Vapor Pressure Deficit & What It Means To You!

[Pennywise]

I'll have to look at it.

 

[Archiweedies]

I paid less than 200 for it. There are better ones I'm sure and maybe I'll upgrade at one point but right now it's doing the job I need from it

 

[Pennywise]

Ok you can run a humidifier and heater with that one?

 

[Archiweedies]

Ok you can run a humidifier and heater with that one?

Yes, but you'll have to change your tactic depending on time of year. Currently Socal is chilly ( yeah 60 is chilly for us okay ) and dry. The winds that often fuel wildfires here, cause a drastic drop in humidity.

So I have my humidifier running nonstop, but my exhaust fan is set to the "dehumidifier" outlet on my controller. So if I set my RH on the controller to 50%RH, the humidifier brings the tent up to 50%, the "dehumidifier" circuit energizes turning on my exhaust fan which lowers humidity by 5%, then de-energizes the circuit turning off the exhaust fan.

I am cycling the air in the room every 3.5 minutes roughly. When the fan comes on it runs for 1.5 minutes roughly.Hope that helps

 

[Pennywise]

Shouldn't be hard to set one up.

 

[Archiweedies]

It's not, especially if you are used to correcting out of range environmental conditions. If you already have a dehu, humidifier, heater, and an ac working for you and your grow, it just takes a little finesse with a controller to take advantage of VPD.

 

[Gazoo]

Looks like I have been hung up on the temp and what I assumed was a good temp/humidity range'

Been keeping my lights on temps at 77/80 thinking this was the best temp to grow at also trying
to keep the humidity between 45/55 in flower and trying to keep it on the low to reduce mold

BUT

Looking at the posted chart, if I wanted to run flower at 50% RH I would need temps no higher than 75 F
This I can do very easily by just lowering the temp the RH will fall into place.

Apparently using this VDP chart I can reduce the day temps to 75
75F @ 50% RH and be in the GOOD ZONE

It would actually be easier to keep everything in Check at the lower temp

 

[Archiweedies]

Looks like I have been hung up on the temp and what I assumed was a good temp/humidity range'

Been keeping my lights on temps at 77/80 thinking this was the best temp to grow at also trying
to keep the humidity between 45/55 in flower and trying to keep it on the low to reduce mold

BUT

Looking at the posted chart, if I wanted to run flower at 50% RH I would need temps no higher than 75 F
This I can do very easily by just lowering the temp the RH will fall into place.

Apparently using this VDP chart I can reduce the day temps to 75
75F @ 50% RH and be in the GOOD ZONE

It would actually be easier to keep everything in Check at the lower temp

You've seen the light!
Yes!I'm glad you see it Gazoo!

 

[Archiweedies]

Consider 20C (70F) at 35% RH. It's still in the VPD zone. I believe if we are looking to express certain traits like purpling or pink calyxes, we can maintain a low temperature, low RH, and still maintain happy stomata. I plan to experiment going forward with just this goal in mind.

 

[Gazoo]

Consider 20C (70F) at 35% RH. It's still in the VPD zone. I believe if we are looking to express certain traits like purpling or pink calyxes, we can maintain a low temperature, low RH, and still maintain happy stomata. I plan to experiment going forward with just this goal in mind.

Thanks I probably could of pulled that off this last week (SF Bay Area) without any equipment at all

 

[Gazoo]

Was not that hard at all to put things in line with that chart

Lights out I got it at 68F @ 50%RH (I can use the de-humidifier to bring this down)
Lights On 75F @ 45%RH
They are Mid to late flower.

These numbers are much easier to hold ? Maybe because its more naturally balanced ?

I like having a clearer target to aim at so I Thank you for the info hoping to put
it to good use

 

[Archiweedies]

Was not that hard at all to put things in line with that chart

Lights out I got it at 68F @ 50%RH (I can use the de-humidifier to bring this down)
Lights On 75F @ 45%RH
They are Mid to late flower.

These numbers are much easier to hold ? Maybe because its more naturally balanced ?

I like having a clearer target to aim at so I Thank you for the info hoping to put
it to good use

Sounds perfect Gazoo! By the way, I’m not sure where else to mention, it but I’m reading Dark Scotias thread with Doc growing alongside. I’m on page 50 I think of 80~ and very post you’ve made in that thread I’m always shaking my head like” this dude Gazoo’s asking the same questions I have man!” So thank you for not being afraid to ask questions, it really does add value to the subject matter for you and those silent warriors out there!

 

[Gazoo]

Sounds perfect Gazoo! By the way, I’m not sure where else to mention, it but I’m reading Dark Scotias thread with Doc growing alongside. I’m on page 50 I think of 80~ and very post you’ve made in that thread I’m always shaking my head like” this dude Gazoo’s asking the same questions I have man!” So thank you for not being afraid to ask questions, it really does add value to the subject matter for you and those silent warriors out there!

Thanks I do find it a bit challenging trying to get answers sometimes it seems like
the answers are to different questions

EDIT: Not specific to any journal it just feels like I am always on a different track

 

[Pennywise]

You guys just have enquiring minds and want answers.

 

[Archiweedies]

Thanks I do find it a bit challenging trying to get answers sometimes it seems like
the answers are to different questions

EDIT: Not specific to any journal it just feels like I am always on a different track

No I get it! You look to be on the right track from where I'm sitting Gazoo!

 

[Archiweedies]

You guys just have enquiring minds and want answers.

Yea, it's a curse sometimes. Especially cause my memory is so bad. I've probably forgotten more than I've learned at this point

 

[Pennywise]

Lol I hear ya!

