VPD (vapor pressure deficit)

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Moe.Red

Moe.Red

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Some climates may produce better yields than others, some may have more potent than plants others, some may result in different cannabinoid profiles. Not sure what you are asking about on the landraces.

You are so close!

What I am saying is that what you call yield (I assume measured in dry weight) is the measurement by which you determine success is irrelevant to me. I can grow a 4 pound tent any damn time I want. But it would take us 10 years to use that. Trust me, I've done it, and it goes to waste.

What I want is quality.

"some may result in different cannabinoid profiles"

^^^THIS^^^

I am in the search for THCv. The bit about landraces is meant to explain why I am looking at climate tweaks to help my journey. Minor cannabinoids have all but been bred out of the current hybrid offerings commercially. But they were there in certain pockets of landraces. Developed due to local environmental factors and thousands of years.

So if I start looking at a Durban Poison for example, why does it have THCv? I don't think anyone truly knows, only hypothesis. But the climate is surely different than the mountains of Afghanistan.

We also have evidence that cooler night time temps effect many plant processes. Could it provide pathways to minor cannabinoids as well as flavonoids, hormones, and enzymes? Dunno. Wanna try.

I really don't think I am hurting anything with what i am trying here. I'm on Version 2 of my Raspberry Pi 4 system controller. With it, I can perform the VPD calculations and control RH to match current temps. Sound cool enough?

I want total control over every aspect of environment, just that simple. I realize this is not necessary for big yield.
 
Moe.Red

Moe.Red

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I thought of a couple parallels.

Ever eat Kale that has been thru a frost vs the same plant where it hasn't? It's a different plant, unmistakable. The one with the frost is waaaay better.

Another - I just started a grow and the temps decided to go to negative numbers here and my basement was hovering around 60* for over a week. The lights in the Veg tent could not keep the tent warm enough like they had every grow before. It was a week before I got a heater in there and the early veg plants were short and squatty and the leaves were huge and very wide. Unmistakable evidence that temps have a very direct influence over plant growth.

With the added heat, they are now growing normally. Further evidence that changing temps can steer plant growth.

What did that do to the plant? Did it change something structurally like the Kale example? Better or worse?

Inquiring minds want to know!
 
FourthCity

FourthCity

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You are so close!

What I am saying is that what you call yield (I assume measured in dry weight) is the measurement by which you determine success is irrelevant to me. I can grow a 4 pound tent any damn time I want. But it would take us 10 years to use that. Trust me, I've done it, and it goes to waste.

What I want is quality.

"some may result in different cannabinoid profiles"

^^^THIS^^^

I am in the search for THCv. The bit about landraces is meant to explain why I am looking at climate tweaks to help my journey. Minor cannabinoids have all but been bred out of the current hybrid offerings commercially. But they were there in certain pockets of landraces. Developed due to local environmental factors and thousands of years.

So if I start looking at a Durban Poison for example, why does it have THCv? I don't think anyone truly knows, only hypothesis. But the climate is surely different than the mountains of Afghanistan.

We also have evidence that cooler night time temps effect many plant processes. Could it provide pathways to minor cannabinoids as well as flavonoids, hormones, and enzymes? Dunno. Wanna try.

I really don't think I am hurting anything with what i am trying here. I'm on Version 2 of my Raspberry Pi 4 system controller. With it, I can perform the VPD calculations and control RH to match current temps. Sound cool enough?

I want total control over every aspect of environment, just that simple. I realize this is not necessary for big yield.
I never brought up my definition of success, I'm talking about and trying to help you reach yours. Without going to far off the vpd topic, part of what I am trying to say is that if you want high thcv for instance then the most important thing you can do is grow strains that are known to have it, if you do this for a few grows with a consistent environment and proper vpd and test the results you will then be able to make tweaks to your settings and have a good idea of what kind of impact those changes may have.

Without a proper baseline what can you compare you results to and how will you be able to determine what changes are getting you closer to your goal? For a specific cannabinoid like thcv, the genetics are going to play an exponentially greater role in its presence (relative to other cannabinoids, not in total) than environmental factors.

