Rocket Soul
Well-Known Member
Id say the higher the intensity is the more important it is to get vpd and transpiration right. I find it more tricky to get right in veg.
Is VPD more important than light intensity for growth?
- Thread starter SmokeyBear11
- Start date
CloudHidden
Well-Known Member
I'd say they go hand in hand, but having enough light intensity is primary. My understanding is having the VPD in the optimal range ensures higher transpiration rates hence more growth. If you think about it, no one is maintaining optimal VPD outdoors and plants grow just fine. They might not be growing / yielding at their maximum possible genetic potential, but they're growing and reproducing nonetheless.
Richard Drysift
Well-Known Member
This makes a lot of sense. It’s hard enough just to grow your own weed; running out of bud is always my main concern. If you have decent lighting and ventilation everything else should kind of fall into place.
My bloom room RH is typically 45-55% with temps ranging from 77 to 84 degF....the VPD chart lists the “leaf temp“ at 79 deg and optimal RH is 77%; that’s ridiculous. I understand why ppl are using misters or what have you to increase RH but this is a recipe for mold; especially if you grow monster size buds like I do. Would not even try to dial this in and neither should anyone growing in a tent at home. Seems to me that VPD is a guideline pertaining for large commercial grows and possibly even unrelated to cannabis. I can see easily hitting recommended RH in a greenhouse.
Blossom21
Active Member
What chart are you looking at? Because that's just spreading some wrong ass information.
With 80F average room temp your humidity should be between 40-60% based on the leaf temperature, usually the leafs are cooler with airflow so with 80F you would be fine between 40-50% RH in mid to late flower. Of course you want more more humidity in the veg and the "optimal VPD range" goes all the way from seedlings to late flower, lowering your humidity towards the end.
So you need less humidity with a cooler room, for example average 75F room temp would mean you should aim between 35-55% RH based on the leaf temps (again referencing mid to late flowering phase). You can measure leaf temps with a infrared thermometer, they aren't even that expensive if you wanna buy one, could be useful/fun for measuring all kind of temperatures, but aren't even necessary unless you wanna be fancy like that
Last edited:
ComfortCreator
Well-Known Member
VPD is a range that should be maintained. There may be an "ideal" figure, but it is a range to help us understand when the stomata are closed from too much or little deficit.
With that said, we don't grow cannabis plants naturally. We grow them with intentions of growing big dense buds that the plant would likely not grow without the special efforts we make.
Because we push the plants hard to their potential, we sacrifice perfect vpd for the relative safety of lower humidity. Whatever we may lose going down to 50% and below in flower, the plants adaptability make the trade off worth it. The plant can adjust, just like plants can grow in extremely low humidity i.e. outdoors in AZ.
Sometimes we see someone who does it all, and targets vpd to the end, running 70% to the end. Thats hardcore, serious focus. It may be worth it too...but almost nobody knows what that difference would be...i don't.
With that said, we don't grow cannabis plants naturally. We grow them with intentions of growing big dense buds that the plant would likely not grow without the special efforts we make.
Because we push the plants hard to their potential, we sacrifice perfect vpd for the relative safety of lower humidity. Whatever we may lose going down to 50% and below in flower, the plants adaptability make the trade off worth it. The plant can adjust, just like plants can grow in extremely low humidity i.e. outdoors in AZ.
Sometimes we see someone who does it all, and targets vpd to the end, running 70% to the end. Thats hardcore, serious focus. It may be worth it too...but almost nobody knows what that difference would be...i don't.
Well he certainly isn't completely wrong. At his maximum lights-on parameters he gave in his post above, he's pushing his girls at a VPD of about 1.85 kPa which is above the 1.5 recommended max for flower. So he can either raise humidity to 70% and keep his temp constant, or lower his temperature another 10 degrees. I honestly don't know what would happen because I've never purposely kept my tent at 70% humidity in flower before.
