VPD - following chart and south they go!

Have2

Well-Known Member
Where is your probe exactly. I like to put mine right below a top leaf.
I have multiple sensors... Some BME280 and 2 from ac infinity controller, I'm testing two controller atm :)

But I usually put it near the top canopy. Reading seems consistent across the area.
 

ProPheT 216

Well-Known Member
Hey everyone!

I'm having some mars hydro led and well.. When I put 78-79F as temp. I measure leaf temp at ~ -3/-4 F from ambient temp. I should raise my humidity at 58% to get 1.

But, around 1, nothing works... As soon as I drop my humidity to 43-46%, they take off immediately.

Anyone experienced this? Trying to follow VPD guidance but shit... it only works when VPD is at 1.5. (when I just put the clones into the dwc bucket)

thanks for any input!
You are changing so many things so fast it's hard to know whats the what in your set up. I think you are over complicating things. Ambient room between 70 and 77 with humidity around 40 to 60 and good light they will grow. I grow all plants on the same feed and in the same environment. I don't change for strains. Find a universal set up.20231211_225259.jpg
 

Have2

Well-Known Member
How’s your feed? When you raise RH they will transpire less. Less food.
Are you sure your RH is reading correct?
I usually am around 360-450 ppm. Auto-replenishing nutrients... Ph is stable, ppm too.

I have multiple RH sensors and they all read within few %, which is normal.
 

Have2

Well-Known Member
The optimal VPD for cannabis as seedlings is 0.8, 1.0 for veg, and 1.2-1.4 in flower. There's simply no need to argue on that point because we know that's how plants work. But VPD isn't the whole enchilada, of course.

High 70's and 58 is the cat's meow. High 70 and mid-40's means that your plants are transpiring a lot so they're bringing in a lot of water and that also means a lot of chemicals.

First, how do you know that your temps and RH are accurate? Are you using a calibrated hygrometer?

Check out this graphic. Check over each of those items in your grow environment. If they're all within normal parameters, you should be good to go.

If you need 1.5 to make things work, then something is out of whack.

Genetics? Fans? Are you soil or hydro?




View attachment 5350710

My current grow is shown here. VPD in range for the entire month. I use an AC Infinity Controller 69 with their T4 humidifier and the only thing I need to do is put water in the humidifier. I set it at 0.8 for seedling and 1.0 in veg. The stupid computer does the work.

View attachment 5350711

Post some info about your grow environment and something will pop up.
Multiple sensors, all reading +/- 4-5%, which is normal. I tested with digital one, bme280, ac infinity. I'll test using the water+salt method and see the numbers.

Hydro setup, water temp at 68° steady. I also tried to play with water temp, it gives a huge difference on leaf temp!

I love this shit, so many things to see/test/change!
 

Drop That Sound

Well-Known Member
The back of your hand was used with gas discharge because they generated IR that would heat the plants. LED's are a different story.
I would set a lawn chair in the middle of the room, right under my hoods, and kick back with my feet up. Chillin' for a good half hour in my undies with my grow shades, and push buttons on all the equipment until the settings feel good.. :leaf::cool:
 

Have2

Well-Known Member
You are changing so many things so fast it's hard to know whats the what in your set up. I think you are over complicating things. Ambient room between 70 and 77 with humidity around 40 to 60 and good light they will grow. I grow all plants on the same feed and in the same environment. I don't change for strains. Find a universal set up.View attachment 5350756
I'm not changing so many things at once... I love to play around.
As I said, it's only a discussion I wanted to have with others. My results are fantastics, don't worry about this.

I also feed the same shit to all strains. I'm against over dosing and adding 294958392 products.

Thanks for your input!
 

Have2

Well-Known Member
I would set a lawn chair in the middle of the room, right under my hoods, and kick back with my feet up. Chillin' for a good half hour in my undies with my grow shades, and push buttons on all the equipment until the settings feel good.. :leaf::cool:
Ahah I love to do that! So relaxing and a nice feeling when the leaves start to be perky! :)
 

Rocket Soul

Well-Known Member
Vpd should not be taken as an end all measurement to chase in itself. Its just how temps and rh compounded in relation eachother, giving you how much water/humidity the air can still take up in absolute terms rather than relative terms (relative would be RH).
But remember when your calculating this youre building in error from several measurements: your thermometer, your rh meter and whatever you use to measure leaf temp. These are three factors that can throw your values off by a lot. So its probably better to tweak your environment until you get the direct plant response, then when you get it you take down temps /rh and calculate VPD and use this as reference for your plants and equipment.

At no time should you treat vpd as something that is more important than the actual visible plant health and general plant posture of your grow. Its more a thing to calculate in which direction to move your temps/rh when you suspect environment being the reason of why your plants are acting poor.

Also remember: apart from the transpiration/vpd: your plants need some baseline temps which is are higher the more intensity you give. Full flower you should go for at least 25C and adapt your rh as needed.
 

cannabiscrusader

Well-Known Member
[


Do you know how much PPFD you're getting at those specifications?

As a previous member said, can't use that when growing with leds...
Just read the plants. A light meter is about as useful as a water meter. It doesn't take rocket appliances
 

Delps8

Well-Known Member
Multiple sensors, all reading +/- 4-5%, which is normal. I tested with digital one, bme280, ac infinity. I'll test using the water+salt method and see the numbers.

