Vapor Pressure Deficit (VPD) and Cannabis Cultivation

A deep dive into Vapor Pressure Deficit (VPD) and cannabis plant growth.
Nadia Sabeh​​, PhD, PE, LEED AP
​Owner, Founder of Dr. Greenhouse, Inc.
According to Dr. Sabeh, vapor pressure deficit (VPD) is the primary driver of evapotranspiration inside the indoor plant environment. VPD is
​a measure of the amount of water in the air versus the maximum about of water the air can hold at a given temperature (the saturated condition). VPD is measured in units of pressure, typically in kilopascals (kPa), but may also be reported as bars, milibars, inches of Hg, or other pressure units.
In general, a low VPD means the air is "humid" and a high VPD indicates the air is "dry." VPD is directly responsible for the stomatal opening of plant leaves. If VPD is too high ("dry" air), stomata will close to conserve water. If the VPD is too low ("humid" air), stomata may be fully open, but evapotranspiration will be slow since the difference in water vapor in the air versus at the leaf is very small. Both conditions can cause leaf wilting, leaf tip burn, and other crop maladies, including susceptibility to pests and molds. On the other hand, when VPD is managed correctly, plants will transpire freely, moving nutrients readily to cells and maximizing CO2 uptake and photosynthesis.
​
The correct VPD level depends on the type of crop, as well as the stage of crop for fruiting and flowering plants.
Dr. Nadia (A.K.A. “Dr. Greenhouse”) is President and Founder of Dr. Greenhouse, Inc., an agricultural and mechanical engineering firm that specializes in the design of HVAC systems for indoor plant environments. Dr. Sabeh first became interested in controlled environment agriculture (CEA) as an undergraduate, while working on a small shiitake and oyster mushroom farm in southern Idaho. She was inspired by the farmer’s way of life - the daily rhythm, the appreciation for the land, the hard work necessary to grow a crop, the friendly competition, and the respect and camaraderie between growers, helping each other to troubleshoot problems, fill orders, and make money. It was then that she decided she wanted to use her engineering know-how to help farmers control their environments. More than 20 years later, she is still on that path, helping farmers grow crops indoors, in greenhouses, and in locations that would otherwise make it impossible or impractical to do so.
This conversation was filmed at the Resource Innovation Institute (RII) Indoor Agriculture Energy Solutions conference, which brought state regulators together with light, plant, HVAC, green house, water, vertical farming experts and cannabis cultivators to talk energy and cultivation best practices. The hope is that when regulators get to learn from experts and the farmers they regulate, they will create sensible agriculture regulatory policy. Farmers (indoor, greenhouse and outdoor), on their part, will hopefully move towards ever increasing energy efficiency and reduced carbon footprints.
Learn more about Resource Innovation Institute
resourceinnovation.org/

[@dh18436]

Here's an easy way to understand what VPD is as a concept if you had trouble with this video. Relative humidity is a percentage because it's the ratio of how much moisture the air contains compared to how much it CAN hold at the current temperature. VPD is basically the other side of that coin - it's a measure of how much MORE moisture the air can hold at a given temperature.

[@nate6748]

Thanks! Super helpful

[@dampbutthole3025]

so she said a vpd of 1 is usually a good set point. but do you try to maintain 1 the entire grow? or change it for different stages?

[@LifeOnceLived]

@@dampbutthole3025seedlings .80, veg .95/1 flower 1 to 1.2

[@nate6748]

1 is generally safe all-around, but there's a range! You can go lower for clones/seedlings, like 0.6 - 1.0 kPa. Vegging plants stay happy around 0.8-1.2 kPa, and flowering plants can go higher - 1.2 - 1.5kPa. I like to envision a spectrum from hotter + wetter, to cooler + dryer, as the plants go from seedling to vegging to flowering @@dampbutthole3025

[@OmerPlayz]

@@dampbutthole3025Different VPD for different stages. You want a low VPD 0.4-0.8 during propagation/early veg so the plant focuses on root development. 0.8-1.2VPD for Early-Late veg and 1.0-.1.5 for early to late flower. She states in the video that having a higher VPD to prevent molds can actually have a ironic effect. So, a VPD of around 1* is good. Each plant is different.

