This is a two parter and I couldn’t find much info searching here but Google brought up a few things. I tend to trust you all here because we are all actual growers with nothing to gain from giving honest non biased feedback.
My first part is if I am understanding correctly using LEDs we should be running a slightly higher temperature in our grow rooms? I have been consistently around 77 or so and my plants have created crazy trichomes but I started doing the math in my head and my leaf and soil temps would be significantly cooler than that. If we are supposed to do this what temps should we strive for in Flower?
Second part is VPD. I know everyone has an opinion on this but it seems impossible or well not impossible because I have a humidifier but potentially problematic to run in flower at 60+% humidity to maintain good “VPD”.
Just wanted to get some feedback about that and what temps / humidity do you all using LEDs that have had amazing finished product strive for?
Good questions, I’m just starting out with LEDs & I’ve been trying to figure out a good strategy for the things you are talking about as well. The idea of higher room temps when using LEDs seems to stem from having less/no IR photons from LED fixtures compared to HID, but still trying to get the leaf temps up to ‘optimal’. The levels of IR from something like an HPS bulb helps to heat the leaves a lot. The difference between fixtures can be felt, my small 270w HPS super agro bulb would feel like it was burning my scalp from up to a couple feet away or maybe even a little more when working under it. My LED fixtures with actual IR diodes from 6 inches away don’t feel anything like that HPS unit did from feet away. I saw a lot of different recommendations for optimal air & leaf surface temps for LED, couldn’t decide what was correct, so went back to my old HID rule of 84f max for air temps, & I read somewhere that leaf surface temps of 70f to 82f are OK, which is good, because right now that is where mine usually seem to be, all at once. My plants leaves run from 70 to 80f from bottom to top of the plant & my air temps & humidity are different from bottom to top of the plant as well. So really there is no way to dial in towards & ‘lock in’ a specific ideal VPD number, & I’m just going to figure out what sort of humidity numbers won’t beat up the plant too badly, based on the range of air temps & range of leaf temps I can realistically provide it, & based on what stage of growth it is in. Or at least I’ll know if I’m in a bad range. In my mind, if I go off of the hottest leaves, that is probably the best thing to do. I also don’t want to be obsessing over VPD. But one thing that is helpful is to get the actual leaf temps instead of guessing, in addition to having the room/box temp & humidity, & then running all of that through the VPD calculator, & then going from there based on some real numbers & where you want to be. Watch out for looking at random VPD charts, they might be adjusted for different leaf temps & they might not be marked, or you might not notice. That got me a few times until I learned to go straight to the calculator with my actual leaf temps & other actual numbers.
The concern ends up being about dialing in the leaf VPD & not really the room VPD, & really you would want to know & not guess the actual leaf temps in addition to the other factors to get the actual leaf VPD. Cheap IR thermometers are out there.
I read somewhere cannabis plants grow best at 76 degree leaf temps so I try to keep my room around 78-79 during day then reduce to around 60 at night during flower
I then make the rh balance to around a 1.5 vpd depending on the room temp using a small algorithm I wrote into the coding of the system, using a dehumidifier and a humidifier depending on where the levels need to be stabilized at.
Simple answer 78-79 degrees F with a vpd around 1.4-1.6 during flower and you’ll have a fragrant product
What grows best at what temp/ humidity/ vpd is probably strain/ pheno-dependent, but the leaf temp is as important as the air temp & humidity if you want to know where you are vpd-wise. Personally I’m not sure that I can manage vpd closely enough all of the time to gain a lot from it since I don’t have a dehumidifier & have to rely on bringing in dry (if available) ambient air to the lungroom if I want to really drop the humidity in the lungroom. So today when it is 85% humidity outside, my box is going to run 55 to 60% RH with 83-84f air temps + 78-80f leaf temps at the top of the canopy, & the vpd could probably be a little lower to be optimal according to the calculator. But VPD is something that wasn’t really talked about back in the day, so I find it interesting to learn about & try to work with what I have to maybe have it be where it might be optimal as much as possible. Do you check your actual leaf temps or going with the estimate?