Blossom21
Active Member
Based on his "readings" he could be anywhere between 1.2 - 2.2 kPa, so it's hard to nail it down. And the recommended range would be between 1.0-1.6 kPA based on the phase his plants are in the bloom. But even at 84F and the same leaf temps he shouldn't need to go above 60-65% RH, unless i'm just too stoned atm to look at the charts. But would be good to know the leaf temps to get accurate readings, it's a little bit a guessing game without them.
But 79F temps with "optimal 77% RH" would be just insane at any point, wouldn't it.
Last edited:
Yeah, according to this chart that's definitely on the low side and outside the recommended range too. lol
Hey, I did say "not completely" now didn't I?
Anyway, I think the whole thing about VPD is a little bit overblown. You can definitely grow weed above 1.5 perfectly fine. As far as below the recommended range (under 1.0 kPh), I don't even know growing colas in an indoor tent develop mold because it's never happened to me even when I had an exhaust fan break. Maybe it's more of an outdoor thing. I'll let someone else try it first though. lol
Last edited:
Kushash
Well-Known Member
Of course he's not wrong, he's talking from experience not charts and science. I think he's spot on.
Considering "science" represents the body of knowledge that humankind has accumulated about the natural world around us, I think it's okay if we talk from that position too now and then.
Anecdotal experience does often prove to have validity, because it leads to a hypothesis that is then tested and repeated by others. It becomes "science and charts" and officially a part of our collective understanding of the world.
Kushash
Well-Known Member
I don't give love often but I love the responce.Considering "science" represents the body of knowledge that humankind has accumulated about the natural world around us, I think it's okay if we talk from that position too now and then.
Anecdotal experience does often prove to have validity, because it leads to a hypothesis that is then tested and repeated by others. It becomes "science and charts" and officially a part of our collective understanding of the world.
You also appear spot on, I just did not appreciate someone who probably doesn't grow at the skill level of Richard referring to him as an ass for sharing his opinion.
I would say to @Richard Drysift it is possible IMHO VPD for HPS is less important than it is for LED and also it may be easier to dial VPD in when growing in an expensive sealed environment vs the situations most of us are growing in.
Well he actually said he was "just spreading some wrong ass information" which is not exactly the same thing as calling him an ass. This isn't Reddit after all. Lol
I go agree it was a harsh way to call someone out over a little exaggeration of what an "ideal" VPD would be and not taking leaf temperature into account. But leaf temperature of course is influenced by air circulation, so it is an important metric. You can get nutrient overloads just by blowing your plants too hard with air (seriously). That's why leaf temperature is a thing and @Blossom21 does have a point there too.
But I think our plants are way more adaptable than we give them credit for, and sometimes we tend to overthink it to the extreme. VPD is one of those things IMO. I really don't bother calculating VPD now in my own grow, but I know it's lower than recommended for veg. But they pray to the lights, and look healthy without any burnt leaf-tips or an curling or whatever. So I figure they're okay. Anyway, I'm not growing for absolute maximum biomass production, it's just a hobby now.
Blossom21
Active Member
Who referred Richard "as an ass" and where? That i would like to know too. IF you're talking about ME, i only talked about the source he got the information from, because it can't be correct with his growing conditions, so you have understood something very wrong in that case. I don't even know him and you don't know me, i'm just trying to share knowledge i've been reading on VPD and can calculate it too including LVPD (leaf vapor pressure deficit), which gives you the kPa you want because plants are usually couple fahrenheit cooler than the surrounding area due to transpiring. Peace out
Blossom21
Active Member
Props to my boy NorthWood too, seems like you know your stuff. And yeah, sometimes it can go to little bit extremes. Some of us are just chasing to minmax every possible thing at the same tame, and at some point you find yourself micromanaging every single thing. Then some just give up on everything and focus on the basics again without anything fancy, because at that point growing just comes from your backbone and it's easy to grow very good bud just being a hobby grower or something else
Rocket Soul
Well-Known Member
If were getting back to ops question: the whole idea of vpd is to maximize transpiration, or at least to get it to the point where its able to support the level of photosyntesis desired. in many comercial settings you try for 600ppfd even (40000lux iirc) and dialing in environment/vpd.
