VPD (vapor pressure deficit)

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C

Cushdy

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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 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
Hi great post btw. I'm confused with the bit @
  1. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide. I thought if the rh is too high with more moisture they would close up. Preventing the extra co2 doing its job?
 
Aqua Man

Aqua Man

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Hi great post btw. I'm confused with the bit @
  1. If you have a low VPD, or high RH, plants will open their stomata and let in more carbon dioxide. I thought if the rh is too high with more moisture they would close up. Preventing the extra co2 doing its job?
Well it will slow transpiration rates but the stomata will remain open the plant will just slow transpiration... Also not good if to high.

In that post I listed the ideal KPA for stages of growth. I should have been more clear typically people run to low of RH and so the point I was making is that a higher RH will help keep stomata open ensuring ootimal CO2 exchange. But you are correct to high while the stomata remain open the transpiration is to low to optimize it also.
 
C

Cushdy

17
3
Well it will slow transpiration rates but the stomata will remain open the plant will just slow transpiration... Also not good if to high.

In that post I listed the ideal KPA for stages of growth. I should have been more clear typically people run to low of RH and so the point I was making is that a higher RH will help keep stomata open ensuring ootimal CO2 exchange. But you are correct to high while the stomata remain open the transpiration is to low to optimize it also.
Nice1 👍 thanks again for all your advice & the straightforward way you have of explaining stuff so it s easy to digest & understand.
Its lights out for the ladies right now been off for bout 5 hours. So thinking on what you've said I popped in ( under green light) to have a look @ the rh , temps & you can also see the night time ppms. Yep they def needed a tweak. Its winter here but not so cold every night. Now its saying 10°c outside but still needed to turn up the heater. But had to remind myself now I've turned up the dehuey, that machine warms up the air also! But should be ok, the heater has a thermostat so as long as it kicks in if it does get too hot we'll be sweet. Did you manage to look @ that link I added? Basically you put you #'s in room temp/ rh then theres a slider for leaf temp & a slider for what week & it gives your current reading & then a target. Is that sort of stuff reliable? Next week or soon as poss I've seen a ir gun with good reviews like you said @ no real cost. You gotta have that simple bit of kit to really catch the vpd window. Cant wait to get 1 now & learn to balance this
 
Aqua Man

Aqua Man

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Nice1 👍 thanks again for all your advice & the straightforward way you have of explaining stuff so it s easy to digest & understand.
Its lights out for the ladies right now been off for bout 5 hours. So thinking on what you've said I popped in ( under green light) to have a look @ the rh , temps & you can also see the night time ppms. Yep they def needed a tweak. Its winter here but not so cold every night. Now its saying 10°c outside but still needed to turn up the heater. But had to remind myself now I've turned up the dehuey, that machine warms up the air also! But should be ok, the heater has a thermostat so as long as it kicks in if it does get too hot we'll be sweet. Did you manage to look @ that link I added? Basically you put you #'s in room temp/ rh then theres a slider for leaf temp & a slider for what week & it gives your current reading & then a target. Is that sort of stuff reliable? Next week or soon as poss I've seen a ir gun with good reviews like you said @ no real cost. You gotta have that simple bit of kit to really catch the vpd window. Cant wait to get 1 now & learn to balance this
The link was broken for me... Idk why but I'm sure it's likely accurate. I use Excel spreadsheet to do mine with the formula I provided.
 
C

Cushdy

17
3
https://vpdcalculator.com/vpd-calculator I'm not sure why it didn't work for you? I just checked it through google seems ok its basically called vpdcalculator.com.
Yep so this dialling in the vpd is that all important vital last missing piece of the jigsaw puzzle that is learning to grow marijuana the best way 💯
 
