Nick Hardy's Skunk Works #3 5x5 - Dissertation Grow

[Gee64]

Huh?! Wasn’t meant like you seem to have taken it at all. Sorry for any misunderstanding. I just think that the changes you’re suggesting a so tiny simply opening the door once a day wipes out any meaningful impact they might have.

For me PPFD for VPD is like changing the type of paint on a sports field looking for a marginal gain in performance.

Leaf temp a 2c input change brought my readings down by about 0.2 - as I said elsewhere over s 1.3 reading when you asked what it told me I said “its a bit high” turns out that was actually 1.1 so it was sweet.

As I said its not something I’m bothered about or chasing.

Sorry for any confusion as to how you interpreted my response.

Nick

Fair enough , but vpd is actually more important with led than hps or cmh.

With LED you can fry your plants with light from 4 feet away. The light from 4 feet away won't change leaf temp by warming the air.

It's not the heat from the light that changes leaf temps, it's the amount of photons hitting the leaf making it work harder. Just like you get warmer walking uphill than down. Even in the dark. How can that be if the sun isn't out?

Photosynthesis creates heat. It's a chemical reaction from splitting molecules. It drives leaf temp.

The difference between leaf temp and air temp drives transpiration. Transpiration pulls in nutes. The flow of nutes needs to match the rate of photosynthesis to maximize the grow.

So VPD ties food, light, and environment together by the flow of water. Earth Wind Fire Water.

What VPD really does, is control the state of the stomata, and they control the grow.

You use CO2. You need your stomata running optimal to intake it.

If you input your data wrong, your pulse meter won't deliver CO2 properly. Maybe 25 pounds could be 30?

Every day after 10 hours of lights-on, you should take a temp reading of the leaf and input it, or better yet, contact Pulse and see when they suggest to take that reading.

If it's not input optimally, your CO2 isn't working properly. It could be better. Plants use a lot of CO2 in veg so it's important in veg too.

From Gavita's site on how to convert to LED.
1. expect different temps.
2. monitor vpd closely....

5 Tips for a Seamless Transition from HPS to LED | Gavita – Retail

gavita.com

Notes 2 and 5 tie it together.

I think Gavita knows light? maybe lol. But there are thousands of articles on it.

I don't care if you pay attention to it or not, it doesn't matter to me. I just wanted you to know you weren't using your meter correctly so CO2 might not be getting delivered optimally.

To be true to your meter you need to do a new leaf temp input every day. Late in the day. Don't pay attention to earlier readings, they will be all over the map.

A plant does a lot of things while we think it is just sitting there.

At night it will raise it's own leaf temps to stop transpiration so respiration can go to work.

If you get it all dialed in you may notice a difference in daily CO2 delivery amounts.

If the plant can use more, thats a good thing. If it needs less then your meter will dial it back for you. If it doesn't change then Nick's hands and eyes got it right.

But the meter only reacts to your inputs. It trusts you to tell it the truth. Otherwise it's just using old or default input data.

But there's nothing wrong with hands and eyes, it's how I grow.

But at least consider this... I took one quick glance at your post of the Pulse readout and knew it was wrong. That means hands and eyes were wrong, and CO2 wasn't being delivered optimally.

Hands and eyes are good for data input too.

 

[NickHardy]

the Pulse has nothing to do with my Co2 delivery. That’s run off an Inkbrid controller with a different type of Co2 sensor.

 

[NickHardy]

I took one quick glance at your post of the Pulse readout and knew it was wrong. That means hands and eyes were wrong, and CO2 wasn't being delivered optimally.

No you have this ass backward. The reading was wrong because I hadn’t entered a leaf offset. When I did it brought the reading down to 1.1 which is about optimal. The Skin & Eyes were correct. The device was out because I had not provided all the data. I didn’t change the environment - I merely changed the reporting tool.

 

[Gee64]

No you have this ass backward. The reading was wrong because I hadn’t entered a leaf offset. When I did it brought the reading down to 1.1 which is about optimal. The Skin & Eyes were correct. The device was out because I had not provided all the data. I din’t change the environment - I merely changed the reporting tool.

Thats exactly what I meant. Without entering leaf offset the CO2 isn't getting optimized by the meter. You need to reenter offset every day.

 

[Gee64]

unless of course you are over riding the CO2 and the meter isn't controlling it.

 

[NickHardy]

The Pulse simply reports the Co2. It does not control its release. It hits 1300 on the Inkbird it cuts out at the cylinder. It goes below 1100 it goes back on.

The Pulse likely reads a little higher as its currently below the inline extraction fan so everything runs past it I mentioned this above.

 

[InTheShed]

Maybe this is helpful to someone. Right now outside the air temp is 64.7º. I just measured the leaf temperature on a Grandpa's Moonshine clone in the sun: 91.5º. I moved it into the shade and immediately took another reading on the same leaf: 73.9º. That's an immediate drop of 17.6º. Radiant heat is not something that LEDs generate.

