.

I make the environmental changes gradually, over the course of a couple weeks.

 

[Douglas.C]

I still stand that the chart and research are wrong for cannabis, at least from a trichome density and overall quality standpoint. Given the choice between hothouse cannabis and cool mountain cannabis (environment wise), I'll take the cannabis grown in the cool and dry mountains.

Looking forward to someone with the setups to do a side by side with the same strain. Come post their results. Working on being able to do that myself in the next year or so. Will definitely be interesting. :)

Douglas

 

[GT21]

So the stomatal density is based on the heat and amount of co2 present and rh? That's cool, the new growth adapts.



I make the environmental changes gradually, over the course of a couple weeks.

Purdy much. This is where the gas off and intake happens (transpiration) .. less co2 in the air means they have to work harder to grab it.



The mechanism by which carbon dioxide traps heat in the atmosphere is commonly referred to as the "greenhouse effect." Stated very simply, carbon dioxide, or CO2, is nearly transparent to the solar radiation emitted from the sun, but partially opaque to the thermal radiation emitted by the earth. As such, it allows incoming solar radiation from the sun to pass through it and warm the earth's surface. The earth's surface, in turn, emitts a portion of this energy upwards toward space as longer wavelength or thermal radiation. Some of this thermal radiation is absorbed and re-radiated by the atmosphere's CO2 molecules back toward earth's surface, providing an additional source of heat energy. Without water vapor, CO2, and other radiatively-active trace gases in the air, the planet's average temperature would be about 34°C cooler than it is at present.

 

[Douglas.C]

which is why I don't advocate changing humidity during flower, or at all really. Were I vegging for indoor and transplanting outside, I would want to duplicate the outside environment as closely as possible.

I'm familiar with the VPD information, I just believe the most commonly referred to chart is either wrong, or cannabis produces higher trichome density at 18+ lol

Cannabis is ridiculously sensitive to changes in environment and nutrition, it just doesn't visibly show it as much as most plants would. The trichome thing though, this is a visible and repeatable change. :)

Douglas

 

[Ignignokt]

I still stand that the chart and research are wrong for cannabis, at least from a trichome density and overall quality standpoint. Given the choice between hothouse cannabis and cool mountain cannabis (environment wise), I'll take the cannabis grown in the cool and dry mountains.

Looking forward to someone with the setups to do a side by side with the same strain. Come post their results. Working on being able to do that myself in the next year or so. Will definitely be interesting. :)

Douglas

I think this may actually be obvious on the face of it - the sort of terpenes we cherish are volatile organics that heat ( long-wave IR ) and too intense PAR are known to degrade and for our purposes 'evaporate'. Ima thinkin' you already have intuited the race results. :D The question then becomes what should be the limits for late season conditions?

 

[GT21]

I still stand that the chart and research are wrong for cannabis, at least from a trichome density and overall quality standpoint. Given the choice between hothouse cannabis and cool mountain cannabis (environment wise), I'll take the cannabis grown in the cool and dry mountains.

Looking forward to someone with the setups to do a side by side with the same strain. Come post their results. Working on being able to do that myself in the next year or so. Will definitely be interesting. :)

Douglas

When speaking about mountain regions we must realize that the amount of ultraviolet radiation(light photons) increase as well. At sea level there are more gasses and air contaminants that a photon can NOT pass through. In the mountains there is less air which means a photon can travel more direct to the surface. This is why our UV index is highest around noon. It is shooting straight down at earth rather than having to fight the curvature of the earth and the thick atmosphere.



So in reality its much more than your vapor deficit

 

[Douglas.C]

Need a Hashish expert in here. Have wondered about this in the past few years.

The best hash in the world comes from Morocco and what other places? Parts of morocco are dry and cool, other parts are hotter and wetter, wondering where these awesome hash plants are grown and their environment. :)

I know Afghanistan is dry and cool.

Douglas

 

[Ignignokt]

When speaking about mountain regions we must realize that the amount of ultraviolet radiation(light photons) increase as well. At sea level there are more gasses and air contaminants that a photon can NOT pass through. In the mountains there is less air which means a photon can travel more direct to the surface. This is why our UV index is highest around noon. It is shooting straight down at earth rather than having to fight the curvature of the earth and the thick atmosphere.



So in reality its much more than your vapor deficit

Yes, yes. Latitude also a factor for what that buys you. I would note that the Original Genetics strains that are closer to 30 degrees latitude come from places of high altitude - the others come from closer to the equator.

 

[Douglas.C]

So in reality its much more than your vapor deficit

Reality does not bear this out though. Same light, same nutes, upside down VPD with high humidity and temps and the same clone comes out VERY differently.

One comes out like the majority of cannabis I come across, aromatic but nothing special. The cannabis grown with 25% humidity and 72F temps, under the same light, grows significantly more trichomes and retains significantly more terpenes. Massive difference in aroma and flavor intensity.

Douglas

 

[GT21]

Reality does not bear this out though. Same light, same nutes, upside down VPD with high humidity and temps and the same clone comes out VERY differently.

One comes out like the majority of cannabis I come across, aromatic but nothing special. The cannabis grown with 25% humidity and 72F temps, under the same light, grows significantly more trichomes and retains significantly more terpenes. Massive difference in aroma and flavor intensity.

Douglas

Well considering VPD was originally studied on populus candicans in 1978 i think ...weed growers read a bit too far into it. Vpd and trich production do NOT go hand in hand.... this is like saying because i can hold my breath longer then i can get also get a great tan hahaha. Trich production and terpenes are a defense response to harsh climates. The dryer the air the more oil the plant will make... the stronger UV the more the plant will flip out trichs to block the rays.