C

Cushdy

17
3
https://vpdcalculator.com/vpd-calculator I'm not sure why it didn't work for you? I just checked it through google seems ok its basically called vpdcalculator.com.
Yep so this dialling in the vpd is that all important vital last missing piece of the jigsaw puzzle that is learning to grow marijuana the best way 💯
Not gonna lie I'm terrible @ understanding charts? That 1 you referenced I'd found just b4 your thread & did bookmark it anyway after having a look. I guess it wasnt so hard to make sense lol. But if you've got the ir gun theres no messing about. What I've got to focus on is these micro climates I've got going on. I'm doing my best to use this added injected co2 efficiently. But def not liking the variables in the environment!? Like I said earlier 3 actual hygrometers. Granted all different brands, none super expensive. The last 1 actually came free with the dehuey, so it's on the floor. Theres a built in temp & obvious current rh level on the dehuey. Then 2 more kinda opposite each other that have the probes hanging in the canopy giving in & out readings. Lol by my calculation that's 6 readings in a 8ft by 8ft x 7ft high room. But still they're all different.. .how mad considering that amount of air movement is going on?? Any ideas , I get what you said about the moisture in the felt pots. Adding to the higher ground rh level. Its amazing how fast they drink when you turn up the rh though! Then with this amount of light I've got with this co2 system. This batch of girls are truly beautiful to see flowers packing it all on! The trich development is advancing like crazy!
Also need to run this by you while I remember? Now I've been reading about this last period of extra darkness b4 harvest. Some agree, lots say about 48 hrs. Lots of stuff ppl say. Some even say well if it was outdoors it's not gonna get no extra dark & that's ok still. But I've heard it pushes extra trich? Also the guy @ my local grow shop said @ that start of the 2 weeks when I'm thinking of dropping co2 levels you should cut 2 hours straight off their time making the extra dark shock them in a way it makes a real difference on trichs? What do you know actually works about how to finish the last two weeks off if you've been using co2. Also the fact of what's it called the point of diminishing returns? Lol cant remember exactly how it goes? But like you've mentioned an over saturation of too much heat whether its directly a result of your lighting or environment as a whole. Surely although this plant & some strains possibly can accept & still reach optimum growth, with high temps. But actually how high is taking the piss outta them? When are you really pushing it all that too much? Surely if the leaves are telling you it's all good. Then whatever it's really @ doesn't matter. If you can manage to feed them in direct correlation with the added co2 plus the added temps. While maintaining ideal rh. Then that's gotta be in your vpd window?
 
Aqua Man

Aqua Man

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Not gonna lie I'm terrible @ understanding charts? That 1 you referenced I'd found just b4 your thread & did bookmark it anyway after having a look. I guess it wasnt so hard to make sense lol. But if you've got the ir gun theres no messing about. What I've got to focus on is these micro climates I've got going on. I'm doing my best to use this added injected co2 efficiently. But def not liking the variables in the environment!? Like I said earlier 3 actual hygrometers. Granted all different brands, none super expensive. The last 1 actually came free with the dehuey, so it's on the floor. Theres a built in temp & obvious current rh level on the dehuey. Then 2 more kinda opposite each other that have the probes hanging in the canopy giving in & out readings. Lol by my calculation that's 6 readings in a 8ft by 8ft x 7ft high room. But still they're all different.. .how mad considering that amount of air movement is going on?? Any ideas , I get what you said about the moisture in the felt pots. Adding to the higher ground rh level. Its amazing how fast they drink when you turn up the rh though! Then with this amount of light I've got with this co2 system. This batch of girls are truly beautiful to see flowers packing it all on! The trich development is advancing like crazy!
Also need to run this by you while I remember? Now I've been reading about this last period of extra darkness b4 harvest. Some agree, lots say about 48 hrs. Lots of stuff ppl say. Some even say well if it was outdoors it's not gonna get no extra dark & that's ok still. But I've heard it pushes extra trich? Also the guy @ my local grow shop said @ that start of the 2 weeks when I'm thinking of dropping co2 levels you should cut 2 hours straight off their time making the extra dark shock them in a way it makes a real difference on trichs? What do you know actually works about how to finish the last two weeks off if you've been using co2. Also the fact of what's it called the point of diminishing returns? Lol cant remember exactly how it goes? But like you've mentioned an over saturation of too much heat whether its directly a result of your lighting or environment as a whole. Surely although this plant & some strains possibly can accept & still reach optimum growth, with high temps. But actually how high is taking the piss outta them? When are you really pushing it all that too much? Surely if the leaves are telling you it's all good. Then whatever it's really @ doesn't matter. If you can manage to feed them in direct correlation with the added co2 plus the added temps. While maintaining ideal rh. Then that's gotta be in your vpd window?
Just out for a few drinks get back at ya in like an hour.

48 hrs. darkness is a yes. Allows the trichs to replenish after light/heat degradation and also moves starches to the roots improving flavor imo.
 
C

Cushdy

17
3
Just out for a few drinks get back at ya in like an hour.