Hydro setup, water temp at 68° steady. I also tried to play with water temp, it gives a huge difference on leaf temp!

I love this shit, so many things to see/test/change!
I appreciate you response and I need to apologize for the tone of my postings. No excuse.

Re. RH 4± - I switched from a Pulse + Inkbirds to AC Infinity and their humidifier. I calibrated using salt but you can also get a Boveda "kit" which consists of one packet and instructions. The Boveda is quicker and reusable.

Water temp changing leaf temp? I could see that being an impact of having a big bucket of water ameliorating temps but, other than that, the two main factors that would drive LST would be ambient temp, IR radiation (very little from LED's), RH, and wind speed. My grow is in a 2' x 4' tent and the res holds 28 gallons so I know if buffers temps for me. In your setup, would you res have enough thermal mass to impact your plants in that way?

Re. VPD being an issue - what I wrote was describing the problem the wrong way. VPD = a combination of RH, and ambient and leaf surface temperatures. I assume that wind speed is assumed to be 0. Given that VPD is just a proxy for, primarily, ambient temp and RH, if your grow isn't doing well in what is considered an optimal range of VPD values, that has nothing to do with VPD. What it means is that your grow isn't growing in what are considered optimal ranges of temp and RH. That's key — VPD is just a way of expressing two numbers (temp and RH) as one number.

If we take the idea of VPD away, the issue is that your grow is not thriving under RH and temp where cannabis is known to thrive. One question to be asked it "Why?" but another, arguably valid question is, "Who cares?". I'm not being facetious, lemme explain.

Metrics are key to reproducibility, with an underlying assumption that reproducibility is, actually, a goal. Unless you're a stickler for detail, as I am, does it matter that this grow/strain isn't growing under what we humans describe as optimal conditions? The plant's happy and, unless you've got to go to extraordinary lengths to keep the plant healthy, then maybe is just the way this grow/strain functions. I'm not advocating "Lie back and think of England.", if you're familiar with that phrase, but, unless there's a need to "fix" this issue, perhaps it's OK to roll with it.

The "If it ain't broke don't fix it." rule be well applied here. Shift the focus from being on VPD to "is the plant healthy". That grates me, personally, because I'm a process-oriented person (software engineer) but, unless this becomes an ongoing issue, how would "Just let it ride." work for you?
 

Rocket Soul

Well-Known Member
The "If it ain't broke don't fix it." rule be well applied here. Shift the focus from being on VPD to "is the plant healthy".
Agree but will rephrase: shift focus from vpd charts to "at what vpd, as measured by my own equipment, is the plants in my grow happy?" It might be that youre measuring equipment isnt giving you perfect results meaning that the chart only steers you away from the right track, and or that you spectrum/intensity/cultivar simply have different needs or actions on the plant. Generally if you reproduce the VPD of "healthy growth" in your grow environment you will get the same healthy growth the next time. Just remembered that there are separate requirements for temps: you can run the right vpd for 60F for full flower intensity and end up with shitty growth cause the metabolism cant keep up.
 

Delps8

Well-Known Member
Agree but will rephrase: shift focus from vpd charts to "at what vpd, as measured by my own equipment, is the plants in my grow happy?" It might be that youre measuring equipment isnt giving you perfect results meaning that the chart only steers you away from the right track, and or that you spectrum/intensity/cultivar simply have different needs or actions on the plant. Generally if you reproduce the VPD of "healthy growth" in your grow environment you will get the same healthy growth the next time. Just remembered that there are separate requirements for temps: you can run the right vpd for 60F for full flower intensity and end up with shitty growth cause the metabolism cant keep up.
Excellent points. It could be cultivar specific of even just one batch of seeds.

How about this for a "Twins" grow. Two plants from seeds in the same seed packet.

IMG_8602.jpeg
 

Delps8

Well-Known Member
Man, what the fuck did we do back in the days when no-one had ever heard of VPD and we had little way of controlling our environment anyway?

o_O
VPD has been around for decades and has been used since soon after people figured it out because it's so important in agriculture.

Back in the day, you had fewer growers, more expensive weed (in real terms), and more variable quality.
 

Prawn Connery

Well-Known Member
VPD has been around for decades and has been used since soon after people figured it out because it's so important in agriculture.

Back in the day, you had fewer growers, more expensive weed (in real terms), and more variable quality.
It was a joke mate. Although it is true a lot of us managed to grow good weed without really thinking about VPD because we read the plant first. If you have to worry about a few degrees or percentages here or there, I'd suggest you have a more direct problem.
 

Lou66

Well-Known Member
Man, what the fuck did we do back in the days when no-one had ever heard of VPD and we had little way of controlling our environment anyway?

o_O
Take longer to become a good grower? I mean just looking at the plant you (eventually) learned that they look bad when you're sweating like a pig or it is so humid that you can't concentrate. That is using organoleptic tools for environmental measurement.
Now you can use some sensors and follow directions in a book. That does not help if you don't know how to collect meaningful data (calibration, proper placement) or ignore your plants. But if you lack that skill you will never learn it.
 
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