So i run my girls in a room with no tent. The room is negative air pressure. Exhaust fan in room blowing out into other side of basement, passive air intake into grow room. This allows the air to cool down without a ac unit. I have a dehumidifier in the room and on the other side of basement. My temps during flower are around 79 to 81 during lights on in the grow room. Humidity is at 50% (1.8 vpd) and that brings me outside the proper vpd in flower. To get the proper vpd i need to raise the humidity to 60% but I’m afraid of bud rot even tho i have a ton of fans in there. What would you do? Raise the Humidity to get perfect vpd or keep it at 50% (1.8vpd)
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Based on your temps, if you are chasing VPD, I would raise your humidity to 55%. That will get you closer to the sweet spot but once you get to week 6 of flower, I would drop the humidity to 35-40% if possible.
What week flower are you in?
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Some regions are more problematic with mold than others. If you live in a naturally dry climate (like me) the incidence of mold/bud rot is very low. I have quit trying to lower the RH until the last two weeks or so of flower, and most of the grow sits at 55 to 60% RH.
If you are in a damp climate then yes; bias toward avoiding WPM (White Powdery Mildew) by getting the RH down below 45% RH.
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I’m right by lake Erie
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I’m in Alabama and it’s beyond humid here, and hot. My room stays around 60-65% during Veg and early flower. Then i take it down to 50% for the rest of the grow. I would go lower but i can’t. It won’t go lower.
I have fans blowing directly into the canopies from underneath and above. Lots of airflow.
I’ve not had any issues with WPM.
Also, I’m set up in a room without tents. The air is also vented into the basement. At up is the same as yours basically.
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I’m worried about bud rot
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Don’t go above 60% and ensure there is always great airflow through the canopy.
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Proactive spraying with a good fungi/insect/miticide helps a lot. Organic spray to harvest day stuff works well. I use Growers Ally. Spray every few weeks at lights out. Takes away a ton of worry. I am dry here so mold not really a thing but i prefer to relax and it helps.
Proper procedure will confirm what your real numbers are. You’re quoting air vpd numbers as a leaf vpd target. VPD targets are leaf vpd numbers. The difference lays in the leaf temperature offset. Leaf Temp - Air Temp = Leaf Temperature Offset. The leaf temperature offset also determines which vpd chart to pick. Or you can use a dynamic vpd chart or a vpd calculator to avoid that confusion.
Adjust your leaf vpd towards leaf vpd targets.
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Leaf temp measured at 78 to 81 depending where I’m aiming at.
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Are you able to lower the vpd by adding an ac in the mix and lowering your lights on temp. Instead of raising humidity to hit sweet spot I believe you can lower the temp as well.
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That’s good that you have a temp gun. For vpd find your hottest leaves along the top of the canopy & run your vpd numbers there. How are you checking your air temp & RH? You want those sensors to be showing you what is going on right around the leaves you are shooting with the gun. Also you want those sensors to be shaded/ not in direct light or they will heat-soak & be off. Also you might want to check everything at different times once warmed up to see if your offset changes, say before watering vs after watering, to see if it moves or if it mostly stays the same. Also, I notice that I have to check the leaves fast within a few seconds of going in there or the air from the open door starts knocking the leaf temps down usually. Maybe double check to see where the offset runs at those hottest leaves, just in case you need to go back & shade the sensors or something. If you did all of that already & your plant does run a tight offset there that would warrant something close to a calculation with a zero offset like you are alluding to, then probably what I would do is watch the air temps & bud temps, & 60% RH probably isn’t going to kill you at that air temp, as long as you don’t cook the buds & everything stays healthy & cool enough. If you want to lower your leaf vpd, then you can raise the rh, &/ or lower your air temps, &/ or if you can figure out how to lower your leaf temps independantly, that would also do it. Then go back & check the offset again after any changes to see where it is. Of course you don’t want to let the leaves get too cool either, so keep that in mind if deciding to lower the air temps.
What I would do is ultimately in addition to watching leaf temps & vpd numbers is to watch your bud temps once they get bigger, since they won’t transpire as much as the leaves & you don’t want to ignore that either & cook the buds. Usually I want to see the actual buds cooler than around 84f. But you wouldn’t want to run your vpd numbers off of the bud temps, just figured I’d mention that.
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