Remember vpd for low/no heat lights (leds) and IR heavy lights (hps) is not the same. Also remember you need a certain heating of the plant to maintain its metabolism rate in high intensity scenarios.
Another thing to take into account is light quality; amount and nm of blue and uv is also responsible for opening stomata which will also affect transpiration, a high blue light will better in part for this: its able to push the stomata open in low light intensity situations. People sometimes miss out on this with leds: in veg we want to give less total light (DLI) as the plants is not yet able to take very much. But the actual opening of the stomata is governed by ppfd of blue light (with certain nm being weighed more). So if your running low intensity with little blue then it can get very difficult to get the plant to transpire without giving it too much light for 18 hours.
So maybe i should revise my original statement: vpd is extra important when you are pushing the limits of DLI of what your plants can take, both in terms of long hours and high intensity. But if you go little by little with this sooner or later you get to a point where the plant is able to take massive loads of ppfd, this is when to really push it. Point is that you need to let the plant show you. Vpd in pure numbers are a bit pointless if you dont consider plant response. Point is: in a grow situation where you can deduce or feel that theres an environment related problem making the plants sag and not transpire, vpd will point the direction of where to take the environment, not an exact value. You slowly take it towards that direction.
Another layer of the oniion is CO2: high CO2 levels tend to close the stomata so you may need to compensate for this. Also, high CO2 levels + high intensity needs a few extra degrees of temps.
In the end the question is not is intensity or vpd more important; having reached basic levels of intensity (flower 600ppfd) most important is to understand how environmental factors, ppm in your feed, light quality all comes together and to be able to see and spot the signs of everything working well.
This is my fav vpd resource, it lets you set leaf temps as + or - degrees oof lleaf temps to account for different types of lighting. Even sso, it only indicates directions not absolute numbers imo.
Remember vpd for low/no heat lights (leds) and IR heavy lights (hps) is not the same. Also remember you need a certain heating of the plant to maintain its metabolism rate in high intensity scenarios.
Another thing to take into account is light quality; amount and nm of blue and uv is also responsible for opening stomata which will also affect transpiration, a high blue light will better in part for this: its able to push the stomata open in low light intensity situations. People sometimes miss out on this with leds: in veg we want to give less total light (DLI) as the plants is not yet able to take very much. But the actual opening of the stomata is governed by ppfd of blue light (with certain nm being weighed more). So if your running low intensity with little blue then it can get very difficult to get the plant to transpire without giving it too much light for 18 hours.
So maybe i should revise my original statement: vpd is extra important when you are pushing the limits of DLI of what your plants can take, both in terms of long hours and high intensity. But if you go little by little with this sooner or later you get to a point where the plant is able to take massive loads of ppfd, this is when to really push it. Point is that you need to let the plant show you. Vpd in pure numbers are a bit pointless if you dont consider plant response. Point is: in a grow situation where you can deduce or feel that theres an environment related problem making the plants sag and not transpire, vpd will point the direction of where to take the environment, not an exact value. You slowly take it towards that direction.
Another layer of the oniion is CO2: high CO2 levels tend to close the stomata so you may need to compensate for this. Also, high CO2 levels + high intensity needs a few extra degrees of temps.
In the end the question is not is intensity or vpd more important; having reached basic levels of intensity (flower 600ppfd) most important is to understand how environmental factors, ppm in your feed, light quality all comes together and to be able to see and spot the signs of everything working well.
This is my fav vpd resource, it lets you set leaf temps as + or - degrees oof lleaf temps to account for different types of lighting. Even sso, it only indicates directions not absolute numbers imo.
Keesje
Well-Known Member
I am still looking for a scientific source of the VPD numbers for cannabis that you see around on the internet.
There are studies done on several plants, like this one for example.
Ideal VPD-numbers for many plants in greenhouses are lower then the numbers you see in all the charts about cannabis.
It is common in the cannabis community to just copy what someone once wrote, and call it 'science'.
