Lets talk Vapor pressure deficit (VPD) in cannabis?

[Fred Sanford]

I used to run co2 and aim for the vpd values and I struggled terribly for a couple years. Then my brother bought a tent and grew better weed than me with nothing but an exhaust fan on 24/7. After that I saif fuck vpd, idled my co2 burner and put the environmental controller on constant exhaust. Problems solved. Anyone looking for a 4 burner propane co2 genetator?

 

[coldcanna]

An easier way to think of this is that your RH should always tail your temp by 5-10 points, so for example and 80* room would perform best with 70-75% humidity, or 75* and 65%, etc etc..... For veg and flower through week 5, you can run with the 80/70 split and you will experience incredible vigor, the plants love the humidity because after all, they are an equatorial plant. For the last 3 weeks of flower when things are really stacking up you should start to dial things back. My rooms are set around 70* and 55-60%RH for the last couple weeks and I never experience botrytis. There is no reason to EVER have your humidity below 50%, it does nothing but stunt the growth. Botrytis needs 65% or better and poor air circulation before you really start to worry. Think about this- the Boveda packs are 62% and you can jar up your bud at this level for months on end.

 

[f-e]

So anything below 100% RH is a vapour pressure deficit that the plant can fill. The bigger the deficit, the faster a plant will drink.

Sound about right?

Turn a healthy plants heat up, and you need to reduce the vpd to keep water use from increasing. Unless you want them to drink more.

One of the many mechanism's that govern water use.

This temperature plotted in the charts is the air temperature at the canopy. Taken in the shade. I was looking at 28c using hids, and 28c using led's, side by side. In my mind, the hid side was drinking more, because it was receiving more heat than the led side. Same air temperature for the charts, but a lot more heat on the hid lit plants. I was in the Goldilocks zone with leds, and couldn't get them drinking enough. While the hid side boomed. I dropped RH 10% at the same 28c and they looked better, but I'm yet to check up on them properly.

I don't think hearing about this meteorologist standard of measurement is going to effect me. Trying to follow the chart caused issues though. It's not all seeing. It's not even that close. They don't even seem to match each other. The first two are 15% RH apart. My nose is more accurate than that.

 

[Mattbho]

Once you go 80/70 in veg you will shoot for that they seriously grow inches per day. Your environment is your environment but good luck vegging in the winter when baseboard heaters have it 20% rh in your apartment . I was scratching my head why wont my plants grow... 8 week to veg out a 4 inch pot lol. Add a 20$humidifier night and day.

 

[keptwalking]

Feel like I need to get this dialed in more and read into it. I know my girls aren’t drinking enough.

 

[bsgospel]

So anything below 100% RH is a vapour pressure deficit that the plant can fill. The bigger the deficit, the faster a plant will drink.

Sound about right?

Not quite. It's more of a sweet spot situation. Don't worry about the differences in the charts- they're using different units of measurement.

Let's consider the first one that spiritz opened the thread with. It's in kPa which is handy to know in itself if you do field measurements.

kPa measures pressure. Regardless of which chart you're looking at, they're basically detailing that too much pressure or too little pressure exerted on the plant incurs different rates at which transpiration can occur effectively. (Note: this is separate from evaporative transpiration.)(Note: I believe the differences between them also assume different leaf temperatures, which affect the outcome. General assumption is that leaf temperature is 2 degrees cooler than air temp.)

The higher the temperature, the more moisture % can be held in the air. Think about that. The air is capturing moisture. But it does this in a relative manner. Relatively, it holds as much at one temperature as it does another. Sorry if that's counter-intuitive.

The deficit is how much moisture is in the air and how much moisture the air can hold. And this creates an atmospheric scenario which ordinarily you wouldn't really be able to consider unless you were thinking about it- but that difference creates pressure. And as we said before, too much or too little affects how water moves through the plant.

It's also worth highlighting for the mental images here, how water moves through a plant. It does this through negative pressure. It seems counter intuitive on the surface, but light and thereby energy are making contact with the leaf- increasing the pressure- but the water nonetheless is drawn vertically up. Whereas, positive pressure would cause the moisture to flow back to the roots, like blowing into a straw. Warmth and a pseudo 'convection', if you will, are pulling the moisture towards a void in the air that can receive the moisture. And at the right temperature/pressure/moisture already contained in the air currently, it moves without any impedance without causing any condensation.

 

[Grapefruitroop]

Im running a sealed room+CO2 and I got really impressed by the growth rate of the canopy when I switched to adapt to the VPD chart...