[@luckycbr600rr]

Ah.. thank you very much for sharing this knowledge to every one.. i have been playing with vpd in my grow tent for a year now.. i have better understanding now..

[@stanbyme3414]

Great information on vpd,I love your speaking voice, easy to listen and compute!

[@magnumopus9631]

when i did vdp my garden went to next level

[@kaybailey2660]

This is completely gold 🙏💗🌱

[@Werdnasemajjamesandrew]

Thank you for your work doctor greenthumb

[@ryanmoore7098]

This was extremely helpful! She’s a godsend.

[@Ghade3245]

Great information very knowledgeable 🌟

[@jonathanschadenfreude9603]

spring in vermont can be interesting .........great video

[@johnkunn2499]

People like Nadia: 10/10. Next level West Coast growers: 10/10. This video as useful educational material: 4/10.

[@markwhitney5821]

Info was great thank you will use!

[@CFDISAAC]

Speaking of double whammy.. Trying to get RH down later flower, when buds are larger and denser, you need HVAC super power to get to a 1.0 vpd due to the fact you need your temps sub 68 degrees, the point at which you can get PM or bud rot if your humidity gets too high due to failure somewhere in HVAC system. Therefor, you need massive HVAC ability to remove quickly enough, the RH in the room from feeding, if you want to run for instance, a 62/63 temp at 45% RH late flower. Having RH above 65 late flower has been no good, giving us yeast and mold issues and bud rot with some. A 1.2 kPa is far easier to get while also doing away with money sucking failed tests. For instance a 75@60. The units can dehum easier, and you have a good RH to deal with. You however lose some of that shock we like to put in the plants to up our test results as well. Lots of variables..

[@laurieedeburn2449]

thanks

[@keepvibin100]

Great info 🌵🌵🌵

[@nihilityjoey]

I would really like to know how defoliating affects VPD. Especially as its a common practice said to increase yields. Leaves are important for so many things so im just wondering why this practice is said to do this.

[@samburgner8179]

Doesnt affect VPD. Plant will evaporate less overall because of less leaf area, so if the humidity drops as a result of that then the VPD would rise, but you could dehumidify less and the VPD would not change. VPD is an indicator, not a measurement. It relies on stomatal conductance, leaf temp, air temp, vapor pressure in the air. Many ways to change VPD, but defoliating is not one unless you change your environmental management too.

[@veela420]

Gold

[@JohnDoe-mc7tp]

NICE

[@allxdup1909]

Not a word about leaf surface temps, which is a major factor of VPD. Think of a triangle, where you have to keep 3 factors constant within a parameter- air temp at top of canopy Relative humidity at top of canopy leaf surface temp at top of canopy you are looking to maintain 1-2 degC cooler leaf surface(LST) than ambient. Everything can look fine in the numbers if you ignore LST, but maybe little growth.. then you check LST and find it is 5-6 degC below ambient, then you realise why LST is also on the VPD chart along with air temp & RH.

[@drgreenhouseinc]

True, I didn't talk about leaf temperature. You are correct: Technically speaking, it is the saturated vapor pressure at the leaf's temperature that we should be using to calculate VPD. But plus or minus 2 degrees doesn't have a major impact on the overall VPD (eg. 1.0 vs. 1.05 kPa isn't going to make a big difference in plant transpiration or health). Also, many people do not measure leaf surface temperature, but they do measure air temperature and humidity, so it's much easier for people to figure out VPD from these parameters. Ultimately, I'll be happy when the day comes that EVERYONE is using VPD as a plant growth metric, whether they use air or leaf temperature.

[@allxdup1909]

@@drgreenhouseinc How can you call it VPD without knowing or addressing LST. maybe .05kPa is insignificant, but you only have to look at a VPD chart to know how much difference 1degC LST makes to the VPD, because they are 2 different charts... I think I'll refer to the Dr that made the charts on this one, his opinion here is obvious. When you have seriously tried to use VPD to guide your grow, including LST, you find out just HOW hard it is to get & keep right. Keeping an ambient temp & RH stable is easy- but that aint VPD.

