Let's talk about VPD while budding ?

[steamroller]

So basically I understand VPD and manage it well .
Now at week 6 almost budding I have a question .
While I can manage to keep leaf temps and RH in line the temp of my buds is not inline with leaves .
Does the bud count or should surface temp be taken only on leaves ?
My buds are close to matching room temp, maybe 1 degree lower thanks to fans . This must be due to mass compared to a leaf ?
I am not having problems since RH is 40% but I just wonder if there is more to VPD in regards to budding .

 

[AdrienneGC]

I would think that the surface temperature should be taken at the leaf. The VPD is the difference between the room conditions and leaf conditions. The bud is probably somewhere in between these conditions (is my guess). Here's a Calculator where adjusting temperature and/or humidity calculates the VPD.

 

[growsince79]

So basically I understand VPD and manage it well .
Now at week 6 almost budding I have a question .
While I can manage to keep leaf temps and RH in line the temp of my buds is not inline with leaves .
Does the bud count or should surface temp be taken only on leaves ?
My buds are close to matching room temp, maybe 1 degree lower thanks to fans . This must be due to mass compared to a leaf ?
I am not having problems since RH is 40% but I just wonder if there is more to VPD in regards to budding .

I ignore vpd chart especially in late flower. I shoot the main cola and try to keep it under 80f @ 45%

 

[kyLman]

I see the VPD calculator, but what is the number trying to achieve? .90 kPA?

 

[steamroller]

You can use the calculator or one of the charts that fits right .

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

The safe range changes as the temp and Rh change .
There are 6 charts there. Just hover the 0-5 degree difference on bottom of chart .
Thanks @growsince79 ,that is what I was thinking but didn't have hard numbers .

 

[Moe.Red]

VPD is calculated at the leaf, not the bud. That said, you need to obviously look at the whole. You can burn your colas if you ignore them and just measure the leaves.

My feeling on VPD is this - it is a good method for newer growers to KNOW they are in the right range with environment. But experienced growers seem to know what the plants need, and respond accordingly, typically in the VPD green range. But certain genetics (i.e. Durban Poison) like it a little hotter and drier than say a Kush, so the experienced grower uses VPD as a start point and then adjusts accordingly. Things like Far Red or UV (A or B) supplementation, media, tendency to get bud rot - all of these factor in and can actually push you outside the green zone in VPD in order to get the best results.

It's a balancing act that you just have to get experience with to ignore VPD and use your senses instead.

 

[steamroller]

tVPD is calculated at the leaf, not the bud. That said, you need to obviously look at the whole. You can burn your colas if you ignore them and just measure the leaves.

It's a balancing act that you just have to get experience with to ignore VPD and use your senses instead.

After making the thread I thought maybe it only applies to leaves ?
But as you mentioned you can burn your buds if you follow the protocol .
Thanks for that !
I am learning to listen to my plants .

 

[Moe.Red]

It’s leaves because that’s where the majority of stomata are. VPD directly interacts with stomata opening and closing thereby setting respiration

 

[phxazcraig]

How are you measuring bud temp? How are we supposed to measure crop temp anyway? I'm hanging an AC-Infinity probe just barely over the canopy. An inch higher is a lot different, as is an inch lower.
I have thermometers at various parts of the tent. Under the canopy is 73-77F all the time, and RH ranges from 33 to 45. In the canopy is about the same temp, higher humidity. at 53-55% Just above the canopy, temp is 80-81F, and RH is 48-50%.

I'm 6 weeks into bud now, and due to inadequate space, I have about 1300 PAR in the center and 400-500 at the edges. The temps and RH are pretty stable since I added a power input fan running 24x7. I'm surprised, but the leaves right under the brightest area don't seem to be suffering.

 

[jguit]

IR thermometer

 

[phxazcraig]

Like one of those $12 jobs? I have one of those...

But still, that can't be the way plant temps have been measured for all the advice of the past.

 

[jguit]

Like one of those $12 jobs? I have one of those...

But still, that can't be the way plant temps have been measured for all the advice of the past.

Not sure how else you'd measure LST if you wanted to calculate VPD. That said, you'll get fairly good at guessing leaf temperature.

 

[phxazcraig]

Not sure how else you'd measure LST if you wanted to calculate VPD. That said, you'll get fairly good at guessing leaf temperature.

I used to be sure, but now I'm not! My method used to be to hang a temp/rh probe an inch or two above the canopy and use that as my measure.

I have one of those remote 3-sensor temp/humidity gauges, and one of the sensors is lying on top of one of my drip halos. A second is tucked in a flap at the side of the tent above the canopy an inch or so. A third is in the same room outside the tent. Temp below the canopy is 74F right now, temp above is 80F. 55% RH below, 50% above. So now what...? Temp in the room is 78F, 43% RH.

The buds themselves looked 'stressed' in the hot area, but the leaves look good, and all is growing well. No bleaching, no tacoing, no praying, fingers crossed. With the 24x7 6-inch input fan, the tent is always bulging a bit and there is a constant flow of air through the canopy to get out via the AC-Infinity. I figure the airflow is keeping me on the good side of trouble.