I have a steady temp of 28 in the room and I pushed a little bit the humidity down to the 60s to get the ladies to drink more but I was wrong...they drank less.....after taking a look at the chart I upped the RH to 72% and baaaam...they drank more...they catched the "wave"..

 

[Veggia farmer]

Just started too read about it... Although noticed some of this before..

Is there any problems with too much RH% except mold and fungus? I have problems getting my RH under 70% no matter the temps and season..

 

[f-e]

Not quite. It's more of a sweet spot situation. Don't worry about the differences in the charts- they're using different units of measurement.

Let's consider the first one that spiritz opened the thread with. It's in kPa which is handy to know in itself if you do field measurements.

kPa measures pressure. Regardless of which chart you're looking at, they're basically detailing that too much pressure or too little pressure exerted on the plant incurs different rates at which transpiration can occur effectively. (Note: this is separate from evaporative transpiration.)(Note: I believe the differences between them also assume different leaf temperatures, which affect the outcome. General assumption is that leaf temperature is 2 degrees cooler than air temp.)

The higher the temperature, the more moisture % can be held in the air. Think about that. The air is capturing moisture. But it does this in a relative manner. Relatively, it holds as much at one temperature as it does another. Sorry if that's counter-intuitive.

The deficit is how much moisture is in the air and how much moisture the air can hold. And this creates an atmospheric scenario which ordinarily you wouldn't really be able to consider unless you were thinking about it- but that difference creates pressure. And as we said before, too much or too little affects how water moves through the plant.

It's also worth highlighting for the mental images here, how water moves through a plant. It does this through negative pressure. It seems counter intuitive on the surface, but light and thereby energy are making contact with the leaf- increasing the pressure- but the water nonetheless is drawn vertically up. Whereas, positive pressure would cause the moisture to flow back to the roots, like blowing into a straw. Warmth and a pseudo 'convection', if you will, are pulling the moisture towards a void in the air that can receive the moisture. And at the right temperature/pressure/moisture already contained in the air currently, it moves without any impedance without causing any condensation.

Hello. Took me a while to spot this. different units? I was shocked there for a moment.. school boy error. But it's not so? There is a 15% difference in the RH guidance at 28c, like I said?

I didn't see anything to move my knowledge along in your post.

I'm looking at Grapefruittoops post with interest. Perhaps you could better explain what's happened for him. I believe his lower RH was causing the stomata to close so much, there was a respiratory restriction. Slowing growth. With the RH increase, the stomata opened further. The primary reason would appear to be opening to attain the same moisture loss. The same opening increases co2 intake, speeding growth. The plant is stepping up all activity.

Grapefruiytroops, do you know your water usage? Mine are using 6L a day, with a further 1.5L going to waste. Per meter. 12/12 LED's 25c canopy 23c pots. 50~55% RH. 600ppfd. Atmospheric co2 but much of the air is recirculated. I can stick a meter in and measure, if I must rise to the occasion 3 fertigations per day to keep an ec of 1.6 at the roots. Lots of numbers there for you to see what I'm doing

 

[djonkoman]

related to the botrytis fears on the previous page, I think 70% rh should still be fine. I agree eventually you'd have to compromise between most optimal vpd for growth vs. botrytis.

I do have to say I grow outdoor only. so I don't have any experience with botrytis indoor. plenty of experience with it outdoor though.

I also did my thesis on a botrytis species(not cinerea, another one, broadly similar though). the spores require a high rh% to germinate, for desease assays(detached plant parts you put in a box and put droplets of spore suspension on it) we used boxes with a layer of standing water or wet paper so humdity is close to 100%. I think I'd start worrying around 80-90%, not 70%. outdoors around here it's also 80% or above for plenty of time, 2nd half flower it's regularly not below 80%(higher, up to 100%, during the nights).

however, plants are more susceptible to botrytis under low light, and seeing that indoor lights never reach the intensity of the sun it might be I'm wrong and the risk-area regarding RH% is lower when growing indoor compared to outdoor.

 

[siftedunity]

It helps tremendously. I prefer this chart. Stay in the gold range.


https://en.wikipedia.org/wiki/Vapour-pressure_deficit

[URL=https://www.icmag.com/ic/picture.php?albumid=51120&pictureid=1650202&thumb=1]View Image[/url]

this is the chart that we need to be following

 

[siftedunity]

this chart......

 

[I'mback]

I prefer this one. Same values but IMHO makes things clearer for newbies like me.

 

[moses wellfleet]

It all depends on how dialed your room is and wether or not you are able to avoid a dew point situation, moisture condensing all over the place, when lights go out and temps drop drastically. That’s the theory anyhow.