[@samburgner8179]

@@allxdup1909 Yeah the leaf temp is critical and even more so between the top and bottom of the canopy. VPD doesnt drive transpiration when the lights are off either, only heat flux.

[@chris432t6]

Good info although i was hoping for more numbers?

[@tomactually9285]

There are lot of free charts showing optimal RH for varied temperatures available on the web

[@lanzjeff0]

Thank you for the knowledge drop I live in canada and in the winter I have a hard time keeping rh over 25 % and definitely noticed a nutrient deficiency I think it's time to get a good humidifier and if I understand right around 50 % rh at 70 degrees Fahrenheit should shock the plants less ??

[@bennyb6071]

Eastern Ontario by late june with the right humidity pressure my plants grow 2' in a week, its crazy

[@MatanuskaHIGH]

I actually feed less when humidity is that low. Plants seem happier than normal feeding as they can’t uptake with low RH.

[@MatanuskaHIGH]

I’m in Alaska and the colder below freezing the drier it gets. So my vpd fluctuates constantly. Humidifier and demumidifier plus AC and heater are needed to keep a constant VPD. Gets expensive lol

[@allxdup1909]

@@bennyb6071 early flower I often see 3-4"/day with my indoor setup

[@natel9019]

There are charts online that tell the RH you want for the temperature your room is. It will optimise the amount of gas your plant can uptake. Which in turn will make you have more nutrient deficiency's because your plant will be able to increase the amount of photosynthetic reactions that take place thereby using more nutrients.Just a heads up.

[@MrBLawson85]

6:45 - experiencing this right now!!

[@OmerPlayz]

How’d you go

[@MrBLawson85]

looking back at it now, my problem was dry environment not enough water@@OmerPlayz

[@RONNIEJNZN]

Since were discussing VPD measures in kilopascals, would an exhaust fan creating a negative pressure in a grow tent effect the VPD measurements ? I have a 4 inch ACInfinity inline fan and on speed 3-4 , if I pull out the tent sides, they suck right in.

[@OmerPlayz]

Interesting. I would like to know this also.

[@jonathansanon157]

where do I put my humidifier and dehumidifier can I put them close to them?

[@koojc7456]

Common sense would say osmotic pressure will have to be taken into consideration as well? How do the 2 relate??

[@samburgner8179]

Common sense says VPD is not that useful since it still is only a reflection of temperature and humidity measurements

[@1ntwndrboy198]

Trying it in a room with a dryer blowing a lot of moist hot air I call it low stress training

[@catpiss32]

I grow outdoors organically,, finishing in 25 gallon Airpots. I intend on using Blumat irrigation. Is transpiration and VPD be something I should be concerned by keeping the medium moist? I understand VPD is to be controlled for indoor conditions, I just get the feeling that I may be disturbing that process that happens whether the plant is indoors or outdoors. Can someone help me with this?

[@SlapRoundTheChops]

So just add humidity?

[@mopeaceproductions218]

Thanks for this now as a person like me a beginner grower I w I uldhave to get a instrument to use the VPD CHART? Is there another way I can do this if I Need,? If I can't equipment? And again thanks for this lesson #YouDig

[@MrPablo1uk]

A thermometer and hydrometer, often packaged together as 1 item

[@surronzak8154]

Hello, could negative pressure ventilation (in a grow room or tent) influence VPD ? Could there be relations between the two we could control ?

[@samburgner8179]

Not really. VPD calcs account for atmospheric pressure so if you drop the pressure it will reduce the vapor pressure in the leaf and outside it.

[@surronzak8154]

@@samburgner8179 yes but the LST and room temp remains the same

[@samburgner8179]

@@surronzak8154 They would both cool down under lower pressure?

[@surronzak8154]

@@samburgner8179 I need to check this but I currently don’t have a powerful enough exhaust fan, you maybe right, could the leaf transpire faster thanks to lower pressure ? Resulting in faster cooling

[@samburgner8179]

@@surronzak8154 Well its all feedback controlled so the faster it cools the faster the transpiration rate will slow. You were right that the light intensity affects the LST more.