 

[delps8]

"I'll take definition of 'leaf surface temperature' for $500"

Gotta agree with you on the first part and there's nothing stopping a grower from guessing any of the parameters of the grow environment. Or all of them for that matter, right? One of the reasons to use a metric like Vapor Pressure Deficit is to remove or reduce the inaccuracies and unknowns. Right now, I've got Gorilla Glue autos seedlings and the LST offset has ranged from +2 to +4 over the past week. There's no way that I could accurately guess the values - humans suck at things like that. Since it only costs $20 for an IR thermometer, why would I not just spent the $20 and remove all doubt?

 

[delps8]

It’s leaves because that’s where the majority of stomata are. VPD directly interacts with stomata opening and closing thereby setting respiration

Vapor Pressure Deficit indicates the difference between the internals of the leaf and the external environment. If VPD is very high, transpiration rates increase which drives water uptake. If nutrient solution (assuming hydro) flows into the plant at an increased rate, it can cause nute burn. The inverse is true, as well. VPD is also a cooling mechanism for plants.
I've never seen anything in VPD discussions about buds. The issue is always leaves, stomata, and transpiration. The folks who sell the Pulse have some good info on this.
And, yeh, I drank the Pulse Koolaid and use it as the primary sensor. My tent is only 2 x 4, I wish I had more room, but it's in an unheated garage in SoCal so keeping things dress right dress can be challenging.

 

[delps8]

How are you measuring bud temp? How are we supposed to measure crop temp anyway? I'm hanging an AC-Infinity probe just barely over the canopy. An inch higher is a lot different, as is an inch lower.
I have thermometers at various parts of the tent. Under the canopy is 73-77F all the time, and RH ranges from 33 to 45. In the canopy is about the same temp, higher humidity. at 53-55% Just above the canopy, temp is 80-81F, and RH is 48-50%.

I'm 6 weeks into bud now, and due to inadequate space, I have about 1300 PAR in the center and 400-500 at the edges. The temps and RH are pretty stable since I added a power input fan running 24x7. I'm surprised, but the leaves right under the brightest area don't seem to be suffering.

Have you checked for fox tailing?

The big light in the attachment was running at about 1200 to try to get light to Mary. Very no bueno. One problem is that > 900± PPFD without CO2 is well into the law of diminishing returns. The bigger issue was that the entire plant on the right fox tailed. I ended up with about 100 linear feet of branches and there was one (1) nug. Ironically, the little plant yielded 181 gm of really nice nugs. :-)

 

[jguit]

"I'll take definition of 'leaf surface temperature' for $500"

Gotta agree with you on the first part and there's nothing stopping a grower from guessing any of the parameters of the grow environment. Or all of them for that matter, right? One of the reasons to use a metric like Vapor Pressure Deficit is to remove or reduce the inaccuracies and unknowns. Right now, I've got Gorilla Glue autos seedlings and the LST offset has ranged from +2 to +4 over the past week. There's no way that I could accurately guess the values - humans suck at things like that. Since it only costs $20 for an IR thermometer, why would I not just spent the $20 and remove all doubt?

If your environment is stable and your plants are healthy, my bet is that you can guess LST within +/- 1F, at least that's been my experience. That's not saying i don't check with a IR thermometer, it's easy enough. Even if your LST offset moves a bit, it only moves your RH sweet spot a tiny bit and your instruments measuring temps/RH and controlling temps/RH are only so accurate anyway.

In veg with LEDs, if your ambient temps are 83 - 85, if you keep your RH around 60% you'll be pretty close to your VPD sweet spot. In flower, just lower your RH.

I think understanding VPD is far more important than chasing it, especially with the instruments we're using to measure and control our environments, at least those of us growing at home.

 

[Blastfact]

Anymore I look at the LVPD/VPD charts and laugh. And unless you have a really high end IR Thermo,,, your just guessing on leaf temps. I just got out the new Inkbird and old Fluke IR Thermos and watched leaf temps swing 5f degrees in 15 mins from 75f/80f. In that time frame tent temp swung between 81f/78f and RH swung between 67%/59%. And thats with my targets set at 79f and 65% rh. And thats controlling temp with the AC Infinity control and exhaust fan and the humidifier on a Inkbird. And with all the tech and BS the best indicator this morning was a few leaf petals twisting and showing a edge to the light and not the surface. Checked light height from the top of the tallest plant and that was 22" so I raised the lights up to 24" from the tallest plant. If that does not calm them down in a few hours I will raise them back up to 28". The thing that gets me is how new and young growers now are having to cut there teeth on all this bred up hybrid goof troop fufu weed. I don't give a shit what any breeder says. The genetics are not stable and it makes growing so much more challenging. I've been growing since the 5th grade and I'm now 61. It's just a big shit show now days.

 

[freezeland2]

Like one of those $12 jobs? I have one of those...

But still, that can't be the way plant temps have been measured for all the advice of the past.

Techniques evolve over time as technology improves. Just my opinion.

 

[jguit]

@Blastfact, that's odd.. My cheap IR thermometer is pretty rock solid, doesnt swing whatsoever. If something changes in your environment (open door, etc), it'll swing. Otherwise, it's pretty solid but yea, I agree.. Chasing VPD is a bit of a waste.