Thoughts on VPD? I'm not sold on it.

I'm back into growing after a 10 year hiatus and have been researching new industry techniques and all anybody talks about now is VPD, but I'm not sold on it.

Anybody else having better results by ignoring VPD altogether and going with temps and humidity levels that are off the recommended "VPD Chart" spectrum?

I tried to chase VPD but found it to be counterproductive and was getting slow growth rates. As soon as I went back to my tried and true methods, and started ignoring VPD, my growth rates and overall plant health improved. I was going off the "Pulse" chart, which had veg pegged at a 0.9 to 1.1 VPD, but that was resulting in underwhelming growth rates and overall plant health for me. I switched to my preferred temps and humidity as a starting point, which hovers around 1.4 VPD on the chart, and all of a sudden my plants are thriving like I'm used to.

I'm interested to hear what other people think about VPD. VPD in my mind seems like marketing since every strain I've ever grown has different needs and can't be confined to a one-size-fits-all chart. I know there's science behind it, but I have a fully controlled environment and had my VPD on point (according to the Pulse chart) and the plants were not loving it. My current veg VPD is more aligned with the recommended flowering VPD numbers and is doing much better than the recommended veg VPD.

I try to follow vpd through veg untill flowers form then drop it to 50-52% to be safe.

nomaninsf said:

I'm back into growing after a 10 year hiatus and have been researching new industry techniques and all anybody talks about now is VPD, but I'm not sold on it.

Anybody else having better results by ignoring VPD altogether and going with temps and humidity levels that are off the recommended "VPD Chart" spectrum?

I tried to chase VPD but found it to be counterproductive and was getting slow growth rates. As soon as I went back to my tried and true methods, and started ignoring VPD, my growth rates and overall plant health improved. I was going off the "Pulse" chart, which had veg pegged at a 0.9 to 1.1 VPD, but that was resulting in underwhelming growth rates and overall plant health for me. I switched to my preferred temps and humidity as a starting point, which hovers around 1.4 VPD on the chart, and all of a sudden my plants are thriving like I'm used to.

I'm interested to hear what other people think about VPD. VPD in my mind seems like marketing since every strain I've ever grown has different needs and can't be confined to a one-size-fits-all chart. I know there's science behind it, but I have a fully controlled environment and had my VPD on point (according to the Pulse chart) and the plants were not loving it. My current veg VPD is more aligned with the recommended flowering VPD numbers and is doing much better than the recommended veg VPD.

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VPD is the difference between the amount of water vapor in the air and the air's saturation point, which is the maximum amount of water vapor the air can hold at its current temperature. That's scientific fact and you see proof of it when you see moisture on a lawn or your car when it hasn't rained.

All you need is a $5 hygrometer and a free chart on the internet to track VPD, so there's really nothing to market. If you spent $500 on a Pulse Pro, then the money was spent for all the automated data it measures in one unit and having all that data in one database with real-time monitoring, as opposed to buying multiple diagnostic tools and having to manually input all your measurements in an Excel file and having no real-time access.

So what you're really saying is that you disagree with botanists on their findings of the scientific effects VPD has on the growth and health of plants.

What is your preferred temp and rh?

I think VPD is a decent guideline, stay “in the ballpark” and you’ll be fine, however, it’s only one piece of the growing puzzle. It might give you more growth, but, conversely, some of the best plants I’ve ever seen/smoked were grown outdoors, where there is no control of VPD,

nomaninsf said:

I'm back into growing after a 10 year hiatus and have been researching new industry techniques and all anybody talks about now is VPD, but I'm not sold on it.

Anybody else having better results by ignoring VPD altogether and going with temps and humidity levels that are off the recommended "VPD Chart" spectrum?

I tried to chase VPD but found it to be counterproductive and was getting slow growth rates. As soon as I went back to my tried and true methods, and started ignoring VPD, my growth rates and overall plant health improved. I was going off the "Pulse" chart, which had veg pegged at a 0.9 to 1.1 VPD, but that was resulting in underwhelming growth rates and overall plant health for me. I switched to my preferred temps and humidity as a starting point, which hovers around 1.4 VPD on the chart, and all of a sudden my plants are thriving like I'm used to.