[@vpdmusic]

Hello! :D

[@IRailroad]

🤔

[@MatanuskaHIGH]

Vpd is scary for cannabis for sure. Finding the happy middle. I like my humidity 55% but temps in mid to high 70s. If I get over 65% I worry and when lights shut off the the humidity should stay below the temp as if it goes above the temp that can cause issues. Juggling the VPD take the proper equipment and can be expensive on a large scale. Humidifiers dehumidifiers a/c and heaters etc are all needed to keep your humidity in check

[@natel9019]

I set the brain to lower the temp down ten degrees or shut off half off the room 10 to 15 minutes before the lights go off. To lower the temperature of the room so less water falls out of the air when the temp goes down. When I run big light depos greenhouses that is one of the things I make sure, that the greenhouse closes slow. 10 minutes or slower and when we do it by hand we do half at a time wait a while and then finish. Otherwise it basically can "rain" on your plants

[@medotaku9360]

So, when she says "VPD is low", doesn't it mean "VP" is high?...

[@surronzak8154]

The deficit of pression is low means the plant transpire less

[@samburgner8179]

Or leaf temperature is low

[@surronzak8154]

@@samburgner8179 not necessarely , you could have to mich light heating the leafs resulting in higher LST. I try to figure this out, i’m not in a battle ;-)

[@samburgner8179]

@@surronzak8154 I was commenting on the one above you sorry. If VPD is low, it could be a high vapor pressure outside the leaf or a low leaf temperature. Speaking from a calculating standpoint

[@patrey6797]

Yes When Vapor pressure is high the Evaporation Rate is low, When Vapor pressure is low the Evaporation Rate is high. So the Deficit Value gives you an indicator of the Evaporation Rate

[@jasonsimmons4319]

Sorry but she doesnt know what she is talking about when it comes to botrytis. Seems like she thinks its just passive mold growth on stressed plants. No, if you have a susceptible variety and inoculum in your growing area, following the vpd curve wont magically protect your plants. Its generally an okay guideline (recommends around 50 percent humidity in warm conditions.) I would say go down to 40 in the last 2 weeks. Saving some on your hvac bill isnt worth losing a full crop due to testing positive to botrytis.

[@drgreenhouseinc]

Hi Jason. Thanks for your comments. I agree that Botrytis is hard to control. Humidity is just one factor in your crop's susceptibility to Botrytis. Botrytis spores are everywhere. The best way to protect your crop from Botrytis is preventing it from entering the grow space at all. If you are lucky enough to be growing in an indoor warehouse type of environment, then installing good filters (MERV 13, for example) will filter out any mold spores that are present. And the most important preventative measure? Good hygiene and clean people, including the clothes that they wear. People are the biggest vectors of all kinds of pathogens, so making sure they "gown up", wear gloves, and don't walk from room to room without taking measures to clean or cover themselves is paramount to prevention. Botrytis needs a wet surface to propagate. So avoiding condensation is also critical. Ultimately, the message I was trying to convey is that VPD affects the plant's health and stress responses. So if there is an abundance of botrytis in the room because measures weren't taken to prevent it, then yes, your plant will be more susceptible to infection if they are stressed.

[@rustinpeace7466]

If you have good air movement and proper plant spacing, 50% is no different than 75% when lights are on. The major point of ingress for botrytis is the major temperature fluctuation (and subsequent relative humidity fluctuation) when light turn off. This rapid cooling can cause condensation on the plants and creates the environment for the botrytis to propagate. This is the most important time frame for control.

[@samburgner8179]

Leaf temperature cannot be near dewpoint of the ambient air. Especially lower leaves.

[@47diodes]

"Do people know about VPD?" What kinda question is that? What amount of quality information can be derived from such an arbitrary question? Her response to that alone consumed 2.5 minutes of your video! Her response in a nutshell: Yeah sure, but lets educate 'em some more.

[@elliottbond6647]

0.50 literally.... not really