VPD vs "acknowledged science"

[Filthy_Casual]

I wasn't sure how to frame the question exactly, so if someone doesn't like the title language sorry.

My question is this:

VPD chart says at about 80f I want an RH of about 60-65%.

"Acknowledged Science" and by that I mean general consensus from decades of info growers use, say I should have it at about 40% for flower.


The 20% discrepancy is what I'm interested in learning about. Is it because people used to use HPS for everything and now LED is in play? Does it differ for the lights or soil source?

Someone who sciences hard, please explain it to one of your less learned brethren

 

[bad dragon bzzz]

if theres something wrong with the language or tone then we are a delicate bunch arnt we
acknowledged science or old wives tale ? just because somebody has been doing something from the beginning of time dont make it perfect right or adequate.Because way back when they didnt have a vpd chart,and ya cant flush an outdoor plant.
early flower,mid flower,late flower.Some folk might want to break up the flower part into 3 parts.
40 % is maybe perfect for late flower and 60 for early/mid flower.
but then again somewhere in the middle is fine.
our we in a position to change temps/humidity ? if not then thers bugger all ya can do about vpd.
on a daily bases my cupboard jumps from 40-65% hum.......but as i have 3 plants at say week 2,5,8 no vpd is perfect (ballpark is good enough for me) temps could be anywhere from 15c to 35c winter summer.
But science is interesting both the theory and practical side,im all ears
good luck n keep er lit

 

[Highway 61]

Late in flower, the same conditions that give good vpd numbers can also be conducive for bud rot. So, some folks like to keep the rh low as they get further into flower. And the temperature that is important for vpd is the leaf temp, not air temp. With led lights, your leaf temp might be a couple of degrees C lower than your air temp which you can check with a laser thermometer. So, that difference can be factored into your vpd calculations to give you a more accurate info.

 

[Filthy_Casual]

Thanks for replying guys! Sounds like maybe at the end as they chunk up, I'll try to get it lower, but for now leave it in the VPD zone.

Appreciate it!

 

[Olderfart]

As I understand the concept, ideal VPD is about growth efficiency, and is all about the RH at which leaves function at their best. Too wet, and transpiration is reduced, too dry and stomata close with the same result. Just right, and stomata are fully open and maximum transpiration and photosynthesis is possible, as determined by whatever the next most limiting factor is. And as pointed out in earlier comments, VPD depends on leaf temperature, and the RH in your canopy.

As to leaf temps, if you use VPD charts, you will see that they are (or should be) labeled as applying to a particular leaf temperature, expressed as the number of degrees cooler than the air temperature. Transpiration is evaporation of water which cools leaves, hence the normal status of leaves being cooler than air. I use charts rather than a calculator. My chart is tacked to my growdrobe door.

The big difference under LEDs vs. HPS is that the latter emit a lot of infra red heat and tend to raise leaf temperature, but LED's emit far less, so leaf temps stay lower. If you want the best info for your grow, check leaf temps with an infrared thermometer, and calculate VPD or use the appropriate chart accordingly. If you don't have a thermometer, assuming a couple degrees cooler will likely get you in the ballpark if you are using LEDs, which quite frankly is all that is required.

As to the issue of flowering, once your buds start getting fat and dense, the importance of VPD for photosynthetic efficiency becomes out weighed by the need to avoid bud rot, and that means keeping humidity lower than VPD charts would otherwise suggest. I have limited experience, but what I do have suggests that all that is required to avoid rot is to keep VPD at the low end of the acceptable range, and make sure that you have consistent and good air circulation through the canopy. Avoiding bud rot may also require some pruning to make sure that air circulation is good, particularly in some Indica leaning strains which can stay bushy and dense as well as developing large fat colas vulnerable to rot.

So, bottom line is that the right VPD chart is your friend during vegetative growth until somewhere around mid flower, but after that, you need to reduce humidity somewhat to avoid bud rot. In effect, you are sacrificing a bit if photosynthetic efficiency to protect your precious bud in it final fattening up.

Happy growing @Filthy_Casual.