I'm interested to hear what other people think about VPD. VPD in my mind seems like marketing since every strain I've ever grown has different needs and can't be confined to a one-size-fits-all chart. I know there's science behind it, but I have a fully controlled environment and had my VPD on point (according to the Pulse chart) and the plants were not loving it. My current veg VPD is more aligned with the recommended flowering VPD numbers and is doing much better than the recommended veg VPD.

Click to expand...

VPD is just a way of using one number to refer to different combinations of temperature and RH.

Overall, cannabis grows optimally, in veg at about 1.0, for example. The VPD chart tells you what combinations of temp and humidity will provide optimal growth for plants. If you're coming up with different numbers, it's not that VPD is "wrong". Instead it's an indicator that your grow environment is such that you're getting good results when your temperature and humidity values are quite different from "standard" values.

VPD is, at its core, a function of physics - if it's hot and/or dry, plants have to transpire more than when it's cool and/or moist. Transpiration is very important because water intake drives about 50% of nutrient uptake. Conversely, a lot of nutrient issues are resolved when VPD is brought back in range.

But if you're getting good results at 1.4 in veg, that indicates that something in your grow environment is allowing that to happen.

You might want to look into it or not but if you're consistently getting good results it might be easier just to take the win.

ec121 said:

VPD is the difference between the amount of water vapor in the air and the air's saturation point, which is the maximum amount of water vapor the air can hold at its current temperature. That's scientific fact and you see proof of it when you see moisture on a lawn or your car when it hasn't rained.

All you need is a $5 hygrometer and a free chart on the internet to track VPD, so there's really nothing to market. If you spent $500 on a Pulse Pro, then the money was spent for all the automated data it measures in one unit and having all that data in one database with real-time monitoring, as opposed to buying multiple diagnostic tools and having to manually input all your measurements in an Excel file and having no real-time access.

So what you're really saying is that you disagree with botanists on their findings of the scientific effects VPD has on the growth and health of plants.

What is your preferred temp and rh?

Click to expand...

I understand VPD and the correlation with the plant stomata, and that following the chart should provide optimal growth. However, last grow I had my veg VPD matching the attached chart and encountered slower growth compared to this round. Same genetics and root development of the cuttings prior to transplant, with no variations to nutrients or lighting.

I've always kept my veg at 82F with 55% RH, which according to the chart is a VPD of 1.4. That's what I'm running now and I'm getting faster growth rates in veg.

Last round I kept the same 82F, but at 67% RH during veg to get a VPD of 1.0. The plants were still happy and healthy, but the growth rate was slower compared to 1.4 this time around.

I'm not doubting the science, especially since it's being implemented in commercial facilities where efficiency is a must, but I'm wondering if there are variables that warrant straying from the recommended VPD, such as genetics or regular foliar feeding.

For flowering I taper down to 78F and 40% RH by the end of flower, which is a VPD of 1.7, higher than the recommended 1.2-1.5 on the chart, but my lower RH at the end of flower is to avoid botrytis, so I'm not concerned with sticking to a recommended VPD as much for flowering.

The charts have a -2F offset, which does align with my plants, according to my infrared thermometer, so I think I'm reading the chart correctly. Either way, I've got favorable results with a slightly higher than recommended VPD, so if it ain't broke, don't fix it I guess. I don't see any differences in node spacing, but then again, I wasn't anticipating varied results so I didn't take any measurements last round.

nomaninsf said:

I'm back into growing after a 10 year hiatus and have been researching new industry techniques and all anybody talks about now is VPD, but I'm not sold on it.

Anybody else having better results by ignoring VPD altogether and going with temps and humidity levels that are off the recommended "VPD Chart" spectrum?

I tried to chase VPD but found it to be counterproductive and was getting slow growth rates. As soon as I went back to my tried and true methods, and started ignoring VPD, my growth rates and overall plant health improved. I was going off the "Pulse" chart, which had veg pegged at a 0.9 to 1.1 VPD, but that was resulting in underwhelming growth rates and overall plant health for me. I switched to my preferred temps and humidity as a starting point, which hovers around 1.4 VPD on the chart, and all of a sudden my plants are thriving like I'm used to.

