VPD and plants response.

Has anyone else tried to dial in their leaf VPD and notice their plants just won’t allow it?
over the past couple days I’ve noticed the more I inch my humidity down the lower the leaf temperature becomes. I understand why this occurs, but it also pushes me further away from proper levels.
Plants are healthy and I’m not going to ruin a grow chasing a number but I would like to know if other people have had similar scenarios?
Tent is currently setting at 82F with 58% humidity under LEDs with a 68.1F leaf temperature, the medium temp is 72.1.
Iv noticed the warmer temps appear to be favorable to this grow but not the lower humidity, seems to have the healthiest looking leaves when at 70% or just above.
This is my first grow after 8-9 years.

keep it real. said:

Has anyone else tried to dial in their leaf VPD and notice their plants just won’t allow it?
over the past couple days I’ve noticed the more I inch my humidity down the lower the leaf temperature becomes. I understand why this occurs, but it also pushes me further away from proper levels.
Plants are healthy and I’m not going to ruin a grow chasing a number but I would like to know if other people have had similar scenarios?
Tent is currently setting at 82F with 58% humidity under LEDs with a 68.1F leaf temperature, the medium temp is 72.1.
Iv noticed the warmer temps appear to be favorable to this grow but not the lower humidity, seems to have the healthiest looking leaves when at 70% or just above.
This is my first grow after 8-9 years.

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I doubt your leaf temps are 14 degrees less than the ambient air.

Regardless I don't chase VPD. It's not needed for a good grow. I deal with what I have.

I’m not going to worry about chasing it either, everything seems good but its weird that the leaf temperature drops that much.. when my humidity is up near 70% it’s only a couple degrees different.

keep it real. said:

I’m not going to worry about chasing it either, everything seems good but its weird that the leaf temperature drops that much.. when my humidity is up near 70% it’s only a couple degrees different.

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I'm going to guess that it doesn't really drop that much. More than likely a measurement issue.

keep it real. said:

I’m not going to worry about chasing it either, everything seems good but its weird that the leaf temperature drops that much.. when my humidity is up near 70% it’s only a couple degrees different.

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More transpiration, more drastic the temp difference would be. Kinda like how swamp coolers work.

I don’t look at those charts but ever since I switched back to hid my plants immidiately looked better I assume from the radiant heat from the lights they have no issues now with low ambient temps and low humidity

PadawanWarrior said:

More transpiration, more drastic the temp difference would be. Kinda like how swamp coolers work.

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I also thought a measurement issue but every leaf is with in a point or two. And when I get those low measurements the leaves just don’t look at healthy. I’m just going to keep doing what I’m doing and not chase a number, read the plant and keep moving.
But If anyone else has experienced this please chime in and let me know how you managed.

As mentioned above, it's physically impossible for the leaf temp to be 14°F lower than ambient.
The leaf simply cant transpire that fast
~3-5°F lower than ambient is typical under LED lighting

I don't chase the number but I do focus on it fairly strongly. I try hard to keep it in a general range close to 1.0 for most of veg and step it up in flower to 1.2-1.4 usually. I care more about how the plants are responding and watch them closely but since I started watching my vpd my plants seem to be healthier and grow faster. I give up on it about week 5 or 6 in flower and worry more about mold and bud rot and lower my humidity to 40-45%.

Billy the Mountain said:

As mentioned above, it's physically impossible for the leaf temp to be 14°F lower than ambient.
The leaf simply cant transpire that fast
~3-5°F lower than ambient is typical under LED lighting

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Are you sure? @PadawanWarrior says it's possible with more transpiration.

My VPD is usually around 2.3-3.3. I've got a infrared temp gun too that I need to test. I should check it out tonight if I get motivated.

twentyeight.threefive said:

Are you sure? @PadawanWarrior says it's possible with more transpiration.

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Never said it could be a 14 degree difference. Just saying that based off logic, a higher VPD would have a bigger temperature difference than a lower VPD.

Billy the Mountain said:

As mentioned above, it's physically impossible for the leaf temp to be 14°F lower than ambient.
The leaf simply cant transpire that fast
~3-5°F lower than ambient is typical under LED lighting

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Agree. I'm running HLG LEDs and my ambient temps are 82-85F during lights on and my LST is -2f to 2.5. Humidity during lights on is 58% and it raises to 62 when it's dark.

