Vpd (vapor Pressure Deficit) What It Means To My Plants

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jumpincactus

jumpincactus

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Good morning Farmers. Just when I thought I knew just about everything about controlling our ladies environments,I stumbled upon this. So what is VPD and what does it mean to my cannabis? Read on.

As mentioned, I am new to this particular term and the associated science data connected to it so if anyone has any personal insight on if you personally use this data in managing your climates what observations have you made and more importantly how you go about getting the sweet spot indicated on the attached graphs. There is also other data out there with claims that less than ideal VPD levels will actually mimic nutrient deficiencies.

I would add that I am in no way advocating this as a must have to grow great cannabis, but I would like to know if anyone who believes in this has seen any measurable results good or bad. Initially my position was "oh great, just one more thing to stress over and control in my gurls world. However is this is solid science and has it been proven to keep our gals happier and healthier than by all means let me have it. Call me a skeptic but I do fare from Mizzourah. I look forward to those in the "know and who actually monitor and use this data for their grows.......... Enjoy.......



Vapor Pressure Deficit - The Hidden Force on your Plants
Once you understand what vapor pressure deficit is, all those environmental factors you're trying to juggle in your mind suddenly click into place and you start to think and feel like a plant. Take a few minutes to understand why VPD management is key to creating the perfect indoor growing environment! Your plants will thank you for it!
Humidity is HUGE when it comes to growing plants. An important milestone in becoming a competent and responsive grower is developing an understanding of what humidity is, how plants respond to it, and how you can manage and manipulate it

Firstly, let's make sure we're all on the same page. When we speak of the "humidity" of or in the air we are basically referring to the amount of water in the air. "In the air?" What do we mean? Well, water can only truly stay in the air when it is in gas form - aka "water vapor". We're not talking about tiny droplets of water in the air here (e.g. fog or mist.)



tomato_plant_suffering_from_high_temperatures_and_arid_conditions_low_humidity.jpg

Unsurprisingly, temperature plays a crucial role when it comes to humidity. The warmer the air, the more water vapor it can potentially hold. As the amount of water air can hold constantly changes with temperature it can be difficult to get a handle on what we need to measure. Fortunately an answer comes in the form of the concept of "Relative Humidity" (RH) - this is a measurement in terms of percentage, of the water vapor in the air compared to the total water vapor potential that the air could hold at a given temperature.


So, when we say there's a relative humidity of 50% - we mean "At this specific temperature, the air is carrying half the potential water vapor possible."

The Effect of Relative Humidity on Your Plants
RH can be easily measured using digital or analogue meters called "hygrometers." They are available for around $15 at your local indoor gardening store. But what do the readings mean for your plants?

Turns out-they mean a great deal! While many novice growers focus solely on keeping temperature in range, many take their eye off the ball as far as RH is concerned-perhaps because they don't fully understand what it is or how to manipulate it to their advantage.

Have you ever been to Florida in July? You'll know that it's not just the heat that's oppressive, it's the humidity! You feel constantly wet with sweat - the whole place feels like a sauna you can't escape from! (Sorry Floridians!)

RH has an ever more direct effect on plants. Plants need to "sweat" too - or rather, they need to transpire (release water vapor through their stomata) in order to grow.

The amount of water plants lose through transpiration is regulated, to a point, by opening and closing their stomata. However, as a general rule, the drier the air, the more plants will transpire.


Under Pressure
All gasses in the air exert a certain "pressure." The more water vapor in the air the greater the vapor pressure. What does this mean? Well, in high RH conditions (think of Florida again) there is a greater vapor pressure being exerted on plants than in low RH conditions. From a plant's perspective, high vapor pressure can be thought of as an unseen force in the air pushing on the plants from all directions. This pressure is exerted onto the leaves by the high concentration of water vapor in the air making it harder for the plant to 'push back' by losing water into the air by transpiration. This is why with high RH plants transpire less. Conversely, in environments with low RH, only a small amount of pressure is exerted on the plants' leaves, making it easy for them to lose water into the air.

What is Vapor Pressure Deficit (VPD)?
Okay, so now that you have RH firmly implanted into your conceptual map, we move on to Vapor Pressure Deficit or VPD. As implied by the word "deficit" we're talking about the difference between two things. In this case, it's the difference between the theoretical pressure exerted by water vapor held in saturated air (100% RH at a given temperature) and the pressure exerted by the water vapor that is actually held in the air being measured at the same given temperature.




vapor_pressure_deficit_explained.gif



The VPD is currently regarded of how plants really 'feel' and react to the humidity in the growing environment. From a plant's perspective the VPD is the difference between the vapor pressure inside the leaf compared to the vapor pressure of the air. If we look at it with an RH hat on; the water in the leaf and the water and air mixture leaving the stomata is (more often than not) completely saturated -100% RH. If the air outside the leaf is less than 100% RH there is potential for water vapor to enter the air because gasses and liquids like to move from areas of high concentration (in this example the leaf) into areas of lower concentration (the air). So, in terms of growing plants, the VPD can be thought of as the shortage of vapor pressure in the air compared to within the leaf itself.

