Leaf temp, ambient temp and VPD

[ATPinMotion]

High all,

I am new to growing in a tent, and like many, I'm trying to optimize my environmentals.

There is a lot of conflicting information out there about VPD and the Boogeyman (powdery mildew, or PM) and I'm looking for some help to make sense of it all.

Here the chart I started working from, published by Fluence;

https://www.kisorganics.com/pages/canna ... episode-10

The values are very percise, but I found it cumbersome to work with.

Here is my interpretation,

I've been trying to maintain a 1.5kPa deficit, and have noticed seedlings at 28°C / 65%RH are growing much more vigorously than those at 25°C / 55%RH.

Those are atmospheric temps. Leaf temps are ~27°C and ~23°C respectively.

Looking forward, I plan to push humidity and temperature in veg. Aim for 28°C on the leaf, and then add humidity until I get a 1.0kPa deficit. Is this relative to ambient air temp? Or leaf temp? Any wisdom in splitting the difference, or 'averaging' as a scientist would say?

I've had a couple of times where the tent hit the dew point at lights out. WOW, there's a lot of water in that hot humid air. Getting a few good airchanges in right at lights out is critical to keeping the dew point low and the humidity stable as the temp drops ~5°C

My real question is; what is the best way to dial a flowering tent in? I'm planning to start out with the 'status quo' 25°C / 55% RH, but that will mean bringing the lights VERY close to get 28°C leaf temp. Is this all there is to it?

Outdoor plants hit the dew point consistently. Air circulation is key. I imagine altitude plays a part here as well, i.e. someone growing at sea level will not have the same results as someone growing at 2500m, so please include your altitude as well. I would love some technical input on how to give my ladies the best environment possible. For reference I am at about 700m / 2300ft,

Thanks, and forgive me if this has been covered elsewhere. I did a search, and if I missed it please point me there!

Any reference material, and links are welcome. Thanks!

[QuantumMechanic]

The whole VPD chart thing scares the crap out of me. When I look at the chart, it appears that I've being growing in the VPD equivalent of a desert for a lot of years. The general consensus at the time I started growing indoors was about 35% RH for flowering. It's what I've tried real hard to achieve. It hasn't been cheap, both for the equipment and the power to run it. I figure that the 35% RH number was a compromise that came about due to someone else having problems. Sort of a balance between power consumption, good plant growth, potency and minimal chance of PM and mold.

There are a couple of things I've have never done: Burning sulfur (or spraying sulfur compounds on the plants). What's that all about anyway? It couldn't make the bud taste better. And powdery mildew. Never seen it in my garden.

I'd guess that the VPD chart was developed for plants that grow differently and are consumed differently than cannabis, the kind of plants that produce fruit or veggies that can be washed aggressively after harvest.

[Professor Xavier]

The whole VPD chart thing scares the crap out of me.

I'd guess that the VPD chart was developed for plants that grow differently and are consumed differently than cannabis, the kind of plants that produce fruit or veggies that can be washed aggressively after harvest.

VPD levels are set around optimal transpiration rates from reactions by the plant's stomata to open or close based on its environment and needs. It indicates optimums for maximum metabolic rates achievable by plants. By no means do you have to follow VPD numbers as you balance other factors like PM, but it is good to know how those VPD tweeks will then affect your plants so you can dial in your space with other variables.
For example, in my last grow I introduced a heater to offset 15C basement temps. This raised temps in the tent from 21C to 28C. My RH went from 45 with a VPD of 1.35 at 21C to 35 with a VPD overy 2.3. What happened? My plants transpired more to deal with the heat. So they started suckling down water and made the root solution quite potent. My runoff solution strength started rising rapidly and salts started forming from what the plant didn't use up. I started to get nute burn as the plant choked on the burning salts.
My solutions: Lower EC/PPM feeding solution after a quick pH balanced flush. Added a humidifier to hit 60-65 RH with a VPD of 1.24 at 27C. I also increased air movement to protect against PM, especially against the wall of my tent that experiences temp shifts. A little fiddling of my exhaust fan allows me to keep these dialed in as plants grow and transpire more. Finally, I cleaned up my plants with a harsh defoliation to prevent leaf overlap and hidey spots for PM.
My elevation is 710 meters or 2350ft. VPD is based on leaf temp. My leaf temp is also lower than my ambient. This is because we run LED'S that aren't pushing high IR light. Most HID tents would be the opposite.