 

[Slowpuffer]

VPD - Overview​

Until the last couple years, Vapor Pressure Deficit, was measured mostly by research universities and agricultural laboratories. The tech available to growers these days has enabled us to dial a grow room in very accurately with regards to heat and RH. We all know that cannabis can benefit from an environment between 70F- 85F or 20C - 30C. We also can benefit from slightly colder temps at night and this is encouraged as it promotes maximum growth during lights out and the potential for certain genetic properties of a particular chemovar to manifest such as purple or pink buds, etc. Temperature and Relative Humidity will allow you to grow cannabis, and grow it well, if the parameters are maintained. When we keep our plants in the "sweet spot" of VPD, the stomata of the leaves are manipulated in a way that allows maximum performance from our plant. One analogy is that the VPD happy plant is like an athlete, whereas a plant outside of the ideal VPD zone is just strolling through the park.

I'd like to first state that I was urged to look into this information from fellow growers @Pennywise and @InTheShed . I am sharing what I learned, in my own words, based off the research I did and then began implementing in my own grow. After dialing in the environmental numbers, my plants looked completely different in less than 24 hours. They were perkier, healthier looking, and just plain looked ready to burst with growth. All anecdotal, of course, but if you have the capability to manipulate your temps and RH, you need to try this!

In order to understand what VPD is, we need to define the variables that go into calculating the VPD. Water vapor is the gas form of water. Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature.
The air inside a grow space can hold a given amount of water vapor relative to the grow space temperature before it condenses to the liquid form of water. As temperature increases the SVP rises and as temperature falls, so does the SVP.
This is why we get dew in the morning after a cool night. The SVP is low, because the temps are low, but the Actual Vapor Pressure ( AVP) is higher than the SVP and boom! We have dew all over everything as the water vapor turned to liquid form!

Did you know that Relative Humidity or RH is actually a comparison of the amount of water in the air relative to the maximum amount the air can hold? That's why we measure it in percentages. 50% RH means that at a given temperature, the air is holding 50% of its maximum amount of moisture possible.

So if Saturation Vapor Pressure, SVP, is the maximum amount of water that the air can hold at a given temperature, and we know Relative Humidity, we can determine the Vapor Pressure Deficit.

Put simply, Vapor Pressure Deficit is the difference, or deficit, between SVP and AVP

VPD= SVP - AVP.

But Archiweedies, I dont want to math! Fear not fellow growers, you only need a couple things to start dialing in that environment so that you can take full advantage of this technique.

What you need:​

Laser Temp Gun I got mine at Harbor Freight for quite cheap. We need this to determine our leaf temps at the top of the canopy. We can't assume that ambient is the same as leaf temps. Remember, the point is to get the stomata working so we care about the leaf temps at the top of the canopy.

IMG_5580[1].JPG

• Archiweedies
• Nov 16, 2018
• vpd

Laser temp gun

Hygrometer - You should have one already! It would be nice to have one with a probe that can be placed at canopy level to get accurate RH measurements.

VPD chart This chart is courtesy of Just4Growers.com There are many online and I suggest you find one you like and print it out for your grow room. The example below is a VPD chart that assumes a Leaf Temperature Deficit of -2 degrees Celsius below ambient.The chart you will need is going to be based on how hot your lights are and what the difference is between ambient grow room temps and leaf surface temps. Ive found, with my LED COBS that they run roughly 2 degrees Celsius cooler.

Now that we have our leaf temperature and our canopy RH, we simply cross reference the chart of your choice. If we are in the red zones above we are outside of the sweet spot for VPD. Once outside of the sweet spot and your plants are not transpiring as efficiently as they could be.

If you are outside the acceptable range, simply determine if it would be easier to manipulate the RH or temperatures to get back in line.

Example: I check my hygrometer and it says 25C. I check the leaf temps they are at 23C. RH is at 37%.
This places us just outside the VPD and it is up to you to determine if adjusting your RH up or your temps down makes more sense for you. For me, and this depends on your unique environment at the time which of course changes daily, I would bump RH up to 45% if I was in late veg/preflower. In flower I would likely choose to maintain the RH and lower temps a couple degrees Celsius to bring it back into line.

Summary​

Dialing in the VPD allows us to turbocharge plant growth by allowing the plant to transpire efficiently. When the plant is transpiring more efficiently, it is drinking more nutrients from the roots. I think that is the goal of growing cannabis. Grow roots in veg and use those roots during bloom to produce flowers. By giving the plant a happy environment with the perfect VPD balance, we accelerate the plants growth across all phases of growth.

Thank you to the wonderful community here at . I hope that by presenting this information, I have in some way given back to the community that has given me so much over the last few years.

Consider 20C (70F) at 35% RH. It's still in the VPD zone. I believe if we are looking to express certain traits like purpling or pink calyxes, we can maintain a low temperature, low RH, and still maintain happy stomata. I plan to experiment going forward with just this goal in mind.

I read your article on VPD and wish you success in your experiment. Very interesting !!

 

[Archiweedies]

Thanks for reading SP. I’ve had my VPD locked in for a couple weeks and my plants are responding quite favorably. Happy Thanksgiving!

 

[Gazoo]

Thought I would give a little update on my progress getting this dialed in

One thing I learned while reading up on all this is the relationship between the outside air and
the leaf temp. From what I have read we are looking for -2C (3.6F) for the best balance
BTW This can be adjusted by raising or lowering your lights.

I like it SINCE ONCE dialed in we have effectively balanced the LIGHT DISTANCE & VPD.
Makes a no brainier on Light distance