In your shoes, if you don't already have one, I would look into purchasing my own cannabinoid tester or sending samples out for testing, from there I would focus on breeding the plants with tests that show the highest levels of thcv and do so in the most consistent environment possible. When the increase of thcv from selective breeding starts to plateau then I would start looking into whether any additional gains can be had from making changes to the environment.

You have a lot of questions in your post that there is not enough scientific research to give a definitive answer on. I'm suggesting that if you are going to try and embark on answering those questions yourself then doing so scientifically will produce the most useful information.
 
Moe.Red

Moe.Red

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Hey Forth, I think we are talking past each other and not really communicating.

There is a very active thread about breeding THCV on which many of the mega minds here at the farm are participating. The things that you talk about have been fully discussed by many already, we are working together to build the proper approach. It's a lot of brainpower and experience talking over there.

I appreciate you reaching out to try to help me, why not join in on the other thread where it might count for something and we can quit polluting this VPD thread with my little issues.

 
FourthCity

FourthCity

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Hey Forth, I think we are talking past each other and not really communicating.

There is a very active thread about breeding THCV on which many of the mega minds here at the farm are participating. The things that you talk about have been fully discussed by many already, we are working together to build the proper approach. It's a lot of brainpower and experience talking over there.

I appreciate you reaching out to try to help me, why not join in on the other thread where it might count for something and we can quit polluting this VPD thread with my little issues.

I get you, I don't have much to add to that thread and thcv is not really a priority for me. I only responded to you because it looked like you were getting hung up on adding more tech to your setup and I thought I could reassure you that it was not needed given the equipment you already have.

I think this thread is the place to discuss vpd and its effect on plants, I don't see why comparing vpd to the natural environment wouldn't be relevant too. You eventually brought up that you wanted this additional control for more than just vpd and that's fine, I'm not trying to stop you, but it's not really fair to criticize me for responding to the statements you have made in this thread in said thread.

If I were to summarize what I am trying to communicate to anyone reading this thread, not just you, its that creating a consistent environment with proper vpd is a crucial stepping stone towards your goal, no matter your definition of success or what you are looking for out of your plant. I also don't want people to feel daunted when they keep seeing the mention of additional equipment. Central air and a dehumidifier (or a humidifier if you live somewhere really dry) is going to be enough for most people.
 
GYOweed

GYOweed

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Aha Leaf surface temp! And I had an infrared pen the whole time lol Makes like easier since leaf shows cooler here atleast.
Thanks op!
 
TheRealM00

TheRealM00

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Because plants don't transpire so there is no need to have it higher... reduces risk of pathogens... Ideally 10% lower at night.


When reversed its like PM heaven... low day and high night humidity = not good.
Hi m8 i have noticed my rh when i open the tent at lights on at around high 60s mayeb even low 70%s when its about 45 give or take in teh lights on phase, 1 fan turns off when light go off as i had issues with fansdrying out the plants at 1 pooint but this seems to be fixed now just not sur eif i shoudl leave teh tent slightly open all night with all fans running like during the day? would this help? i have no money for a dehumid all i could add isa cheap air purefier would this be worth it? cheers
 
Aqua Man

Aqua Man

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Hi m8 i have noticed my rh when i open the tent at lights on at around high 60s mayeb even low 70%s when its about 45 give or take in teh lights on phase, 1 fan turns off when light go off as i had issues with fansdrying out the plants at 1 pooint but this seems to be fixed now just not sur eif i shoudl leave teh tent slightly open all night with all fans running like during the day? would this help? i have no money for a dehumid all i could add isa cheap air purefier would this be worth it? cheers
That's because RH is the saturation % of water in the air relative to temperature. So we all know warm air hold more moisture. When the lights go out and the temperature drops so does the amount of water holding capacity that the air has so the % of saturation (RH) goes up since cooler air hold less moisture. If anything I would run 2 fans for a few hrs after lights out and maybe only one during lights on.

Plants generally close thier stomata at night so a low humidity at night is not nearly as bad as low humidity during lights on.
 
TheRealM00

TheRealM00

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That's because RH is the saturation % of water in the air relative to temperature. So we all know warm air hold more moisture. When the lights go out and the temperature drops so does the amount of water holding capacity that the air has so the % of saturation (RH) goes up since cooler air hold less moisture. If anything I would run 2 fans for a few hrs after lights out and maybe only one during lights on.

