HR and VPD?

If you are already reaching 64% RH, you will almost certainly need dehumidification in the coming weeks. The canopy is still getting larger, even after flip to flower - it takes 3-4 weeks for vege growth to stop. So, the plants will continue to release more moisture per day until that point. You want to avoid anythign too far below 1.0 kPa if not talking about clones or seedlings. Short-term needs of those contexts outweigh the risk. It is important to remember leaf temp is cooler than the atmosphere due to evaporation occuring at the stomata. This is the temperature that you use to calculate VPD correctly. You might be closer to 0.9 kPa based on leaf temp, but that's still 'okay.' I wouldn't go below that with mature plants in either phase. Leaf temp is -1C to -2C relative to atmosphere temp on a vpd table. 3-4F... it's a guesstimate so don't overthink it.. .get a infrared thermometer if you want to be accurate. Not a suggestions.. but to quell any OCD :P Separate from VPD It's wise to keep RH below 65% to avoid microbial issues. VPD dropping too low is also an infection risk. What exactly is best for marijuana isn't well researched. I see wide range of suggestions from one vpd table to the next. If you don't tightly control these things, you merely do as best you can. Download some tables and experiment.... 1.0-1.5 in flower is a wide range i see suggested. Generally they suggest a little lower in vege phase but it overlaps a good bit. With a mature plant i'd suggest 1.0 or higher initial target. Comparing a handful of plants from seed across multiple grows will not be very definitive or give much confdence. There's a ton of genetic variety involved, but you can at least be fairly confident about avoiding anything significantly bad. Smaller effects, positive or negative, will be lost in a fog of war - hidden by normal volatility that results due to numerous uncontrolled factors that vary each grow. Basically, your own biases will likely influence what you want to see on the smaller effects that cannot be distinguished from normal variance grow-to-grow for a plethora of reasons. Check out this video - 15 of 17 by bugbee/athena www.youtube.com/watch Anecdote (*beware): I have experienced issues with anything going too far above 1.5 kPa, but there may hve been coinciding factors i didn't recognize. 1.8-2.0+ causes issues in my environment for one reason or another. They are fine for a short time in that VPD, but long term they get stunted in my experience. This has happened when my wick was fouled and humidifier was down for a while. Maybe, it's because he's speaking in a generalized way and it's simply a matter of rounding, or there's something else in my garden contributing to the increased sensitivity. Can't be certain,but i try to keep my mind open. I'd stick to 1.5kPa and below baed on my experiences. I shoot for 1.0-1.2 kPa, nowadays, but it spikes into the 1.3's. I really can't see a difference in expected yield or quality, but a bit cheaper running the dehum less. Browse over the other 16 vidoes in this series. they are all short 3-5minute presentations. I'd also check out his videos on the apogee instruments youtube account. Especially the one labeled 'maximizing yield...' .. these are much longer videos. goes into media composition, nutrients, irrigation habits etc... Some good ones on light and other topics too. cocoforcannabis.com - guides articles and dr. photon's corner is also a good place to learn without corruption of 'bro-science' nonsense. Whether you are soilless or not, there's good info there.