Dear Have2 and everyone else here at rollitup,
I would like to share with you my post here,
https://www.rollitup.org/t/temps-with-leds-range.1048110/post-16245201 as it references the UC gen rec chart and my experience using it with HID growing and LED growing. In short, I have found that increasing the day temperatures +4f across the board works best for me in terms of quality, quantity and growth rates in all stages when using LEDs instead of HIDs. LEDs produce less IR radiation and therefore heat your plants' surfaces less than what other light types will. To compensate for this, a grower switching to LEDs will need to raise his or her room temps, keeping all other things constant, to achieve the same (or likely better) results with the LEDs. In my experience, keeping all parameters equal and switching from HIDs to LEDs and increasing the amount of light on my canopy on average from 1000 ppfd with HIDs to 1500 ppfd with LEDs, my yields were greatly diminished. After increasing the temps to +4f from what I ran previously (UC gen rec chart), my results are much improved. Plants grow the fastest with more heat (82f leaf temp with LEDs) more light (1500ppfd) more co2 (1500ppm) proper vpd (1.0kpa for veg) and proper nutes and watering (growth stage and water delivery system dependent).
I see you are also asking what the benefits are to lowering the temperatures gradually as the plant's life cycle progresses and from my own research I have found that the answer is that by decreasing the day and night temperatures the plant responds by increasing the effect of certain hormones that increase "rippening." For cannabis that may mean increased bud developement in size and essential oil production including terpenes and cannabinoids. I have found that the frostiest, stickiest and most dense buds are the ones that grow lower on the plant even though they get less light than the tops under LEDs. The only reason for this could be because of the plant surfaces have lower surface temperatures in those areas farther from the heat source (the lights). Further evidence that I have found supporting this conclusion is that for my last two LED cycles, during which I followed the UC gen rec chart to the T, gave me leaf surface temperatures at the top of the plant of 66f (70f - 4f) over the last week or "final ripening" stage. The buds turned out FIRE extremely frost coated stinky and sticky. The only issue was that they were all too small and the yield was dissapointing.
Additionally, it's funny that you mentioned the "stagnation" of growth that "some" have reported when plant surface temperatures drop below 73f. I have certainly found this to be true! That means, in my flowering room, that lowering average day room temps below 77f would mean focussing all my plant's attention to ripening instead of growing. Which I reserve for weeks 8 and 9 for a 9 week variety or weeks 9 and 10 for a 10 week variety.
I blieve that I can finely tune my results have an emphasis on increased yield at the sacrafice of deceased quality by running a flower cycle with higher temperatures and I find that I can achieve higher quality results by running lower temperatures during flower. I now like to achieve a balance between best yield and best quality and I find that sweet spot for my current plants (Zkittlez from clone) to be 84f 84f 82f 82f 80f 78f 77f 76f 75f for each week of flower. If you wanted to emphasize yield, the highest I would recommend going would be a finish at 78f and the lowest I would recommend going (with LEDs) would be 74f for an emphasis on quality. Note that these values are from my experience in an LED flower room with an average canopy top temperature differential of -4f.
My best,
Tbone