Plants generally close thier stomata at night so a low humidity at night is not nearly as bad as low humidity during lights on.
thanks you made this so easy to understand.
 
Marcmiller32

Marcmiller32

41
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Ok i was gonna sit down and write out a whole article but i have been lazy so instead instead I'm going to copy paste because im lazy. This is not my work so let start. You may find this very long but its worth the read. Some may want to skip ahead. I have added here and there to what i feel is missing but im going to skip alot of the more in depth stuff to keep this simple.

WHAT IS VPD:
VPD stands for Vapor Pressure Deficit. All gases have vapor pressures, but when we’re growing, we’re interested in water vapor. Vapor pressure deficit is the difference between the pressure of water vapor in 100% saturated air at a given temperature (basically a leaf’s vapor pressure) and the air’s actual vapor pressure. A high VPD value raises a plant’s transpiration rate and increases nutrient movement through the the plant’s xylem, while a low VPD value slows the movement of nutrients through the plant.

View attachment 910696

WHY SHOULD WE CARE:
VPD control is related to the following:
  1. Increasing or decreasing metabolic rate
  2. Improvement in yield quality
  3. Determining plant stresses
  4. Pathogenesis (more on this later)
  5. Carbon dioxide injection (more on this later)
Growers should care about VPD because it impacts yield quality, overall plant vigor, and nutrient utilization. Managing VPD lets me get away with using fewer nutrients, which improves my bottom line. I’m also seeing increased trichome production in the plants, which naturally follows better health.
You reach expert growing level when you learn to manage humidity and VPD. Everybody spends their time managing temperature, nutrients, and whatever else, but the last little thing you learn to manage is humidity, and it is significantly more finicky.

HOW TO STEER PLANT GROWTH:
In order to stay on the same page, I should point out that relative humidity (RH) andVPD are inversely related. This means that when relative humidity (RH) is high, VPD is low, and vice versa.
When growers want to know how to steer plant growth, they are interested in maximizing growth. So let’s start with how the plants react to changes in VPD:

  1. The bulk flow of water changes within a plant’s xylem as VPDchanges.
    1. If you have a high VPD, meaning that the RH is low, the plant will increase its transpiration rate and start pulling water faster from the substratein an effort to stay cool and moist.
      1. If the VPD is too high, the plant will become stressed, leading to inefficiencies.
    2. In the same vein, if the VPD is too low, meaning that the RH is high, the transpiration rate will decrease, slowing the flow of water through the plant.
  2. Nutrients follow the flow of water through the xylem and into its various tissues. Nutrientslike calcium primarily move with the bulk flow of water through the arteries of the plant.
    1. Therefore, as VPD rises (and the bulk flow of water increases), nutrient uptake will also rise.
    2. If VPD falls (and the bulk flow of water decreases), nutrient uptake will also fall.
  3. If you’re injecting carbon dioxide, you want the plants’ stomata to stay dilated for as long as possible. Ideally, the stomata would be fully dilated at all times to maximize carbon dioxide use. Plants open and close their stomatato regulate moisture loss.
    1. If you have a high VPD, or low RH, your plants are going to close their stomata to reduce water loss. When the stomata close, you’re not getting adequate gas exchange, and you’re not making the most of your carbon dioxide.
    2. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide.


Effective VPD control is about balancing gas exchange. There is a “Goldilocks” zone where the plant is getting everything.
If your VPD is too low, then your plants aren’t going to acquire enough nutrients, slowing growth; if your VPD is too high, you’re going to stress the plant and the stomata will close, rendering your extra carbon dioxide ineffective. Like everything else with growing, there’s a Goldilocks zone. One VPD is too high, one VPD is too low, and one VPD is just right. There are charts of a VPD curve with a three way graph of humidity, temperature, and growth. There’s a sweet spot along the center of the chart.

vpd chart.jpg


DO NOT USE THIS CHART ITS REFERENCE ONLY
A VPD chart for a hypothetical plant, image courtesy of Argus Controls. The far left side of the graph is too wet for the plant,
and nutrient uptake is inhibited. The right of the graph is too dry for the plant and stomata close, inhibiting CO2 uptake.

