HR and VPD?

Unless you’re in an extreme climate area, cold and frozen, or hot and dry, chasing numbers that you can do little to correct, will lead to high or low cost fixes that amount to little. Keep your inside grow within the boundaries of a cannabis plant with lighting, and fans, hopefully your outside environment cooperates with temperatures and RH. The room the tent is in will dictate just as much influence on the plant as the inside of the tent. If your RH is high and dew is on your shaded leaves, add a small fan and exchange more air.

What I run at where you are right now, I consider it to be highly optimal and efficient. But it can be taxing on water intake. Day:18 84°F and 65% RH (VPD) for the vegetative stage. Approximately 1.15kPa(assuming leaf temperature is about 2°F cooler than the air), which falls right into the ideal vegetative sweet spot (0.8kPa to 1.2kPa). At 1.15kPa, plants can draw water and nutrients efficiently without risking stress or wilting. It keeps the leaf pores (stomata) open, allowing for ideal carbon dioxide intake and maximizing vegetative growth. VPD is determined by the leaf's temperature, not just the ambient air. Because leaves usually run 1° to 3°F cooler than room air under bright grow lights, my actual VPD will be slightly lower, closer to the 1.0kPa mark. As she transitions from vegetative growth to flowering, one can gradually lower the humidity (to around 45–60%) and drop temperatures slightly to prevent disease from settling inside dense buds when they appear. Night:6 At 70°F and 60% relative humidity, Vapor Pressure Deficit (VPD) is 0.86 kPa. This is right on the cusp of what's optimal for the vegetative stage. During the nighttime, plants generally close their stomata and undergo cellular respiration rather than photosynthesis. Transpiration slows to a near stop, making VPD less critical at night than during the day. However, maintaining a nighttime VPD between 0.8 and 1.0 kPa is highly beneficial in that it ensures the air is dry enough to prevent powdery mildew or bud rot, but moist enough to keep the plant from undergoing unnecessary stress. This range keeps the environment comfortable for cellular processes and prevents large atmospheric swings. Keeping it all flowing. Your biggest pitfall will be making sure that nighttime RH doesn't shoot over 70% RH in the first two hours after the lights switch off and temps drop; this is the time when dew point can be missed, and pockets of moisture will form. Always make sure you have your ambient 0.1-0.3ms/cm airflow.

Don't worry, it's well within range and a dehumidifier is not necessary as long as you have good air circulation

Your going to want lower temps and humidity as well. What i follow is 40-50% humidity and 60-70 degrees. My exhaust fan lowers humidity 5-10% and temp 5 degrees or so but I still use a dehumidifier in the room the tent is in. Yeah, I never follow VDP because if your temp and humidity are in proper range for stage of growth then the VDP is good.

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.

Hi thought I would give my 2 cents worth.. never used vpd.. you want a temp of 22-28 with high RH in veg and low RH in flower to stop rot etc..... I thing vpd is an average of both tmp and rh but its a load of crap in my eyes. Always work from rh and temp as separate measurements and defo don't add an average meter called vpd