 

[Filthy_Casual]

As I understand the concept, ideal VPD is about growth efficiency, and is all about the RH at which leaves function at their best. Too wet, and transpiration is reduced, too dry and stomata close with the same result. Just right, and stomata are fully open and maximum transpiration and photosynthesis is possible, as determined by whatever the next most limiting factor is. And as pointed out in earlier comments, VPD depends on leaf temperature, and the RH in your canopy.

As to leaf temps, if you use VPD charts, you will see that they are (or should be) labeled as applying to a particular leaf temperature, expressed as the number of degrees cooler than the air temperature. Transpiration is evaporation of water which cools leaves, hence the normal status of leaves being cooler than air. I use charts rather than a calculator. My chart is tacked to my growdrobe door.

The big difference under LEDs vs. HPS is that the latter emit a lot of infra red heat and tend to raise leaf temperature, but LED's emit far less, so leaf temps stay lower. If you want the best info for your grow, check leaf temps with an infrared thermometer, and calculate VPD or use the appropriate chart accordingly. If you don't have a thermometer, assuming a couple degrees cooler will likely get you in the ballpark if you are using LEDs, which quite frankly is all that is required.

As to the issue of flowering, once your buds start getting fat and dense, the importance of VPD for photosynthetic efficiency becomes out weighed by the need to avoid bud rot, and that means keeping humidity lower than VPD charts would otherwise suggest. I have limited experience, but what I do have suggests that all that is required to avoid rot is to keep VPD at the low end of the acceptable range, and make sure that you have consistent and good air circulation through the canopy. Avoiding bud rot may also require some pruning to make sure that air circulation is good, particularly in some Indica leaning strains which can stay bushy and dense as well as developing large fat colas vulnerable to rot.

So, bottom line is that the right VPD chart is your friend during vegetative growth until somewhere around mid flower, but after that, you need to reduce humidity somewhat to avoid bud rot. In effect, you are sacrificing a bit if photosynthetic efficiency to protect your precious bud in it final fattening up.

Happy growing @Filthy_Casual.

awesome, thanks for laying out some of that in a little more detail, including the distinction between air temp and leaf temp. I try to keep my sensors around the tops of the canopy, but maybe I'll let one dip in a bit and see what the differences is in there!

 

[Olderfart]

awesome, thanks for laying out some of that in a little more detail, including the distinction between air temp and leaf temp. I try to keep my sensors around the tops of the canopy, but maybe I'll let one dip in a bit and see what the differences is in there!

As long as your air circulation is good, being close to the canopy is good enough. You are already way ahead by just using vpd information, lots of growers don't pay attention to it, which works sometimes, but sometimes not.

As long as you keep VPD somewhere in the right zone, you will be fine, and other aspects of your grow will determine your outcome.

Good luck with it.

 

[Filthy_Casual]

As long as your air circulation is good, being close to the canopy is good enough. You are already way ahead by just using vpd information, lots of growers don't pay attention to it, which works sometimes, but sometimes not.

As long as you keep VPD somewhere in the right zone, you will be fine, and other aspects of your grow will determine your outcome.

Good luck with it.

Okay great. I have exhaust set to trigger at humidity and temp levels, as well as 3 smaller fans. I am going to add a bigger floor fan as well soon.

Finally getting a lot of things wrapped around my head, but still have a way to go!!

Thanks again!

 

[Olderfart]

Okay great. I have exhaust set to trigger at humidity and temp levels, as well as 3 smaller fans. I am going to add a bigger floor fan as well soon.

Finally getting a lot of things wrapped around my head, but still have a way to go!!

Thanks again!

Sounds like you are getting set up great. Better than I am in fact because my humidity control is, shall we say, a tad more primitive - consisting of manual adjustment of a baffle controlling growdrobe exhaust. All I manage to do with this is, usually, keep the VPD somewhere in the ballpark. But like you, I am at least paying attention. If I didn't, I would be in the glue because RH here in the winter is consistently too low. The other side of that is that it makes dealing with RH easier because I only need to deal with raising RH, never lowering it, which is easy by just adjusting the air exchange rate.

 

[Death The Cultivator]

I manage my VPD from seed to harvest and I have never gotten bud rot. Running 79f and 60% RH near the end currently and no mold. My understanding of mold and pathogens is that frequent swings and or cold moist environments will breed these issues.

Everyone has a unique situation and being able to optimize that through control of VPD is going to improve growth, nutrition uptake, etc.