There are some old time threads on here where massive yields were achieved maintaining lights on RH of 85%.

Wouldn’t like to try it myself. Have done in the past with success in terms of yield. Below 60% is a safer area to stay in if there is any chance of dew point being reached @ 70%. Massive yields are a throwback to the nineties anyhow. Nowadays the focus is on quality!

 

[I'mback]

It all depends on how dialed your room is and wether or not you are able to avoid a dew point situation, moisture condensing all over the place, when lights go out and temps drop drastically. That’s the theory anyhow.

There are some old time threads on here where massive yields were achieved maintaining lights on RH of 85%.

Wouldn’t like to try it myself. Have done in the past with success in terms of yield. Below 60% is a safer area to stay in if there is any chance of dew point being reached @ 70%. Massive yields are a throwback to the nineties anyhow. Nowadays the focus is on quality!

You said it Moses

 

[Ibechillin]

Room temp should be 84F under LED to match the transpiration rates and leaf surface temperature under HID @ 77F. No way to stay in VPD range with LED at 84F unless you want to flower at 75% RH.

It all depends on how dialed your room is and wether or not you are able to avoid a dew point situation, moisture condensing all over the place, when lights go out and temps drop drastically. That’s the theory anyhow.

There are some old time threads on here where massive yields were achieved maintaining lights on RH of 85%.

Some of you may remember me from a ‘private’ tree growing site a few years back. I’m new to this site and am here for a very specific reason. Meanwhile, in the hope of getting my ban lifted on PM’s and posting in general I’m offering a large summary of my growing knowledge. It’s not comprehensive nor do I intend to stir any crap of other peoples styles. This is just what I believe to me true & works for me.

Below are about 15 pages of summary from some 100 pages of notes from a PhD consultant I hired a few years ago. I hired this person as I’d gotten tired of only marginal success growing MJ. There is much good information out there. However, books, journals, and def the web are full of bad information too. I’ve done my best to organize this and have only included what I think important.

Optimum humidity range is 70 - 85% for plant growth. Disease risk starts at around 88%. The humidity level range stays the same throughout the plants' life, expect in the cloning stage when a higher humidity levels prevent desiccation and excess water lost from the leaves at a time when there is no or little root system to take up water. So long as the humidity is below 85% (preferably 80% or below) during flowering this reduces the risk of disease infection – also good air movement up and under the plants during this time also helps prevent areas of higher humidity that might form. If you can handle the amount of water usage by the plants, then humidity in the lower end of the range 70 - 75% is the best level to run. Yes, I realize these ranges are way higher than most MJ growers run. I run in the 70% realm.

Too low RH is stressful on plants - if the air gets to dry, they will shut up the stomata in an attempt to slow the rate of water loss from their leaves and this slows photosynthesis which slows growth etc. You can't force a plant to increase transpiration too much, they will just shut up the stomata to prevent a high rate of water loss as they can only draw water up through the xylem from the roots at a certain rate. If the transport of water up the plant can't keep up with transpiration from the leaf, the stomata shut and that's why we have set points for RH around 60% minimum.

 

[Douglas.Curtis]

Leaf surface temperature is more important than VPD when growing under LED IMO, cant have both.

Thank you for this. I'm switching to LED now and this is useful.

FYI, none of the VPD charts online (that I've seen) show my flowering values. I'm almost 20% drier than the charts list for each temp. I believe it's one of the key reasons why I get drastically different results than everyone else, with the same clone they're running.

 

[moses wellfleet]

There is a thread called ‘coco trees’ or something similar by a guy called DJM. VPD is discussed in detail in that thread. DJM has a way of explaining things and there is photographic evidence.

 

[Ibechillin]

Post #43 from coco trees.

im a firm believer in VPD, its a game changer in any room..i wont try to convince people on it, so please no one come in and try to tell me im crazy, as I assure you this is not my first rodeo...I run 84f and 75% rh through the whole grow, 70f- 74f and 65-70% at night...its hot and wet..glasses fog up when you enter...only the last week of flush will I drop it to 75f and 65rh ...during veg I run it even higher around 86-88 and 80%rh

I don't hold onto veg nutes through stretch unless Im either running a haze that stretches 3x + or im running 6/9...unless its a very stretchy cut, it can hurt yield

hope that helps

 

[moses wellfleet]

Hey man thanx for digging up those posts. Will rep you when I have some again.

It would be interesting to know if DJM still grows within those VPD parameters. He is on Instagram these days. @thelonetree_