VPD IN DIFFERNT STAGES OF GROWTH:

Ideal KPA (kilopascals) ranges for different stages of growth.
Seedling/clone 0.4-0.8
Veg 0.8-1.1
Early flower 1-1.4
Late flower 1.3-1.5
As a matter of fact, most growers use some form of VPD control already, without even knowing it. When you put clones under a dome, you’re keeping the RH high and the VPD low. This, in turn, slows transpiration to a crawl, greatly reducing the stress on the cuttings, which need time to form roots. Typically, most growers will keep their vegetativehumidity a little bit higher as well, which reduces stress.

Domes are a form of VPD control.
Most growers are concerned about the flowering cycle because that’s where the magic happens. You want to keep your VPD relatively high (low RH) during the flowering cycle. If you assume an average flowering cycle of 8 weeks, start with a moderate VPD (medium RH) during the first 3-4 weeks of your flowering cycle, then increase your VPD (lower your RH) towards the end of flowering. This reduces pathogenesis.

One thing you can do when a plant is stressed, say from moving from one room to another, is to raise the humidity. This lowers the transpirational stress and eases their transition into whatever phase or room you have set up. Additionally, HID lights can be stressful for plants, and VPD control gives you the ability to reduce their stress. If you have a dry environment and bright lights towards the top, you’ll see canopy leaves fold in like a taco. Plants do this to reduce light capture and reduce their internal temperature. If you see this happening, you need to ease up on the plants and reduce their stress.

CONCERNS WHEN USING VPD:
Pathogenesis is a big issue, which we’ve touched on briefly. The biggest drawback to running a low VPD (high RH) is that you can run into a lot of problems with pathogens if your rooms aren’t clean. As a result, many growers reduce their humidity as much as possible. Some growers brag that their humidity is as low as 20%, which is really bad for the plants and slows their growth.
Homogenizing a room’s environment is a struggle. In my experience, there are always new micro-environments forming in your room due to the nature of working with living organisms. Keeping on top of it all takes a lot of effort.

Good ventilation/circulation is necessary for VPD control.
Accurate sensor readings are also a problem I keep running into. Keeping the environment at your desired setpoint of temperature and humidity can be tricky. Having the right equipment and the right room layout can make a big difference.

WHAT EQUIPMENT DO YOU NEED TO EFFECTIVELY CONTROL VPD:
You’re going to need a humidifier for starters. You want to be able to inject humidity into the room without causing any problems such as being too close to one plant. If you have your humidifiers spraying plants directly with vapor, you will end up with undesirable microclimates which could favor pathogenesis. Personally, I think that ultrasonic humidifiers work best.

You are going to need a way to measure the leaf temperatures in order to accurately calculate VPD. This is where the online charts cause many growers problems and botrytisis becomes of real concern when not taking leaf temps into account. A simple $15 Infrared Temp gun will do the job quite well.

If you’re going to manage VPD, you’ll also want a controller that integrates your humidification and dehumidification systems. You want your controllers set up in such a way that when the lights are off, the humidification setpoints for the dehumidifiers are different if possible. An RH of 10-15% lower at night is ideal but not required.

Paying attention to RH after the lights go out is a big concern. As temperature drop the RH increases (ergo relative humidity) Slowing the temperature drop will aid in the dehumidifiers ability to keep the humidity in range. I would recommend checking humidity from 20-40 mins after lights out to ensure RH is not spiking.

Temperature are also important to control using a temp controller that controls both heating and cooling is ideal. This could be done by controlling fans, heaters, ac etc.

If you don't have all the fancy stuff listed you can still use VPD to to make adjustments to your setup that will improve the VPD for your stage of growth.

It’s important to note that plants are their own internal humidifiers, depending on how many plants are in a room and what stage of growth they’re at. Small plants have less surface area and transpire less. Small plants in a big room will require humidity injection to keep the humidity up, whereas plants at full size don’t need as much humidity injection because they’re already transpiring at an increased rate. When you hit the final stages of growth, you may have to run dehumidifiers to take water vapor out.