48 hrs. darkness is a yes. Allows the trichs to replenish after light/heat degradation and also moves starches to the roots improving flavor imo.
🎯 thanks 4 the confirmation bro ✌
 
C

Cushdy

17
3
🎯 thanks 4 the confirmation bro ✌
Do you also agree about reducing ppms incrementally @ some point during flowering? Perhaps bit by bit even over the last 4 weeks? I dunno not going to lie no clue on that 1 yet? I'm sure I've read it in 1 of your threads. If not ethylene is over produced or something? Takes taste & flavour , aroma away & doesn't help in ripening?
Also what about taking 2 hours off the day cycle shut them down for longer @ the start of the last 2 weeks. Apparently that's meant to punch out more trichs coz plant def thinks its gonna die?
🎯 thanks 4 the confirmation bro ✌
 
Aqua Man

Aqua Man

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Do you also agree about reducing ppms incrementally @ some point during flowering? Perhaps bit by bit even over the last 4 weeks? I dunno not going to lie no clue on that 1 yet? I'm sure I've read it in 1 of your threads. If not ethylene is over produced or something? Takes taste & flavour , aroma away & doesn't help in ripening?
Also what about taking 2 hours off the day cycle shut them down for longer @ the start of the last 2 weeks. Apparently that's meant to punch out more trichs coz plant def thinks its gonna die?
Ethylene is if your using CO2 burner it's actually a plant hormone. Depends on the media in hydro you just need to cut ppm to about 600 second last week and 300 ppm for the last week's. A flush is for soil to accomplish the same thing. In hydro we just need to reduce the ppm. A flush with just water in soil lowers the ppm to similar levels.
 
C

Cushdy

17
3
Ethylene is if your using CO2 burner it's actually a plant hormone. Depends on the media in hydro you just need to cut ppm to about 600 second last week and 300 ppm for the last week's. A flush is for soil to accomplish the same thing. In hydro we just need to reduce the ppm. A flush with just water in soil lowers the ppm to similar levels.
Sorry this has me confused? I meant with regards to the ppm levels of co2 over the last 2 weeks or so? So your saying if it were you in a soil grow you wouldn't begin to taper off the cf food level in conjunction with a daily flush, whilst reducing the ppm level of co2 & heat down also. I thought it didn't matter whether you used a burner or compressed co2? just too much co2 over the last 2 weeks brings about too much ethylene production which in turn can affect the ripening process?
I thought the best move was (pls advise me if I've got this wrong?) @ Start of last 2 weeks was to bring it all incrementally down together? The plants food cf levels then to no food just a flush for last couple days. Whilst maintaining a gradual reduction in co2 ppm levels all the way down to zero obviously by the time they go into the 48 hrs dark phase b4 harvest. Also what do you think about what my local grow shop guy said. @ the start of the last 2 weeks, taking 2 hours off the day cycle making night time 14 would have any positive impact on extra trich production ? Is this possible? Like the extra dark @ the end?
 
AuntieKush

AuntieKush

35
18
But still they're all different.. .how mad considering that amount of air movement is going on??

That's pretty normal, i'd try to place em all in the same spot if you want to compare how similar the readings are. Rooms have little microclimates everywhere, even 1 foot apart you might get different readings.



This site has a VPD calculator that lets you take into account leaf temps, no messing with formulas, etc: https://pulsegrow.com/blog/vpd#calc

Also they have a thing for you to make your own custom charts like this one
1580145318153
 
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SupaC

SupaC

26
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Resurrecting this interesting thread, because when using the online Vpd calculators , you are presented with a Vpd for air and a Vpd for leaf.
Which one should we be paying attention too?
 
ru knuts

ru knuts

254
63
Resurrecting this interesting thread, because when using the online Vpd calculators , you are presented with a Vpd for air and a Vpd for leaf.
Which one should we be paying attention too?
Cannafused.life uses room temp, leaf temp and RH. I'm not sure if it's the experts way.
I like the fact I can punch numbers in and see what changes I need to make to dial in my grow.
Hopefully someone can be more precise and give us both a definitive answer.
 
FourthCity

FourthCity

778
143
Resurrecting this interesting thread, because when using the online Vpd calculators , you are presented with a Vpd for air and a Vpd for leaf.
Which one should we be paying attention too?
Is there some reason why you don't want to use the vpd calculator I posted? It uses the correct algorithm for vpd that Aquaman provided, not sure what calculators you are looking at. As Aqua explained in the initial post, it is necessary to look at three figures when calculating vpd, leaf temp, ambient temp, and rh, failure to accurately measure and use any of those three numbers will cause problems.
 
SupaC

SupaC

26
13
Is there some reason why you don't want to use the vpd calculator I posted? It uses the correct algorithm for vpd that Aquaman provided, not sure what calculators you are looking at. As Aqua explained in the initial post, it is necessary to look at three figures when calculating vpd, leaf temp, ambient temp, and rh, failure to accurately measure and use any of those three numbers will cause problems.
Much obliged FourthCity, I will be referencing your spreadsheet.
Just for the sake of clarity could you comment on the vpd calc from pulsegrow that I posted.
 
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