Another way of thinking about VPD is the atmospheric demand for water or the 'drying power' of the air. VPD is usually measured in pressure units, most commonly millibars or kilopascals, and is essentially a combination of temperature and relative humidity in a single value. VPD values run in the opposite way to RH vales, so when RH is high VPD is low. The higher the VPD value, the greater the potential the air has for sucking moisture out of the plant.
As mentioned above, VPD provides a more accurate picture of how plants feel their environment in relation to temperature and humidity which gives us growers a better platform for environmental control. The only problem with VPD is it's difficult to determine accurately because you need to know the leaf temperature. This is quite a complex issue as leaf temperature can vary from leaf to leaf depending on many factors such as if a leaf is in direct light, partial shade or full shade. The most practical approach that most environmental control companies use to assess VPD is to take measurements of air temperature within the crop canopy. For humidity control purposes it's not necessary to measure the actual leaf VPD to within strict guidelines, what we want is to gain insight into is how the current temperature and humidity surrounding the crop is affecting the plants. A well positioned sensor measuring the air temperature and humidity close to, or just below, the crop canopy is adequate for providing a good indication of actual leaf conditions.

Managing Humidity
Managing the humidity in your indoor garden is essential to keep plants happy and transpiring at a healthy rate. Transpiration is very important for healthy plant growth because the evaporation of water vapor from the leaf into the air actively cools the leaf tissue. The temperature of a healthy transpiring leaf can be up to 2-6°C lower than a non-transpiring leaf, this may seem like a big temperature difference but to put it into perspective around 90% of a healthy plant's water uptake is transpired while only around 10% is used for growth. This shows just how important it is to try and control your plants environment to encourage healthy transpiration and therefore healthy growth.
So what should you aim to keep your humidity at? Many growers say a RH of 70% is good for vegetative growth and 50% is good for generative (fruiting /flowering) growth. This advice can be followed with some degree of success but it's not the whole story as it fails to take into account the air temperature.



humidifier_increases_relative_humidity.jpg


Photo credit: Aquaculture Hydroponics, UK.




vapor_pressure_deficit_relative_humidity_chart_small.jpg

By looking at this example we can see that at 70% RH the temperate should be between 72-79°F (22-26°C) to maintain healthy VPDs. If your growing environment runs on the warm side during summer, like many indoor growers, a RH of 75% should be maintained for temperatures between 79-84°F (26-29°C.)


The problem with running a high relative humidity when growing indoors it that fungal diseases can become an issue and carbon filters become less effective. It is commonly stated that above 60% RH the absorption efficiency drops and above 85% most carbon filters will stop working altogether. For this reason it is good practice to run your RH between 60-70% with the upper temperature limit depending on your crop's ideal VPD range, in the example it would be 64-79°F (18-26°C.)

The table also shows that if your temperature is above 72°F (22°C), 50% RH becomes critically low and should generally be avoided to minimize plant stress.
Please understand that by presenting this information we do not want you to go to your indoor gardens and run your growing environment to within strict VPD values. What's important to take from this is that VPD can help you provide a better indication of how much moisture the air wants to pull from your plants than RH can.
If you want to work out for yourself the VPD of your plants leaves you can follow the steps below:

Measure the leaf temperature and look up the vapor pressure at 100% RH on the table below.



vapor_pressure_deficit_relative_humidity_chart_small2.jpg

Measure the air temperature and relative humidity and look up the nearest vapor pressure figure on the above table.


Subtract the air vapor pressure from the leaf vapor pressure:

Example:
Leaf Temperature = 24°C (100% RH) Leaf VP: 29.8
Air Temperature = 25°C @ 60% RH Air VP: 19.0
VPD= 10.8

Humidity's Effect on Plants
Plants cope with changing humidity by adjusting the stomata on the leaves. Stomata open wider as VPD decreases (high RH) and they begin to close as VPD increases (low RH). Stomata begin to close in response to low RH to prevent excessive water loss and eventually wilting but this closure also affects the rate of photosynthesis because CO2 is absorbed through the stomata openings. Consistently low RH will often cause very slow growth or even stunting. Humidity therefore indirectly affects the rate of photosynthesis so at higher humidity levels the stomata are open allowing co2 to be absorbed.



thai_basil_leaf_curl_localized_low_humidity_stress.jpg



When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light, as shown in the photo.