Realize that at this level you are doing some serious high performance fine-tuning of your gardening operation. You could be adding a few percent to the final weight of your yield, but it’s going to take some work and you are going to need the proper equipment to measure and control your garden at this level.

The fan system is required because we know botrytis and other fungi are always waiting to pounce. Botrytis establishes itself best between 50 and 70°F, in still air having humidity above 55%RH. We especially want to avoid condensation; this means watch out for uncontrolled temperature drops between daytime and night.

You will also need some type of computer system capable of running a modern spreadsheet program. This is not rocket surgery, but you (or someone you know) will need to know how to use some basic features of a spreadsheet. This is useful to display the logged files from a data acquisition setup, as well as for calculating VPDs and other moisture quantities. Consider it the entry stakes to quantifying and visualizing the performance of your growing operation.



HOW TO CALCULATE YOUR OWN VPD:
If you don't like math your in luck here is a formula you can put into a spreadsheet to do it for you. I use excel personally.

Enter the formula on the next line into spreadsheet cell A10 (copy and paste it).

=3.386*(EXP(17.863-9621/(A7+460))-((A6/100)*EXP(17.863-9621/(A5+460))))

You will type-in 3 values into 3 other cells:

  • Cell A5: The air temperature (A5 in the formula)
  • Cell A6: The air %RH (A6 in the formula)
  • Cell A7: The leaf temperature (A7 in the formula)
Cell A10 will then give you the total VPD for that grow room condition.

Example:

Room temperature= 80°F

Room %RH= 47%

Assumed leaf temperature= 75°F

VPD= 1.34 kPa (a little too dry for best growth)

Calculating Individual Vapor Pressures

For those interested in further exploring water vapor pressure.

Enter the formula on the next line into spreadsheet cell A20 (copy and paste it).

=3.386*(A17/100)*EXP(17.863-9621/(A16+460)))

You will type-in 2 values into 2 other cells:

  • Cell A16: The air temperature (A16 in the formula)
  • Cell A17: The air %RH (A17 in the formula)
Cell A20 will then give you the water vapor pressure for that temperature and %RH combination.

Examples:

1)

Room air temperature= 80°F

Room air %RH= 47%

Water vapor pressure= 1.67 kPa

2)

Leaf temperature= 75°F

%RH of the air inside the leaf = 100%

Water vapor pressure= 3.00 kPa

These 2 examples show the “long way” to calculate the VPD given in the VPD equation section above this one: Subtract the room condition from the leaf condition to come up with the room-to-leaf water vapor pressure deficit (3.00 – 1.67 = 1.33 kPa).

Ok well that the long/short version and I hope this helps. If ya have any questions I will do my best to answer
Oof I wasn't expecting it to be super scientific haha, I'll go out a buy a few things I don't already have that you mentioned, I appreciate you guys thoe! Glad I joined this group!
 
Marcmiller32

Marcmiller32

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thanks you made this so easy to understand.
That's because RH is the saturation % of water in the air relative to temperature. So we all know warm air hold more moisture. When the lights go out and the temperature drops so does the amount of water holding capacity that the air has so the % of saturation (RH) goes up since cooler air hold less moisture. If anything I would run 2 fans for a few hrs after lights out and maybe only one during lights on.

Plants generally close thier stomata at night so a low humidity at night is not nearly as bad as low humidity during lights on.
So one thing I've been confused about, the ph of my water, I've been giving tap water. The tap water in va isn't super high, usually reads 6.5-6.8, should I stop? I heard it was a highly debated topic, and when I add nutrients to my water I need to adjust the ph right? Like I shouldn't just add them in and then water?
 
Aqua Man

Aqua Man

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So one thing I've been confused about, the ph of my water, I've been giving tap water. The tap water in va isn't super high, usually reads 6.5-6.8, should I stop? I heard it was a highly debated topic, and when I add nutrients to my water I need to adjust the ph right? Like I shouldn't just add them in and then water?
What media are you in? Ph is important but so is the stability. If in soil I wouldn't worry but others like hydro, coco, rockwool it's absolutely crucial.

Have a read here may help you understand ph a bit better. It's often misunderstood and ppl don't realize how important alkalinity is.

 

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