I'm interested to hear what other people think about VPD. VPD in my mind seems like marketing since every strain I've ever grown has different needs and can't be confined to a one-size-fits-all chart. I know there's science behind it, but I have a fully controlled environment and had my VPD on point (according to the Pulse chart) and the plants were not loving it. My current veg VPD is more aligned with the recommended flowering VPD numbers and is doing much better than the recommended veg VPD.

Click to expand...

Master the basics, that’s what will take you places, not vpd.

Mason Jar 92705 said:

Master the basics, that’s what will take you places, not vpd.

Click to expand...

Yep. I went down the YouTube rabbit hole of people boasting VPD as this holy grail of environmental parameters. Sounds good on paper, but it seems like a lot of people get hung up on it instead of sticking to what has worked for decades, before VPD became the big industry buzzword.

nomaninsf said:

I understand VPD and the correlation with the plant stomata, and that following the chart should provide optimal growth. However, last grow I had my veg VPD matching the attached chart and encountered slower growth compared to this round. Same genetics and root development of the cuttings prior to transplant, with no variations to nutrients or lighting.

I've always kept my veg at 82F with 55% RH, which according to the chart is a VPD of 1.4. That's what I'm running now and I'm getting faster growth rates in veg.

Last round I kept the same 82F, but at 67% RH during veg to get a VPD of 1.0. The plants were still happy and healthy, but the growth rate was slower compared to 1.4 this time around.

I'm not doubting the science, especially since it's being implemented in commercial facilities where efficiency is a must, but I'm wondering if there are variables that warrant straying from the recommended VPD, such as genetics or regular foliar feeding.

For flowering I taper down to 78F and 40% RH by the end of flower, which is a VPD of 1.7, higher than the recommended 1.2-1.5 on the chart, but my lower RH at the end of flower is to avoid botrytis, so I'm not concerned with sticking to a recommended VPD as much for flowering.

The charts have a -2F offset, which does align with my plants, according to my infrared thermometer, so I think I'm reading the chart correctly. Either way, I've got favorable results with a slightly higher than recommended VPD, so if it ain't broke, don't fix it I guess. I don't see any differences in node spacing, but then again, I wasn't anticipating varied results so I didn't take any measurements last round.

Click to expand...

You all forget the vpd chart is made for leaf temperature not the room temperature

No proof but i think if your hydro you want the plant to evaporate more soo she drinks more.soo chart cant be the same for soil and hydro

amneziaHaze said:

You all forget the vpd chart is made for leaf temperature not the room temperature

No proof but i think if your hydro you want the plant to evaporate more soo she drinks more.soo chart cant be the same for soil and hydro

Click to expand...

The chart compensates for that. That's what the "Leaf Temperature Adjustment: -2F" in the top right of the chart is referring to. When I checked leaf temperature with my infrared thermometer, it was exactly -2F below the room temp, so that should mean everything corresponds correctly with the chart.

As far as "evaporate more so she drinks more", that is my understanding of why I'm seeing better growth rates with a higher VPD than what's recommended on the chart. If it's drinking more, and the root system can take in more nutrients without causing toxicity, it would make sense why my higher VPD is growing faster. Regardless, if 1.4 is outperforming 1.0 in veg, I see no reason to drop down to the recommended 1.0 VPD if that's only going to slow growth, at least for these particular genetics.

Temp and relative humidity, are a couple of the limiting factors. Either one too far off, is going to give you problems.

You can get good results, staying in the ballpark, but on point is what will give the best results. Strain related of course.

The medium growed has also probably alot to do with it.
If the medium is allowed to dry between waterings and the humidity is too low and the plant transpires more,
then there might be too dry parts in the medium which would affect negatively on the nutrient/water uptake and thus growth.

But in any kind of hydro type it don't matter too much, I'd rather go closer to dry end, especially later in the parts of bloom.
Just need to adjust nutrient levels to match the transpiring levels, mainly Calcium since it's the most passive uptake that correlates with the amount of transpiring. But more transpiring -> little less calcium since more water is pulled and vice versa.

