Humidity vs air movement and bud rot/mold

So I am having issues with my humidity as is often the case even with a dehumidifier I can't get much below 60% in my flower room I have a lot of air movement though as with large fans mounted inside moving air as well as a 6 inch fan on filter and can add another. Am I still at a high risk of mold / bud rot even with all my air movement? Where is the line here?

Some folks follow the VPD chart through the whole grow, and if you're doing that then you might actually want to be in the 60-70% range. I watched a video of big commercial grow recently and they kept RH at 70% even in late flower.

I think if you have sufficient air movement, the actual number is less important than the variation in it. Like if the lowest is 60%, what does it spike to after the lights go out?

I believe botrytis requires liquid water on the leaf surface to germinate, which can happen when lights go out - temps suddenly drop, so RH jumps up, which can cause condensation on or inside the buds. PM does not always need liquid water to get started, but it favors big swings in humidity, so the same logic applies.

Huh thanks guys! These numbers give me incidence that I won't lose this amazing harvest!!!

weedstoner420 said:

Some folks follow the VPD chart through the whole grow, and if you're doing that then you might actually want to be in the 60-70% range. I watched a video of big commercial grow recently and they kept RH at 70% even in late flower.

I think if you have sufficient air movement, the actual number is less important than the variation in it. Like if the lowest is 60%, what does it spike to after the lights go out?

I believe botrytis requires liquid water on the leaf surface to germinate, which can happen when lights go out - temps suddenly drop, so RH jumps up, which can cause condensation on or inside the buds. PM does not always need liquid water to get started, but it favors big swings in humidity, so the same logic applies.

Click to expand...

Running the dehumidifier at night and AC during the day to try to eliminate large swings.

weedstoner420 said:

Some folks follow the VPD chart through the whole grow, and if you're doing that then you might actually want to be in the 60-70% range. I watched a video of big commercial grow recently and they kept RH at 70% even in late flower.

I think if you have sufficient air movement, the actual number is less important than the variation in it. Like if the lowest is 60%, what does it spike to after the lights go out?

I believe botrytis requires liquid water on the leaf surface to germinate, which can happen when lights go out - temps suddenly drop, so RH jumps up, which can cause condensation on or inside the buds. PM does not always need liquid water to get started, but it favors big swings in humidity, so the same logic applies.

Click to expand...

I do try to follow the VPD model now also, now that I am running all cobs.

meangreengrowinmachine said:

So I am having issues with my humidity as is often the case even with a dehumidifier I can't get much below 60% in my flower room I have a lot of air movement though as with large fans mounted inside moving air as well as a 6 inch fan on filter and can add another. Am I still at a high risk of mold / bud rot even with all my air movement? Where is the line here?

Click to expand...

Great question! I was just talking to someone about this. I agree limiting swings and air movement are vital to keeping pathogens at bay. Esp if the environment is sealed.

Here is my room currently. I have the dehumidifier on today since it is cool out.

meangreengrowinmachine said:

Huh thanks guys! These numbers give me incidence that I won't lose this amazing harvest!!!

Click to expand...

Confidence* smh

meangreengrowinmachine said:

eliminate large swings

Click to expand...

One way to help control temp swings is to "ramp up the lights slowly to full"
@ lights "ON" and "ramp them down slowly to "OFF" @ lights "OFF".
If you have the capability.

meangreengrowinmachine said:

Here is my room currently. I have the dehumidifier on today since it is cool out.

Click to expand...

I only run my dehumidifier at lights out and keep it around ~60% during late flower. I haven't had issues with daytime humidity getting as high as ~75%. Good air movement and temperature control is equally as important as the humidity level. When the lights go out and the temp drops, the humidity spikes. I use a minisplit that keeps the room temp pretty constant even when lights go out (the dehumidifier adds heat on its own)

OSBuds said:

View attachment 4909865

Click to expand...

thats an awesome chart. i think i am going to print that up and put it on the wall next to my grow tent lol

If its possible try to have your dehumidifier coming on before the light goes off, you'll be close to your dew point with a high humidity and water can/will form.

I've been having trouble getting temps up so vpd suits me, it means the cooling/humidity fan is on less and better temps because of it,

In nature mold can form when there is more than 50% humidity. I do run up to 55% esp lights out, but no higher.

There have been lots of these discussions and what the gist is....well it's fine until it's not. Someone growing for a long time has a blow out mold out throw it out moment, and feels that sxxx feeling of failure.

There are even some threads about devices to prevent mold and mildew problems at higher rh...mixed results.

It's dangerous above 55% in flower imo, but very doable. I would want airflow behind every side of every plant.

When I started growing it was with the absolute basics, a light, desk fan and a extractor fan/thermostat.
The UK (North) is humid and cool its fucking perfect for mould but I was oblivious to this at the time.
Ignorance is really is bliss, I grew for a few years summer and winter with temps down to 4/5c rh% 70/80% during the night and never had any problems.
I don't why that is exactly, was it strain related, was because the fan was on for almost all the dark period idk it's still a mystery to me?