For most plants, growth tends to be improved at high RH but excessive humidity can also encourage some unfavorable growth attributes. Low VPD causes low transpiration which limits the transport of minerals, particularly calcium as it moves in the transpiration stream of the plant - the xylem. If VPD is very low (95-100% RH) and the plants are unable to transpire any water into the air, pressure within the plant starts to build up. When this is coupled with a wet root zone, which creates high root pressure, it combines to create excessive pressure within the plant which can lead to water being forced out of leaves at their edges in a process called guttation. Some plants have modified stomata at their leaf edges called hydathodes which are specially adapted to allow guttation to occur. Guttation can be spotted when the edges of leaves have small water droplets on, most evident in early morning or just after the lights have come on. If you see leaves that appear burnt at the edges or have white crystalline circular deposits at the edges it could be evidence that guttation has occurred.



tomato_plants_exhibiting_guttation_excessive_humidity_levels.jpg



Most growers are well aware that with high humidity comes and increased risk of fungal diseases. Water droplets can form on leaves when water vapor condenses out of the air as temperature drops, providing the perfect breeding ground for diseases like botrytis and powdery mildew. If humidity remains high it further promotes the growth of fungal diseases. The water droplet exuded through guttation also creates the perfect environment for fungal spores to germinate inviting disease to take hold.



powdery_mildew_takes_hold_after_poor_humidity_control.jpg



Low VPD / High RH High VPD / Low RH

Mineral deficiencies Wilting
Guttation Leaf roll
Disease Stunted plants
Soft growth Leathery/crispy leaves
 
geologic

geologic

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When humidity gets too low plants will really struggle to grow. In response to high VPD plants will try to stop the excessive water loss from their leaves by trying to avoid light hitting the surface of the leaf. They do this by rolling the leaf inwards from the margins to form tube like structures in an attempt to expose less of the leaf surface to the light,
During Santa Ana Wind events,
when temps are above 100 and RH is below 5%;
my plants will turn their plate leaves sideways so as to get the least sun possible--
that's time for overhead watering...
 
jumpincactus

jumpincactus

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So based on this information, I would liken less than ideal VPD to why humans find 90 deg F and 90 % Rh humidity unbearable. With too much moisture in the air our cooling system via evaporation and cooling thru sweating is inhibited thus keeping our cooling system compromised. Same thing with plants correct???
 
midwestdensies

midwestdensies

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I dont need to read this to tell you to rock 70-85% humidity with 80 degree temps and co2 1000-1100 ppm. Ive seen the growth before my own.eyes. we run sealed with equipment to extract or add humidity so its ALWAYS perfect. That temp and humidity is used until end of.week 4 then down to 50-60% humidity and 75 degree temps. That is the secret recipe thanks @JACKMAYOFFER
 
seaslug

seaslug

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I'm a new believer in the power of VPD. I've been fighting tipburn even though ppms are at the low range in my Blumat reservoirs (going into gallon smartpots with bagged coco).

My new baby comes tomorrow, an ESSICK AIR Space Saver 821 000 evaporative humidifier:
Essick Air.jpg
WhooHoo, coming up from the new Amazon distribution center in Dupont, free Prime for 30 days!

I'm near Seattle so I didn't expect to need humidity but my (outside air exchange) flower room averages 78F and 40%rh during lights on. Lights are up to 2800W of LED wall power. I bought a decent dehumidifier but I've never plugged it in during the 14 months I've run this room.

. . . or maybe I just need more calcium, from MGRox's Ratio thread (from another source):
By contrast, several costly Ca‐deficiency disorders occur in horticulture. . . These generally arise when sufficient Ca is momentarily unavailable to developing tissues. Deficiency symptoms are observed (a) in young expanding leaves, such as in ‘tipburn’ of leafy vegetables.
 
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ShroomKing

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I'm in an unfortunate situation and cannot control humidity or run my temps lower than 82f. So I'm able to see the effects of VPD as the weather changes. I can say that Afghan hybrids are less effected by low humidity than Thai hybrids, but I guess that's a no brainer.
Great article.
Peace
20150216_101840.jpg
This RugBurnOg is most unhappy with VPD, Nothing else.
 
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fishwhistle

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I'm a new believer in the power of VPD. I've been fighting tipburn even though ppms are at the low range in my Blumat reservoirs (going into gallon smartpots with bagged coco).

My new baby comes tomorrow, an ESSICK AIR Space Saver 821 000 evaporative humidifier:
View attachment 486235
WhooHoo, coming up from the new Amazon distribution center in Dupont, free Prime for 30 days!

I'm near Seattle so I didn't expect to need humidity but my (outside air exchange) flower room averages 78F and 40%rh during lights on. Lights are up to 2800W of LED wall power. I bought a decent dehumidifier but I've never plugged it in during the 14 months I've run this room.

