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Late flower mold control: the numbers that actually matter
We're getting into the window where bud rot can take a whole plant in 48 hours out here in the PNW. I've lost harvests to it and I've also learned to stop it cold. The difference usually comes down to hitting specific environmental targets in those last 3-4 weeks, not just hoping for dry weather.
Your two numbers are: relative humidity below 45% during lights on (or daylight hours outdoors), and below 55% overnight. Once you're past week 5 of flower, dense colas stop drying themselves out naturally. Air can't get through. That trapped moisture is where botrytis starts.
Anyways, if you're outdoors and can't control the RH, you control airflow. I run a strong oscillating fan on a timer from about 6pm to 11pm, aimed at canopy level. Evening is when dew point drops and buds hold moisture the longest. That window matters more than mid-day airflow.
For strains finishing in 7 weeks or less you have more margin. Anything running 9-10 weeks outdoors in a maritime climate is a liability by late September. I've started pulling fast-finishing cuts at 85% cloudy trichomes just to beat the weather. Slightly early beats total mold loss every time.
Check your big dense colas every single morning. Peel back a leaf or two near the base of the cola and smell it. You'll catch botrytis before you see it. Gray, musty, almost sweet smell means it's already started inside and you need a blade immediately.
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6 Comments
Experienced2d ago
The smell test thing is real and i don't think enough people talk about it. By the time you see gray fuzz it's already spread further than what's visible.
One thing i'd push back on slightly, the 45% during lights on target is solid but i've seen people obsess over RH numbers while ignoring vapor pressure deficit, and those aren't the same thing. A room sitting at 43% RH but running warm can still have conditions that stress the plant and slow surface drying. VPD gives you the fuller picture in late flower.
My main add here is about the cure window connection. If you're pulling early to beat rot you need to be even more careful going into the dry and cure. Slightly underdeveloped calyxes hold moisture differently and i've had early pulls go sideways in the jar because i cured them the same way i would a fully ripe harvest. Slower initial dry, more burping, longer before you seal. The terp profile on an early pull is already compromised, last thing you want is to also end up with hay smell from a rushed cure on top of it.
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Experienced2d ago
VPD point is valid indoors but outdoors its kind of academic. you cant dial in temp and humidity out there, you just watch the sky and manage airflow best you can. the cure advice though, that tracks, early pulls need more patience in the dry room, not less.
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Expert Grower2d ago
The outdoor VPD point isn't fully academic though. You can't control ambient conditions, sure, but you can manipulate the microclimate around dense colas with a well-placed oscillating fan, and knowing your current VPD tells you exactly how aggressive to be with defoliation in those last weeks. Going in blind versus going in with a target is a different decision-making process even if you can't hit the number perfectly.
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Experienced2d ago
The fan point is solid, I do the same thing indoors and it makes a real difference on tight colas. But I'd push back a little on late-stage defoliation based on VPD readings, because once you're past week 5 or 6 pulling leaves stresses the plant at a point where it can't really recover and you might do more damage than the humidity would have.
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Experienced2d ago
The early pull point is real but i'd push back a little on treating 85% cloudy as a universal bail-out number. On fast autos and short-season strains that window is genuinely fine, maybe even ideal depending on the effect you're after. But on a longer photoperiod strain that's been running 9+ weeks and still has clear trichomes mixed in, you're leaving more on the table than you might think. sometimes the early pull costs you more than a targeted cut would.
That's the thing i do with mold in my tent too, not weather-related but same logic. I don't pull the whole plant when i find botrytis, i cut the affected cola with a clean blade, bag it immediately so spores don't travel, then reassess. Lost maybe a quarter of a plant that way a few runs ago and saved the rest. treating it surgical instead of nuclear.
The morning check habit is underrated though, fully agree there. i do mine right when lights kick on before the canopy warms up, that's when you catch anything that developed overnight.
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Expert Grower2d ago
The surgical cut approach is solid but i'd add one thing -- after you bag and remove the infected cola, hit the wound site and surrounding area with a light spray of diluted hydrogen peroxide or a lacto serum if you're running living soil. Botrytis spores are already in the air by the time you see it, and giving the plant's surface microbiome a quick boost helps the remaining tissue resist secondary infection. Skipping that step is where a lot of "surgical" saves still fall apart two weeks later.
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