Late flower mold prevention, specific numbers that actually work

We are deep into the window where botrytis kills outdoor runs. For anyone in a wet climate, here is what i target in the last three to four weeks of flower: daytime RH below 50%, nighttime below 60%. If you cannot hit those numbers, you need airflow and you need it constant. The overnight temperature drop is the real killer. Warm days, cold nights, and dense colas equal condensation sitting on your buds for hours before the sun burns it off. I have started pulling a cheap wireless sensor from my shed and hanging it at canopy height overnight. You will be shocked what the numbers look like at 4am versus noon. Airflow is the mechanical fix when RH is out of your control. I run two oscillating fans on timers starting at sundown, pointed low through the canopy, not straight at the buds. Moving air at the stem level breaks up that stagnant humid pocket that forms underneath the canopy overnight. That is where botrytis starts every single time. Anyways, the other thing I do around week six of flower is a selective defoliation pass. Not a heavy strip, just opening up the interior of dense colas so air can move through. Any leaf that is folded over a bud and trapping moisture comes off. Takes maybe twenty minutes per plant and it has saved me more than once when a wet week rolls in unexpectedly. If you see one grey fuzzy spot, that cola is already gone. Cut it, bag it before you walk it through the garden, and hit the surrounding tissue with a diluted hydrogen peroxide spray. Do not wait and hope. The PNW does not reward optimism in late October.

2 comments

2 Comments

Expert Grower12d ago

The airflow advice is solid, but I want to add something that doesn't get mentioned enough in these threads: your soil biology can actually help with disease suppression from the root zone up. Dense, living soil with active trichoderma populations produces volatile organic compounds and systemic signals that prime the plant's own immune response. It's not magic, and it won't save you from a week of solid rain, but plants grown in healthy living soil tend to handle late-season stress better than plants running on bottled nutrients. They're not burned out trying to process salts when they need all their energy to finish strong. The condensation point you made is spot on though. I hung a sensor at canopy height for the first time two seasons ago and the 4am readings genuinely surprised me. I was sitting at 78% RH at canopy level on nights I thought were fine because it hadn't rained. That data changed how I managed the last few weeks completely. One thing I'd add to the defoliation pass: if you're seeing any yellowing or dying fan leaves tucked into the interior, those come off even earlier than week six. Decaying plant material is basically a welcome mat for botrytis, and living soil growers should know that applies above the soil line too, not just in your mulch layer.

Expert Grower12d ago

the fans and the sensor stuff is all real, nobody's arguing that. but i'll push back a little on the defoliation timing. week six is late if you're already dealing with density problems. i do a lighter pass around week three, four. by week six the plant's stressed enough finishing up, you're just adding to it. earlier is better, less shock, same result. the thing nobody mentions is variety selection doing half the work before you ever get to late flower. been running tighter structure cuts outdoors for years specifically because i got tired of losing dense sat-dom colas to botrytis every wet october. some genetics just aren't built for a wet maritime finish and no amount of fans fixes that. hydrogen peroxide spray is fine as a reactionary move. i prefer copper soap earlier in flower as a preventative when the forecast looks ugly. not the same thing, just worth knowing both tools exist.