My VPD remains in the .9 to 1.4 range.

PadawanWarrior said:

My VPD is usually around 2.3-3.3. I've got a infrared temp gun too that I need to test. I should check it out tonight if I get motivated.



Never said it could be a 14 degree difference. Just saying that based off logic, a higher VPD would have a bigger temperature difference than a lower VPD.

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That's crazy, I'd struggle to grow in an environment like that for sure. I'm sure you get used to it, but damn. I know vpd isn't necessary to grow but it seems like it's helping me up my results as far as grams per watt efficiency and overall plant health. It's probably because I'm just focused on the overall environment more.

FirstCavApache64 said:

That's crazy, I'd struggle to grow in an environment like that for sure. I'm sure you get used to it, but damn. I know vpd isn't necessary to grow but it seems like it's helping me up my results as far as grams per watt efficiency and overall plant health. It's probably because I'm just focused on the overall environment more.

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It's definitely a little easier with proper VPD. At my VPD nutrient burn is really easy to get. I'm in CO and have one fan that's exhausting air outside constantly. I could put it on a controller, but I haven't yet, even though I've got one here. Cloning is also more difficult.

Before I started exhausting outside I needed to run a dehumidifier, especially in flower.

VPD is a constant battle of dehumidification, humidification, cooling and heating. Personally I just try to keep my rooms at the following.

LED VEG room:
80 °F : 55% RH day
70 °F : 50% RH night

HPS BLOOM room:
78 °F : 50% RH day
68 °F : 45% RH night

In the last 3 years of playing with temps and humidity this is the sweet spot for most plants with my setup.

LED & HID are going to be different because of Leaf Surface Temperature. Play around with it and find out what works don't let an APP or hype lead you away from experimentation.

I appreciate everyone’s input and struggles and work around for it. Makes me feel better. I’ll keep an eye on it and the plants and keep them happy.
I could definitely imagine how easy burning could be with 3.3.. you’d have to cut your nutes in a 1/3?

calvin.m16 said:

VPD is a constant battle of dehumidification, humidification, cooling and heating. Personally I just try to keep my rooms at the following.

LED VEG room:
80 °F : 55% RH day
70 °F : 50% RH night

HPS BLOOM room:
78 °F : 50% RH day
68 °F : 45% RH night

In the last 3 years of playing with temps and humidity this is the sweet spot for most plants with my setup.

LED & HID are going to be different because of Leaf Surface Temperature. Play around with it and find out what works don't let an APP or hype lead you away from experimentation.

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I was actually shooting for the 80 at 55% but they really didn’t seem to like it. And the leaf temp was Super low.
I also found this article.

keep it real. said:

I appreciate everyone’s input and struggles and work around for it. Makes me feel better. I’ll keep an eye on it and the plants and keep them happy.
I could definitely imagine how easy burning could be with 3.3.. you’d have to cut your nutes in a 1/3?

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less is more do not run 3 EC like these shitty nutrient lines recommend. If you need 3 EC of strength their nutrients suck dick. I feed plants that are 5-6 ft tall 2.0-2.2 EC with 1200 PPM Co2 and 1.6 to 1.8 VPD with 1000w Double Ended HPS and get 21 oz a plant. more EC will just cause problems and if you get a deficiency at 2+ EC throw the nutrients in the nearest trash bin and find something else.

calvin.m16 said:

less is more do not run 3 EC like these shitty nutrient lines recommend. If you need 3 EC of strength their nutrients suck dick. I feed plants that are 5-6 ft tall 2.0-2.2 EC with 1200 PPM Co2 and 1.6 VPD and get 21 oz a plant. more EC will just cause problems and if you get a deficiency at 2+ EC throw the nutrients in the nearest trash bin and find something else.

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I would never haha! He was talking about his VPD of 3.3. That’s a lot of water uptake, and anything else in that water.

keep it real. said:

I would never haha! He was talking about his VPD of 3.3. That’s a lot of water uptake, and anything else in that water.

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Ahh my bad, yeah 3.3 VPD is dangerously high. I would try to stay well under 2.0 VPD. Hey then again I've grown plants in 20 gallon pots of coco/perlite and had great results, some say a 5 gallon pot of coco is too big. Many variables.