Originally posted on 04/17/2021:

Quote Originally Posted by KVB View Post
In my opinion you guys are way to high unless you have great ventilation and even then you are too high.

We have found that this is strain specific. They really do react to the humidity differently. Take note of how temp and humidity interact. Obviously you know that warm air holds more water than cool air.

But operating at higher than 50% and close to 60% is unnecessary in any stage and the higher runs a risk of mold. You can do it, with good circulation and temp regulation, but you can seriously drop those humidity levels.

In early stages when roots aren't established you can kick the humidity up to 70% or more, it's how it gets water. Watch that temp though, keep it under 25C with lights on.

It changes in veg as the roots start taking on more water and the plant itself uses evaporation to cool. Lower the humidity about weekly through veg. You guys might take it to 50, but so many strains do better at lower. Take it to 40%.

With temps slightly higher here evap is good, stay under 27 or 28C

Keep dropping that shit for flowering. If you're not at 40%, get there, obvioulsy temp is dropping a bit for flowering too.

This is where I differ from you guys. About two weeks from cutting, maybe while flushing, I'm dropping humidity to 30% definitely less than 40%.

There are some plants that we are putting at 20% humidity in flower and those are about 40% during veg.

Keeping temp and RH down during the flowering period can be a bitch and getting the late stage lights on/off temp difference to increase with these lower levels of humidity are very important.

And a bitch to do sometimes. I think my thoughts on how it can be and is done is for another post though.

I don't have great ventilation, I have no ventilation. You turn off the exhaust fans in a sealed room. Once every 2-3 hours during lights on I turn on the exhaust fan for a few minutes to recirculate fresh air.

I have also read about different strains performing better in different environments, I can see that. When people breed landrace strains around the world, they select the best performing ones in their environment, which breeds plants that favor conditions in a specific local region.

Most people following the VPD chart would say that operating below 50% RH is too high of a VPD and too stressful to the plants in any stage beyond late flower. Not sure why you are saying it is unnecessary if that is the recommendation of countless experts for optimal parameters. I doubt you are pulling 3+ pounds per light using that growing style, maybe you are hurting your yields by a dry environment. Why do you run at a high VPD? There are other ways to combat mold like the ProGuard. What benefits have you found from lower humidity besides avoiding mold?

People who buy the ProGuard supposedly never get mold and pass all the microbial testing.
https://craft-farmer.com/products/in...-dxm-100-w-mci

Humidity and temp need to be looked at together. Ambient temps aren't the only factor, you need to look at leaf temp. Depending on your lighting technology, that would change ambient temperature suggestions, higher for LED as there is less radiant heat. LEDs need hotter rooms.

Keeping temp and RH% down in flowering in a sealed room can be easily accomplished with a properly sized mini split A/C and industrial dehumidifier.

I turn down exhaust duty cycle and use a space heater to adjust night time temps up/down if needed.

I have tons of sources I can share I've come across in my research that would suggest your rooms are much too dry and your VPD is too high.