[ATPinMotion]

Thanks for the replies, guys.

Here is a good resource I found (possibly linked somewhere else on this site)

http://fluence.science/wp-content/uploa ... Gr6swZ1RXi

It provides a good overview of optimal and limiting environmentals.

I remain unconvinced that VPD is based on leaf temp. My understanding is that ambient air temp must be considered, because we are trying to find the vapor pressure in the air.

One can see on the VPD chart, that close to the saturation point ,(aka Dew point) temperature has little effect on humidity. At the dew point there is no VPD - vapor pressure has been equalized and now the plant is unable to transpire. Speaking of plants, they should be 97% water. The temperature of the plant should have zero effect on the vapor pressure in the air.

There is one other contributing factor to VPD, and nowhere do I see it mentioned. Air speed. This is analogous to 'windchill'. It's why a summer breeze cools your skin, or why a winter wind will burn it. Airspeed contributes to VPD and I'm guessing those who are pushing their levels (high temps and humidity) are airflow artists!

Looking forward to hearing from more growers - it's the experience In looking for. I'm not expecting an 'expert' to drop from the heavens and provide the perfect environmental recipe. That is the art of growing. As temps and humidity go up, pests start to thrive too, so there is always a compromise...

I have grown plants outdoors, and they have gone from desert like conditions in the day, hit the dew point in the evening, got chilled overnight and done it all over again. This obviously stresses the plants (they can take hours to perk up in the morning), but they survive and produce... One of the things that excites me about growing indoors is the control, and the possibility of creating a stress free environment.

I see HID as having a major advantage here. As @professor xavier pointed out - it's easier to get leaf temps above ambient temps with HID light. This could allow a grower to run a more standard 24°C/50%RH, while still keeping the plant at 'optimal' temperature for development (28°C).

That beings said, I know there are LED gardeners out there matching HPS yields per area (per square foot or per meter), and I KNOW there are LED gardeners KILLING HID in yield per watt.

Looking forward to more tips and data points!

[Professor Xavier]

Thanks for the replies, guys.

Here is a good resource I found (possibly linked somewhere else on this site)

http://fluence.science/wp-content/uploa ... Gr6swZ1RXi

It provides a good overview of optimal and limiting environmentals.

I remain unconvinced that VPD is based on leaf temp. My understanding is that ambient air temp must be considered, because we are trying to find the vapor pressure in the air.

One can see on the VPD chart, that close to the saturation point ,(aka Dew point) temperature has little effect on humidity. At the dew point there is no VPD - vapor pressure has been equalized and now the plant is unable to transpire. Speaking of plants, they should be 97% water. The temperature of the plant should have zero effect on the vapor pressure in the air.

There is one other contributing factor to VPD, and nowhere do I see it mentioned. Air speed. This is analogous to 'windchill'. It's why a summer breeze cools your skin, or why a winter wind will burn it. Airspeed contributes to VPD and I'm guessing those who are pushing their levels (high temps and humidity) are airflow artists!

Looking forward to hearing from more growers - it's the experience In looking for. I'm not expecting an 'expert' to drop from the heavens and provide the perfect environmental recipe. That is the art of growing. As temps and humidity go up, pests start to thrive too, so there is always a compromise...

I have grown plants outdoors, and they have gone from desert like conditions in the day, hit the dew point in the evening, got chilled overnight and done it all over again. This obviously stresses the plants (they can take hours to perk up in the morning), but they survive and produce... One of the things that excites me about growing indoors is the control, and the possibility of creating a stress free environment.

