The Gee Spot - You Finally Found It

[Gee64]

i like using this chart for VPD

VPD Charts in Fahrenheit and Celsius With Leaf Temperature

We made some easy-to-use VPD charts for both Celsius and Fahrenheit, for clone, veg, and flower stages. Hope you find them useful. For more information about VPD, and to make your own custom charts, check out our VPD Guide. VPD Charts for Fahrenheit VPD Chart for Clones in Fahrenheit -2F...

pulsegrow.com

There's a link to a good guide in this post too. Thanks Con

 

[Kaldean]

If you google VPD you will get ons of info on it. You want the info on VPD that deals in RH, air temp, and leaf temp. VPD that only deals in RH and air temp is for weather forcasting, not growing plants.

@Gee64
Iv just got the dr greenhouse one, the pulse one I’m lost with, and iv put in my temp and rh, and like you say, when I calculate, it’s saying dry and stressed , I think it was on about me thou to be honest
But when I put in the one with leaf temp, that iv guessed at, cos I see someone mention 6 degrees lower
it’s saying I’m ok
But Gee, you know it’s coming don’t ya
Howd I measure leaf temps
And iv just realised my tents hot

 

[Kaldean]

Play around with it Kal. Run some scenarios in the app. Then decide what variables are hard to change, such as ambient room temp, and adjust the other 2 parameters in the calculator.

If you look at this that I posted for Azi you can see at the bottom what your targets are for the stage of the grow you are in.

Leaf temp is the tricky one. If you raise room temp it will also raise leaf temp but if both go up 1 degree not much difference is noticed. Like this

raising both did raise VPD but if room temp stayed at 90 and you only raised leaf temp by 1 degree you would get this

which is a much greater rise in VPD.

To raise leaf temps to close the gap on air temps, you add more light and the radiation in the light excites molecules in the leaf just as exercise does in our muscles, and heat is formed.

You need to be careful you don't light burn the leaves so don't get carried away, but once you calibrate your PPFD app you can guard against that.

A healthy plant under optimal conditions will try to get to 2 degrees under room temp after 10 hours of lights on. That is where optimal transpiration occurs.

It's important that you take your readings late in the plants day as they are all over the place in the mornings. You want readings from when the plant is working it's hardest or you may end up over revving your plants late in the day. Never react to earlier readings. Always from the 10 hour mark.

Then there is RH. Once your light is set properly to your room temp you can speed up or slow down the plant by lowering or raising RH.

It's not as complicated as it sounds if you run some scenarios in the app before you start actually adjusting things in the grow room.

Right now in my grow when the plants stretch and PPFD hits 1100 the leaf temp differential is 2 degrees, and when I drop it back to 950-1000 PPFD it goes to a 3.5 degree differential which tells me she may be able to handle 1100 PPFD, but I will wait until stretch finishes and she stops growing taller before I sneak in on that one, as I don't want to burn my leaves or yield and quality will suffer.

VPD is an extremely powerful tool. It's actually the difference between outdoors and indoors. Outdoors you get what nature dictates.

It surprises me that it isn't followed and used by everyone. We all have lights and nutes, so environment is all thats left.

And as always, the proof is in the pudding so lets look at the pudding. We have all read/heard numerous times that in flower 76F is the ideal temp, and most would say that after stretch 48RH is perfect. Well if you toss 76 and 48 into the calculator with a 2 degree spread you get this...

and 1.4 is perfect VPD for flower after stretch. The plants are working hard but not too hard and you still have buffer room if conditions fluctuate.

So here is the warning. If you raise VPD too high the atmosphere will suck too hard on the plant and draw water in too fast at the roots. The water coming in will not contain adequate food as it's getting sucked in too fast, and a deficiency will occur, so try to stay under 1.5 always, and 1.4 in late flower is plenty.

Don't get greedy here. Over 1.4 and you are one RH drop or temp spike away from screwing up your grow for a few days until it recovers, and possibly damaging leaves that won't recover. That comes out in yield.

In a nutshell, it's the throttle for the grow.

