Late flower humidity control, what actually works outdoors

We're at the point in the season where your buds are stacking up and also at their most vulnerable. Botrytis and powdery mildew don't need much invitation this time of year, especially if you're anywhere near the coast or in a valley that traps morning fog. The number I chase is 50% RH or below during lights off, or in my case, overnight. Once you're consistently above 55% through the night in late flower, you're playing with fire. For outdoor plants I watch the dew point more than the humidity percentage honestly. If the dew point is within about 4 degrees of your overnight low temp, you will get condensation on your buds. That moisture sitting on dense colas for 6 to 8 hours before the sun burns it off is enough to start a botrytis pocket in the center of the canopy where you won't even see it until harvest. What i do from week 5 of flower onward: I run a battery powered oscillating fan under the canopy on a timer from 10pm to 8am. Keeps air moving through the lower bud sites. I also defoliate more aggressively than most people are comfortable with at this stage, pulling any large fan leaves that are trapping moisture or blocking airflow through the interior. If rain is coming, I pull a simple greenhouse cover over my plants the night before. Not a sealed tent, just a clear poly tarp on a frame that lets sides breathe. Keeps the direct wet off and lets humidity escape. It's not pretty but it has saved more than one harvest. Anyways, the window between now and chop is short and this is when people lose the most weight to invisible rot. Stay on it.

4 comments

4 Comments

Seedling18h ago

Agreed. I just chopped 3 this morning. Better to be safe than sorry. We have Southern Monsoons that pop up (like this morning) plus coastal fog. The sun is struggling to come out and its 78 degrees right now with 64% humidity. Waiting for amber trichromes could cost you the entire plant. Don't roll the dice and gamble with botrytis. Like PNW_Sungrower said, "stay on it".

Experienced18h ago

The dew point tip is legit and I don't see enough people talk about that instead of raw RH. That said I'd push back a little on the aggressive late defoliation for everyone. I've gone too hard on stripping in week 6 before and stressed the plant at exactly the wrong moment. Lost some terp development on a pheno I was really excited about, plant just kind of stalled. It's a balance. What actually saved me more than the defoliation was the canopy airflow piece. That overnight fan is the real move. I run mine from about 11pm through sunrise and it's made a bigger difference than any spraying or stripping I've done. Also if you're pheno hunting and you have multiple plants, keep an eye on which ones trap moisture worst. Dense, tightly stacked structures are beautiful but they're also the first ones to go. I've lost some genuinely incredible smelling phenos to botrytis in week 8 because I wasn't respecting how tight those colas were. The gassy open-structured ones almost always make it. The loud fruity brickhouse phenos are the ones you have to babysit.

Experienced17h ago

The poly tarp trick is solid but i'd add one thing people skip -- angle matters more than people think. I've seen setups where the tarp is basically flat overhead and it just pools condensation and drips right back onto the canopy. Get some slope on it so moisture runs off the sides. Learned that one the hard way. The other angle i don't see mentioned here is timing your watering if you're in pots or raised beds outdoors. Watering in the evening going into a humid night is just asking for trouble in late flower. Morning watering only from week 6 onward has quietly been one of the better changes i made. Soil surface evaporation overnight is a free source of humidity right at plant level that nobody accounts for. Running autos i don't always have the luxury of timing my harvest window to dodge a bad weather stretch, so the micro-environment stuff matters even more to me than it might to photoperiod growers who can read the season and plan around it. You get what you get with the finish date.

Seedling10h ago

First time tarp users should also be aware of their local wind conditions. Secure it well so it doesn't blow away.