Then again, if you grow LEDs with dry air with lower end temps like 22celcius.
Then your leafs transpire quite alot and thus dropping the leaf temps to 20celcius or below and then you would start seeing slower growth.

No one said vpd could be why my colas are dying and drying out but no mold whatsoever even used a magnifying glass.

My vpd is in the red. I guess this is my first summer grow of this magnitude. I may of only one other time on a hobby one plant scale but didnt have heat issues or I simply never grew during summer.

nomaninsf said:

I understand VPD and the correlation with the plant stomata, and that following the chart should provide optimal growth. However, last grow I had my veg VPD matching the attached chart and encountered slower growth compared to this round. Same genetics and root development of the cuttings prior to transplant, with no variations to nutrients or lighting.

I've always kept my veg at 82F with 55% RH, which according to the chart is a VPD of 1.4. That's what I'm running now and I'm getting faster growth rates in veg.

Last round I kept the same 82F, but at 67% RH during veg to get a VPD of 1.0. The plants were still happy and healthy, but the growth rate was slower compared to 1.4 this time around.

I'm not doubting the science, especially since it's being implemented in commercial facilities where efficiency is a must, but I'm wondering if there are variables that warrant straying from the recommended VPD, such as genetics or regular foliar feeding.

For flowering I taper down to 78F and 40% RH by the end of flower, which is a VPD of 1.7, higher than the recommended 1.2-1.5 on the chart, but my lower RH at the end of flower is to avoid botrytis, so I'm not concerned with sticking to a recommended VPD as much for flowering.

The charts have a -2F offset, which does align with my plants, according to my infrared thermometer, so I think I'm reading the chart correctly. Either way, I've got favorable results with a slightly higher than recommended VPD, so if it ain't broke, don't fix it I guess. I don't see any differences in node spacing, but then again, I wasn't anticipating varied results so I didn't take any measurements last round.

Click to expand...

You've got the data points nailed down for VPD and it's interesting that you're getting higher growth rates at 1.4 than at 1.0. At 1.4, your plants are transpiring more so water uptake and nutrient consumption will increase. Are you seeing a change in those parameters or are you changing EC perhaps?

The chart below is handy guide for a grower. Two of the variables that you're not touching on are light (PPFD, schedule) and wind speed. For my most recent grow, I bought an anemometer and got 2 meters/second on my plants. That dropped my leaf temperature offset from -2 to -4. Have you changed how your grow is ventilated?

"I'm wondering if there are variables that warrant straying from the recommended VPD, such as genetics or regular foliar feeding."
Genetics yes but foliar feeding…hmm, perhaps in that it would temporarily slow down transpiration (because the leaves have liquid on them) and raise RH a bit.

Back to the "I'm getting faster growth rates in veg." - you're measuring or Mark IV eyeball?

Another metric is to measure flower yield and above ground mass at harvest time.

"so if it ain't broke, don't fix it I guess." - there's merit in that.

medidedicated said:

No one said vpd could be why my colas are dying and drying out but no mold whatsoever even used a magnifying glass.

My vpd is in the red. I guess this is my first summer grow of this magnitude. I may of only one other time on a hobby one plant scale but didnt have heat issues or I simply never grew during summer.

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Those plants look like they have a lot of N in them and the burned leaf tips and leaf discoloration indicate a nutrient imbalance which is typically caused by a VPD that's high.

One of the leaves is completely desiccated.

I've never paid any attention to it.
People get way too technical........it's just a plant.

Delps8 said:

You've got the data points nailed down for VPD and it's interesting that you're getting higher growth rates at 1.4 than at 1.0. At 1.4, your plants are transpiring more so water uptake and nutrient consumption will increase. Are you seeing a change in those parameters or are you changing EC perhaps?

The chart below is handy guide for a grower. Two of the variables that you're not touching on are light (PPFD, schedule) and wind speed. For my most recent grow, I bought an anemometer and got 2 meters/second on my plants. That dropped my leaf temperature offset from -2 to -4. Have you changed how your grow is ventilated?