. . . or maybe I just need more calcium, from MGRox's Ratio thread (from another source):
By contrast, several costly Ca‐deficiency disorders occur in horticulture. . . These generally arise when sufficient Ca is momentarily unavailable to developing tissues. Deficiency symptoms are observed (a) in young expanding leaves, such as in ‘tipburn’ of leafy vegetables.

There is a thread on here somewhere that shows how to build a humidifer from mainland mart parts,Maybe @Capulator posted it i think?Maybe he will chime in and link that thread or post a couple pics.
 
seaslug

seaslug

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That would be a very different tech, the mainland mart stuff is ultrasonic. I don't have RO so I'm not willing to buy distilled water to feed a humidifier. I bought a swamp cooler.

Reportedly, another tech gives you a wet room from too-big water droplets, being a centrifigal type: http://hydrofogger.com/

My air handling, a pair of S&P 200x eight inchers (discontinued model)--the exhaust fan in the attic is wired to the high speed windings, the intake is switched to low. The Phason controller is commercial/farm duty equipment, very happy with it.
Air_0001.jpg Air_0004.jpg
Pics won't upload with correct orientation after changing with MS Office 2010?
Room size is about 1500 cubic feet.
 
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fishwhistle

fishwhistle

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very clean setup seaslug,trying to wrap my head around the essick air humidifier,what makes it different?
 
sixstring

sixstring

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take the article with a grain of salt JC,notice the plants pictured in it are mostly water loving veggies lol.lettuce for example has a much higher value when its bloated with water and nice n green. im not gunna sit in here and argue with these guys as many do pretty darn good using higher rh to grow.ill just say its not as important as some at this site might have you believe.my plants are happy and my patients love the smoke and i havent seen 50% rh in any of my rooms since oh about august.even then ill fight to keep it down around 40%.this time of year im lucky to see 25% rh but the plants chug along,my 8 wk strains still finish in 8,big girls still get big :)
i cant speak on landrace sativas as i have none growing,but all these hybrids dont mind none ;)

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mary jane is a forgiving plant,but if i had to put a number on r/h and its importance in growing weed it would be way down the list .peace
 
seaslug

seaslug

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An evaporative humidifier is like putting a wet burlap bag over a fan. I bought a replacement wick and fungicide and I plan to fill the six gallon reservoir with 0.90mS/cm tap water.

Edit to post #12: the Phason is wired to a duplex outlet and powers the fans in parallel.

Same plant as above:
IMG_0005.JPG
IMG_0020.JPG
 
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symbiote420

symbiote420

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take the article with a grain of salt JC,notice the plants pictured in it are mostly water loving veggies lol.lettuce for example has a much higher value when its bloated with water and nice n green. im not gunna sit in here and argue with these guys as many do pretty darn good using higher rh to grow.ill just say its not as important as some at this site might have you believe.my plants are happy and my patients love the smoke and i havent seen 50% rh in any of my rooms since oh about august.even then ill fight to keep it down around 40%.this time of year im lucky to see 25% rh but the plants chug along,my 8 wk strains still finish in 8,big girls still get big :)
i cant speak on landrace sativas as i have none growing,but all these hybrids dont mind none ;)

View attachment 486369 View attachment 486370 View attachment 486355 View attachment 486356 View attachment 486359 View attachment 486368 View attachment 486371 View attachment 486376 View attachment 486377

mary jane is a forgiving plant,but if i had to put a number on r/h and its importance in growing weed it would be way down the list .peace

You ain't never lied, we deal the same type of conditions bro and I had never seen PM in my room until I tried to use that VPD chart as suggested. My more sativa leaning plants which need more humidity to thrive get sprayed down with plain water a few times a day to avoid those humidity related issues mentioned. My Ghash loves higher temps and lower humidity, right now my room is around 80 - 82 degrees and the RH ranges from 25% - 33% with the lights on..... during these subzero days like we're having now when my hygrometer is reading 0% I have to spray them all down several times daily, no biggie!
 
sixstring

sixstring

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See there,and sym posts some of the dankest pics around here :)
yeah if you got all your shit on lock you can up yields a bit running that high rh but i just cant advise a hobby grower to run those numbers.its just asking for trouble imo and for what? All that water needs to be removed during the drying/curing process plus ime i get less oily goodness on my buds when i ran higher rh and co2.
not saying it dont work,just stating cannabis needs less attention to vpd than lettuce and maters :)
 
jumpincactus

jumpincactus

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No, you would liken it to a human's heart not working properly, like... a valve not closing all the way, or arterial sclerosis. VPD *is* the plant's circulation system.
As always thank you @Seamaiden for your clarification. Much appreciated!!! :D
 
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