To be true to your meter you need to do a new leaf temp input every day. Late in the day.

Just so I understand, you're saying that air temps don't affect leaf temp? Because if they do then who among us has an environment so stable over a 24 hour period that it experiences absolutely no change in temperature?

As I said when I first posted, I have no deep understanding of VPD, nor do I have any means of controlling it, but I do like to do some research when something doesn't pass the skin and eyes™ smell test.

 

[Gee64]

Maybe this is helpful to someone. Right now outside the air temp is 64.7º. I just measured the leaf temperature on a Grandpa's Moonshine clone in the sun: 91.5º. I moved it into the shade and immediately took another reading on the same leaf: 73.9º. That's an immediate drop of 17.6º. Radiant heat is not something that LEDs generate.


Just so I understand, you're saying that air temps don't affect leaf temp? Because if they do then who among us has an environment so stable over a 24 hour period that it experiences absolutely no change in temperature?

As I said when I first posted, I have no deep understanding of VPD, nor do I have any means of controlling it, but I do like to do some research when something doesn't pass the skin and eyes™ smell

I forgot that you only grow outdoors so you would have never had a need for it.

What I'm saying if you increase light, but not room temp, the leaf temp will rise. It's interesting.

When a plant photosynthesizes, it splits a CO2 molecule into a C and an O2. That split causes heat. Room temp can certainly add to that or subtract from that.

What the plant wants is a 2 degree difference. So if you set your ambient room temp, set your RH, and then set your light at 50% you will get a leaf temp reading that is a temp combined from the room temp plus the heat being created by photosynthesis.

If you leave the room temp the same but turn the light to 100%, the leaf temp will increase because it is photosynthesizing harder.

That increase will change the difference in temperature so science says that the rate of transpiration will change. If transpiration changes, so does intake at the roots.

So adding or subtracting air temp will change things, changing the RH will change things, and changing the amount of light will change things.

Just google something like What is VPD, or How does VPD work and you will get lots of info. Light companies and controller companies quite often have good articles on it too. It's way to much for me to explain, I'm a slow typer and it's a lot of words lol. But it's only pertinent for enclosed grows, outdoors gets what Mother Nature is serving.

Grabbing a VPD calculator app with 3 inputs, air temp, leaf temp, and RH, will help your brain out alot as you read up on it, as you can quickly run scenarios to see the results.

 

[InTheShed]

I usually grow indoors and out but this time I'm keeping them indoors while they veg. These are autos. Even if I grew 100% indoors I would not be exercising the environmental control needed to intentionally maintain a target VPD.

What I'm saying if you increase light, but not room temp, the leaf temp will rise. It's interesting.

I know you're saying that, I just didn't see it in practice. Nor do any of the sites I quoted and linked.

What the plant wants is a 2 degree difference. So if you set your ambient room temp, set your RH, and then set your light at 50% you will get a leaf temp reading that is a temp combined from the room temp plus the heat being created by photosynthesis.

If you leave the room temp the same but turn the light to 100%, the leaf temp will increase because it is photosynthesizing harder.

These are complete hypotheticals though. Just because it's possible to raise leaf temp with a huge increase in PPFD doesn't make it a good idea. No one bumps their light from 50 to 100%, so whether that will change leaf temperatures is meaningless. We're growing in the real world rather than in a theoretical one, and since it's very important not to burn your leaves with too much light, it's a much better approach to raise the temperature in the grow room than double the amount of the light (or decrease the distance to the tops by half). Raising the temp to get your VPD to your goal is much safer than trying to raise leaf temps by slamming them with added photons they often don't appreciate.

As I said, a 17% decrease in distance resulted in a 0º change in leaf temp. Less than 10" to the light and the leaves show signs of distress, as they would if I turned up the wattage. Would you have me sacrifice the plant on the altar of light-based VPD?

 

[NickHardy]

I’ve just spent twenty minutes with Google and if I have a work super power is formulating search terms. No where is 2 degrees mentioned as being optimal - several places “under LED lights 3 degrees is common” NB these were mostly UK sites or where celsius is defined.

I went deep even learning how to calculate the offset for the IR gun (typically 98% emissary versus 95% for leaves, use a black tape wrapped piece of cardboard to calculate, some IR guns can be calibrated/set)

Everything is change room temperature. Change Rh.

Anything that takes 24 hours to have an impact when simply opening the door to that tent can swing the temperature up by 5c in 2 minutes cannot have a meaningful impact. Its like the Coriolis effect of growing for me.

Be sure to spin your seeds 24 times anti clockwise in the Northern Hemisphere (or clockwise in the Southern Henisphere) before popping them.

This is quite interesting, interplay between blue and IR lights in an LED and their impact on VPD. It does sum up with but just use Temp and Rh.