I see HID as having a major advantage here. As @professor xavier pointed out - it's easier to get leaf temps above ambient temps with HID light. This could allow a grower to run a more standard 24°C/50%RH, while still keeping the plant at 'optimal' temperature for development (28°C).

That beings said, I know there are LED gardeners out there matching HPS yields per area (per square foot or per meter), and I KNOW there are LED gardeners KILLING HID in yield per watt.

Looking forward to more tips and data points!

Anecdotally, I too have found my plants like it on the dry side of what is recommended in my VPD chart. If I were running at 75% RH, I'd start to see condensation on the far wall of my tent without my upgraded airflow. A neat Hypothesis to test would be that IR, being commonplace in outdoor sunlight and indoor HID setups when much testing was done, affected the results such that they cannot be replicated in modern full-spectrum white light LED grows and may need updating (species specific testing might help too). OR, it could be that I'm just running at the perfect point for MY situation with as high of an RH as is safe in my tent and my 90% of plant VPD optimization growth is just seen as acceptable to my naked eye.
I will also agree that higher temp/humidity growing requires more skilled airflow design and that adding or removing CO2 or dimming overpowered lights is another set of dials with which we can tinker to find that perfect environment our plants thrive in.

[unkle_psycho]

I always understood that the heavy airflows were just a necessity of growing with HID fixtures. Using LEDs I find it way easier to maintain humidity levels where I want. The lungroom rh is between 20-30%, and I can set the tent anywhere between that, and dew point with fan speed.

If I want to really run humidity low, I need to add heat to the lung room, or the heavy airflow will cool the tent lower then I want.

With HID I had to use a crazy airflow, which always pulled the RH low, close to lung room levels. Now there is absolutely nothing that needs 'violent solutions'. A computer fan at an exhaust keeps temperatures and moisture where I want. An extra fan blowing in gives enough control for humidity.

70-80% humidity until around week 5-6 when serious bulking is going on.

You dry weed at over 50% humidity, so it should easily make it without molding in that humidity.

[unkle_psycho]

I checked the RH% of the Bovida humidity control bags designed to keep cigars and weed at the right moisture levels for storage. They keep your jars at 62% humidity. Is there any reason to grow at a lower rh then used for drying/ curing/ storage?

[QuantumMechanic]

Seems like when I search the web for VPD, I get a whole lot of recent search results involving cannabis. One could get the idea that it's a recent thing, like someone invented it just for cannabis. But looking further, this started a long time ago, with other greenhouse plants, here's a link to tomatoes:

http://csbe-scgab.ca/docs/journal/41/41_3_135_ocr.pdf

I'm still using HPS, mostly because it will take some relearning to convert. So when I change over to LED in my garden, I'll need to raise the temp a bunch to compensate for the lack of IR on the leaves. With the raise in temp will come a drop in humidity, which, according to the chart, I'll have to increase really a lot compared to what I've used in the past. Yep, I'm skeered. Kind of like learning how to grow all over again.

[unkle_psycho]

Seems like when I search the web for VPD, I get a whole lot of recent search results involving cannabis. One could get the idea that it's a recent thing, like someone invented it just for cannabis. But looking further, this started a long time ago, with other greenhouse plants, here's a link to tomatoes:

http://csbe-scgab.ca/docs/journal/41/41_3_135_ocr.pdf

I'm still using HPS, mostly because it will take some relearning to convert. So when I change over to LED in my garden, I'll need to raise the temp a bunch to compensate for the lack of IR on the leaves. With the raise in temp will come a drop in humidity, which, according to the chart, I'll have to increase really a lot compared to what I've used in the past. Yep, I'm skeered. Kind of like learning how to grow all over again.

For me, taking replacement air from a dry room, I find it easier to get humidity up with the LEDs, and easier to get temperatures up with the HPS. When I switch to leds I just reduce airflow, it brings up both temperature and moisture.