"I'm wondering if there are variables that warrant straying from the recommended VPD, such as genetics or regular foliar feeding."
Genetics yes but foliar feeding…hmm, perhaps in that it would temporarily slow down transpiration (because the leaves have liquid on them) and raise RH a bit.

Back to the "I'm getting faster growth rates in veg." - you're measuring or Mark IV eyeball?

Another metric is to measure flower yield and above ground mass at harvest time.

"so if it ain't broke, don't fix it I guess." - there's merit in that.


View attachment 5418150

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I'm keeping all variables identical to my previous grow where I was locked on to 1.0 during veg. My EC/nutrients are the exact same down to the ml. I kept paper notes so I'd be able to do a 1:1 comparison. The only thing that changed is the RH, which in turn results in the higher 1.4 VPD value.

I guess one other change is the feeding frequency, but that's out of necessity since they drink more at 1.4 and need to be watered more often as the coco dries out faster. So even though they're getting the same nutrient ratios to the ml, the 1.4 veg is technically getting more accumulated nutrients since the feedings are more frequent. I water on an as needed basis, judging by the weight of the pots. Before each new feeding, I always let the coco get fairly dry, but not bone dry to the point where there's any wilting.

The veg solution they get is Canna Coco A + B, Rhizotonic, Roots Excelurator, CalMag, ProTekt silica, Cannazym, Thrive Alive B-1 Red (nutrient and foliar), and Great White. The Great White isn't as effective in coco as it would be in a soil blend, but it still helps the root zone to a degree. I've noticed I get finer root hairs with Rhizotonic and more hearty roots with Roots Excelurator, so I rotate between the two every other feeding. I don't combine them in the same solution.

The previous 1.0 and current 1.4 both receive the exact same foliar feeding, which is a daily foliar feeding of Thrive Alive B-1 Red. They get a foliar feed each day, right when the lights come on. The light is dimmed to 25% for the first hour to avoid burning the leaves. After an hour, there are no more droplets on the leaves acting as magnifying glasses, so it's safe to bump up the light intensity, which I dial to 50% for the first week of veg and then bump to 75% for the remainder of veg.

I'm not measuring height with a ruler, but I'm able to tell that the growth is faster because the plants reached my trellis grid sooner, which remains at the same height as last round. In other words, I know the growth is faster because with VPD at 1.0, the plants reached the trellis on day 16, whereas the current plants at 1.4 reached the trellis in 13 days. It's not a huge difference, but mildly substantial with the plants at 1.4 reaching the same size around 19% faster.

Another upside is that at 55% RH, it's not too far off from my air conditioned lung room, so my humidifier only kicks on sparingly to counter the incoming dry AC air which floats between 45-50% RH. With 1.0 VPD, my humidifier was almost always running to maintain 67% RH, so I was having to buy a lot more distilled water.

MickFoster said:

I've never paid any attention to it.
People get way too technical........it's just a plant.

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Agreed. It's not like people didn't grow good bud before vpd was a thing.

cage said:

The medium growed has also probably alot to do with it.
If the medium is allowed to dry between waterings and the humidity is too low and the plant transpires more,
then there might be too dry parts in the medium which would affect negatively on the nutrient/water uptake and thus growth.

But in any kind of hydro type it don't matter too much, I'd rather go closer to dry end, especially later in the parts of bloom.
Just need to adjust nutrient levels to match the transpiring levels, mainly Calcium since it's the most passive uptake that correlates with the amount of transpiring. But more transpiring -> little less calcium since more water is pulled and vice versa.

Then again, if you grow LEDs with dry air with lower end temps like 22celcius.
Then your leafs transpire quite alot and thus dropping the leaf temps to 20celcius or below and then you would start seeing slower growth.

Click to expand...

I have my fan blowing the heat of the light onto the leaves like a hps bulb
Cause the bulb was raising the temp of the leaves ..instead of having to raise the whole tents temperature for the leaves to be a certain temperature

Are these clones of the first round? The exact same plants as the slow growth round?