Intensity and Spectrum: The Role of Lighting in Vapor Pressure Deficit

This article delves into the aspects of horticultural light intensity and spectra as well as their relative influence on cannabis photosynthesis, photomorphogenesis, and overall yield.

www.cannabissciencetech.com

 

[InTheShed]

Be sure to spin your seeds 24 times anti clockwise in the Northern Hemisphere (or clockwise in the Southern Henisphere) before popping them.

I'm filing this away for the spring grow.

 

[NickHardy]

If you can pop exactly on the Spring Equinox then give them an extra half turn, at 34 degrees latitude where you are and the tilt of the earth, you won’t regret it!

 

[InTheShed]

If you can pop exactly on the Spring Equinox then give them an extra half turn, at 34 degrees latitude where you are and the tilt of the earth, you won’t regret it!

That date probably won't work for me. Is there a chart where I can calculate the different angles on different dates?

 

[NickHardy]

Loads out there, but be sure to chose one with leap year offset.

 

[Gee64]

I usually grow indoors and out but this time I'm keeping them indoors while they veg. These are autos. Even if I grew 100% indoors I would not be exercising the environmental control needed to intentionally maintain a target VPD.

I know you're saying that, I just didn't see it in practice. Nor do any of the sites I quoted and linked.

These are complete hypotheticals though. Just because it's possible to raise leaf temp with a huge increase in PPFD doesn't make it a good idea. No one bumps their light from 50 to 100%, so whether that will change leaf temperatures is meaningless. We're growing in the real world rather than in a theoretical one, and since it's very important not to burn your leaves with too much light, it's a much better approach to raise the temperature in the grow room than double the amount of the light (or decrease the distance to the tops by half). Raising the temp to get your VPD to your goal is much safer than trying to raise leaf temps by slamming them with added photons they often don't appreciate.

As I said, a 17% decrease in distance resulted in a 0º change in leaf temp. Less than 10" to the light and the leaves show signs of distress, as they would if I turned up the wattage. Would you have me sacrifice the plant on the altar of light-based VPD?

lol the doubling of the light was purely an example. You obviously want to believe this is untrue. Follow what you find, do what your comfortable with.

 

[InTheShed]

You obviously want to believe this is untrue.

I obviously want you to show me how it is true. All of the evidence, whether empirical or from research, shows it to be untrue. Not that increased light will raise leaf temps, but that going that route is the way to adjust VPD.

What I found troubling is that you're making a very specific suggestion and then are unable to back it up with any piece of evidence other than your inclination.

 

[NickHardy]

lol the doubling of the light was purely an example. You obviously want to believe this is untrue. Follow what you find, do what your comfortable with.

I just can’t find anything suggesting what you are so if you have links please share. I’ve found several like this one:

LED Lights Improve Environmental Control in 4 Ways

This blog explains four significant ways LED grow lights can improve environmental control and create an ideal environment for your plants.

californialightworks.com

That suggest just by not releasing a bunch of radiant heat like an HPS LEDs allow to just focus on more simple HVAC to improve VPD.

Not one suggesting a) 2 degrees is optimal b) managing leaf temperature via PPFD levels is a way to manage VPD.

At the moment the only thing I have read saying that is you, here. I’ve really looked Gee but I’m not finding it, so enlighten me (easy pun )

 

[Gee64]

Heres a screen shot from Pulses manual. Read note 3. Ask them where they get their data from. They have more skin in the game.

 

[NickHardy]

In a multi page document its the only mention. They go into a lot of detail on everything else but that is it. I suggested a read in the comments of that document looking for a guy called Noah, commercial grower who gives a lot of great advice. I can’t link as its their community forum.

But those two lines do not get anywhere close to what you are suggesting. They give zero further guidance on light in the document. Why I’ve been calling it of “tertiary importance”

 

[InTheShed]

Heres a screen shot from Pulses manual. Read note 3. Ask them where they get their data from. They have more skin in the game.

Is that the entirety of the basis for your post in Jon's thread:

So light is the driver behind leaf temp, not air temperatures.

Because that's not what the Pulse manual says. Nor does it mention light source.

Again, are you suggesting I induce light stress in my leaves to raise my VPN to a better level:

You're VPD is 0.3. Thats a bit too low. The plant wants you to keep moving the light closer until it's leaf temps are 2 degrees less than ambient. Your light isn't close enough to be "in the ballpark".

If you move it incrementally closer until the leaf is only 2 degrees different, your VPD would be 0.4, which is marginally low for that stage, but getting into the ballpark.

Only after putting the light mere inches away to raise the leaf temp (if it even would) do you suggest changing the environment in the room, and then raising the light back up to compensate for more heat/less RH:

Then as you raise room temps and bring the humidity down, your VPD will climb, and you may have to start raising your light.

What would be lost if your advice was:

"Make sure your plants are getting the correct PPFD, and then raise room temps and bring the humidity down to get into the correct VPD range."