Thank you @Gee64
There’s a lot there for me to go over to get a better understanding of it all, and your absolutely correct, if your growing weed and you understand this, it’s making it much easier in it, it’s a great tool that I need to learn, thank you mate for that run through

 

[Kaldean]

i like using this chart for VPD

VPD Charts in Fahrenheit and Celsius With Leaf Temperature

We made some easy-to-use VPD charts for both Celsius and Fahrenheit, for clone, veg, and flower stages. Hope you find them useful. For more information about VPD, and to make your own custom charts, check out our VPD Guide. VPD Charts for Fahrenheit VPD Chart for Clones in Fahrenheit -2F...

pulsegrow.com

Thank you @con for that link, now to get me head round it

 

[Gee64]

@Gee64
Iv just got the dr greenhouse one, the pulse one I’m lost with, and iv put in my temp and rh, and like you say, when I calculate, it’s saying dry and stressed , I think it was on about me thou to be honest
But when I put in the one with leaf temp, that iv guessed at, cos I see someone mention 6 degrees lower
it’s saying I’m ok
But Gee, you know it’s coming don’t ya
Howd I measure leaf temps
And iv just realised my tents hot

you need an infra red thermometer like this one

and you zap the highest mature leaves and get the leaf temps to plug into the calculator.

If you do it at about 10 hours after lights on you get the reading that is where they are at when working the hardest.

Not saying you should run out and spend money, but for how cheap they are they're worth it. This, a water stik, and a refractometer are my 3 main tools that I use all the time. Daily for the gun and water probe. I actually zap my leaves for temp every time I open the tent. A sick plant will suddenly run hot. So will a dry one.

 

[Gee64]

Thank you @Gee64
There’s a lot there for me to go over to get a better understanding of it all, and your absolutely correct, if your growing weed and you understand this, it’s making it much easier in it, it’s a great tool that I need to learn, thank you mate for that run through

You don't really need to know it deeply, just in general and then let the app do the work. You plug in values that you think you can achieve and see what you get before you make changes.

 

[Kaldean]

you need an infra red thermometer like this one

and you zap the highest mature leaves and get the leaf temps to plug into the calculator.

If you do it at about 10 hours after lights on you get the reading that is where they are at when working the hardest.

Not saying you should run out and spend money, but for how cheap they are they're worth it. This, a water stik, and a refractometer are my 3 main tools that I use all the time. Daily for the gun and water probe. I actually zap my leaves for temp every time I open the tent. A sick plant will suddenly run hot. So will a dry one.

Wow, big part of my life them and I didn’t even think of it
Thank you Gee

 

[Kaldean]

You don't really need to know it deeply, just in general and then let the app do the work. You plug in values that you think you can achieve and see what you get before you make changes.

@Gee64
That’s exactly my intentions, wing it, and the more I use it the better (hopefully lol) I will get with it
Could you do me a favour if you don’t mind please Gee, if I show you this pic, could you check yours just to see if the one iv got is roughly on track
And thank you again Gee for all the time you’ve spent trying to educate an idiot

 

[Azimuth]

It could be cold water, but likely it means you need more light to drive them harder.

No, not cold water and the ppfd is north of 1,000.

Or it could be that you need better ventilation to lower air temps. 90 is a bit high.

Yeah, I don't have any meaningful control of the environment, but I'll turn up the extract fan to see if I can lower temps a bit. I'd rather be mid 80's at the peak, but, you know...

What is the RH in your grow space?

Between mid 20's and mid 50's which I think is likely my issue. But no room for a humidifier...

Leaf temps drop quickly so you need to open your grow space and zap a leaf temp quickly.

Ok, makes sense. I'll try again tomorrow.

An RH of 55 is even better as it would get your VPD down to 1.33 which is a great sweet spot to avoid stress and leaves room for fluctuations.

I think that that mid 50's in rh is during lights out, and likely the mid 20's number is lights on. It was a steady 45 with the doors open but that's not typical obvs.

So close!

Remind us again please Azi, which mix is this?

This is the one with all of the mineral calciums but relying on flowers and nettle for much of the P value, so no bone meals, and growing in my first NetPot SIP.