Cannabis is not a special plant (in a biological way)
So why would the numbers be different from the majority of other cultivated plants?
I am not saying the numbers are wrong, but it would be so nice to see a scientific source and not some anecdotical evidence.
There are studies done on several plants, like this one for example.
Ideal VPD-numbers for many plants in greenhouses are lower then the numbers you see in all the charts about cannabis.
It is common in the cannabis community to just copy what someone once wrote, and call it 'science'.
Cannabis is not a special plant (in a biological way)
So why would the numbers be different from the majority of other cultivated plants?
I am not saying the numbers are wrong, but it would be so nice to see a scientific source and not some anecdotical evidence.
Rocket Soul
Well-Known Member
It may have to do with exhausting a tent and negative pressure, this will change vpd aswell.
Keesje
Well-Known Member
There are lots of circumstances that can influence your VPD.
But the numbers... how did they come up with them?
With cannabis there is also another important factor: Although bud rot en mildew are not caused by high RH, it sure can speed it up.
So although a lower VPD would be better for growth and yield, there are many growers who prefer a higher VPD to prevent disease.
Very legitimate argument to keep the RH low.
In the discussion about VPD (not necessarily in this thread) it would also be nice if people understand a little more about RH.
Because often people indicate that their humidity is low, but they do not indicate at what temperature.
I also wonder whether people actually measure their leaf temperature.
But the numbers... how did they come up with them?
With cannabis there is also another important factor: Although bud rot en mildew are not caused by high RH, it sure can speed it up.
So although a lower VPD would be better for growth and yield, there are many growers who prefer a higher VPD to prevent disease.
Very legitimate argument to keep the RH low.
In the discussion about VPD (not necessarily in this thread) it would also be nice if people understand a little more about RH.
Because often people indicate that their humidity is low, but they do not indicate at what temperature.
I also wonder whether people actually measure their leaf temperature.
CloudHidden
Well-Known Member
I have fairly limited means to control my environment. So I try to keep things in the ballpark, but don't stress about it. I don't use charts.
I use this formula: =3.386*(EXP(17.863-9621/(leaf temperature +460))-((%RH /100)*EXP(17.863-9621/(air temperature +460))))
And these ranges:
Seedling/clone 0.4-0.8
Veg 0.8-1.1
Early flower 1-1.4
Late flower 1.3-1.5
.Smoke
Well-Known Member
I'll throw in my 2 cents and probably piss some people off in the process.
I'm a newish grower (1.5yrs). From what I've seen across the interweb, 80-90% of home growers probably shouldn't even know or worry about what the definition is for VPD.
Insufficient lighting is 9 out of 10 times why people are getting bad harvests.
Whatever LED manufacturer it was that managed to convince people you can grow cannabis and get good yields using less wattage than most people's living room lighting, really pulled the wool over people's eyes.
It kills me to see people spend 3+ months of their lives growing beautiful looking plants, only to realize from another angle, that "beautiful plant" produced a whopping 2-4oz.
Worrying about VPD when a 4 pack of incandescent light bulbs can out power your "grow light", is like worrying about whether or not you need to put a new set of racing tires on your Ford Pinto...
Lights>VPD
Get #1 right before worrying about 2, 3, 4 etc...
I'm a newish grower (1.5yrs). From what I've seen across the interweb, 80-90% of home growers probably shouldn't even know or worry about what the definition is for VPD.
Insufficient lighting is 9 out of 10 times why people are getting bad harvests.
Whatever LED manufacturer it was that managed to convince people you can grow cannabis and get good yields using less wattage than most people's living room lighting, really pulled the wool over people's eyes.
It kills me to see people spend 3+ months of their lives growing beautiful looking plants, only to realize from another angle, that "beautiful plant" produced a whopping 2-4oz.
Worrying about VPD when a 4 pack of incandescent light bulbs can out power your "grow light", is like worrying about whether or not you need to put a new set of racing tires on your Ford Pinto...
Lights>VPD
Get #1 right before worrying about 2, 3, 4 etc...
Last edited: