Was wondering about everyone’s thoughts on Temp versus VPD. Does Temp really matter as long as the VPD is correct? Or should you keep your grow under a certain Temp with correct VPD?
Yes.
VPD is a calculated proxy value for values of RH and leaf surface temperature that will provide an "optimal" growing environment. That raises the question - what does "optimal" mean in this context? My take on it is that that "optimal" means that the plant will function at the the highest level of photosynthesis. Research tells us that increasing photosynthesis ≠ maximum yield or maximum "quality".
I've grown cannabis in the low to mid-80's in seedling and veg but for my upcoming grow, I'm going to try to keep temps in the high 70's. My rationale is that even my little 2' x 4' tent will generate a prodigious amount of cannabis so I'm willing to sacrifice quantity to improve quality. A big driver for looking to make this change was the comments by Mitch Westmoreland in a You Tube video "Does Hemp Need Extra Water and Fertilizer?".
My notes from that video:
"Discusses temperature - Optimum temp to maximize CBD and yield and keep THC below 3%
Plants grown at 73 and 84 degrees. Plants at 84 were taller but colas were smaller in girth.
At 84 little but taller, a bit more vegetative biomass. Yield wasn't all that affected by the increase tempearature. At 84%, cannabanoids were roughly 1/2 of what they were at cooler temperatures.
Suggest that the optimum is between 70 and 80.
Fertilizer
You've probably come across hundreds of companies promising to boost yield. They "lack a theoretical basis" and you'll probably end up just throwing away money and excess nutrients that will end up polluting our nice rivers.
We recomend that you fertilize it just as you would tomatoes. This is a good comparison there's nothing particularly special about hemp.
One of the big claims that you'll see coming out out of the cannabis industry is that high phosphorus will increase yield and will increase cannabanoid content. We've tested this.
We look at a range of phosporus treatments ranging from borderline deficient to excessive. We found no significan difference, especially as we move toward the rates recommended by the cannabis industry. This has implications because phosporhus is a potent pollutant in our environment.
What we're trying to do is get growers to use as little P as we can while still getting a good crop.
Drought stress
In veg - they're very resiliant. Got to the point where they were severely wilted. They recovered so that it seems as if nothing happened. Hard to see which one's were stressed and while ones were well watered.
In flower - yellowing and dropping of leaves and reduction in yield. CBD and THC did not increase.
Temperature - yield increased slightly. CBD and THC plummeted - "cut in half basically". We don't have a good explanation for this right now. We're going to see if we can reproduce this and see if we can come up with an explanation for what's going on." (emphasis mine)
