Bud rot prevention during flowering comes down to four controls: hold relative humidity under 55 percent, push air through and above the canopy, never let water sit on buds, and inspect every cola once a day as they tighten up. Botrytis cinerea, the gray mold behind bud rot, needs hours of surface moisture and cool temperatures to take hold. Break any one of those conditions and the infection stalls.
how to prevent bud rot on cannabis plants
What causes bud rot in flowering cannabis?
Botrytis spores are already everywhere: in soil, on dead leaves, and drifting in outdoor air. They germinate when buds stay wet or sit above 85 percent humidity for six hours or longer, usually between 59 and 72 F.
cannabis bud rot early signs and prevention
Dense colas trap that moisture in the middle. That is why rot almost always starts inside the largest top bud, close to the stem, where you cannot see it until the sugar leaves turn brown and pull free with a light tug.
Cannabis Bud Rot Prevention Tips for Indoor Growers: Airflow, Dew Point, and Canopy Control
What humidity and temperature should flowering cannabis be kept at?
- Weeks 1 to 4 of flower: 45 to 55 percent relative humidity.
- Weeks 5 to harvest: 40 to 45 percent relative humidity.
- Daytime temperature: 68 to 78 F, with no more than a 10 F drop at night.
- VPD target: 1.0 to 1.5 kPa during lights on.
Nighttime is the danger window. Air cools, humidity climbs, and buds sit wet for eight to twelve hours. Run a dehuey on a controller so it kicks on before lights out, not after the spike has already happened.
How do you keep airflow strong enough to stop mold?
Strip the bottom third of each plant and any fan leaf shading a bud site around day 18 to 21 of flower. Then set one oscillating fan above the canopy and one below it, moving air across the stems rather than blasting a single cola.
Exhaust must cycle the whole room volume every one to three minutes. Stagnant corners, tent walls that bow inward, and pots packed shoulder to shoulder create the dead zones where mold starts.
Should you water differently during flowering?
Water the medium, never the plant. Use a wand, drip line, or bottom feed so leaves and buds stay dry, and water early in the light cycle so any splash evaporates before lights out.
Stop foliar feeding by week 3 of flower. If you must spray for pests late, do it at lights on with the fans running and no bud contact.
How do you treat bud rot once you see it?
- Shut off circulation fans so you do not launch spores across the room.
- Slide a bag over the infected cola and tie it off before cutting.
- Cut two inches below the rot with alcohol-sterilized pruners.
- Bag the tool wipe and the bud, then remove it from the grow space entirely.
- Inspect the two nearest plants, then check them again in 48 hours.
Never compost infected material near a grow. If rot has reached the main stem or shows up in more than a few colas, cut your losses and harvest early, because Botrytis moves faster than buds ripen.
Which products help prevent botrytis during flower?
Biological options include Bacillus subtilis, Bacillus amyloliquefaciens, Trichoderma harzianum, and potassium bicarbonate. They work as preventives on healthy tissue, not as rescues on active rot.
Check your state's rules and the product label before using anything in flower, and respect the pre-harvest interval. Skip oils, sulfur, and heavy peroxides once buds have formed, since they can burn pistils and leave residue.
What changes for outdoor growers?
Cover plants before forecast rain in late flower, and shake stems gently in the morning so buds dry fast. Choose airy, mold-resistant cultivars instead of rock-hard indica domes if your climate is wet.
Control caterpillars with Bacillus thuringiensis through mid-flower. Budworms and corn borers chew entry wounds that give Botrytis a free door, and the caterpillar frass feeds the mold.
When should you harvest to avoid rot?
If buds are mostly cloudy under the loupe and the forecast shows three or more days of rain or rising room humidity you cannot fix, harvest. A slightly early crop beats a moldy one.
Dry at 60 F and 60 percent humidity with steady indirect airflow, and check jars daily during the first week of cure for that telltale ammonia smell.
Bud rot prevention checklist
- RH under 55 percent at all times, under 45 percent late flower.
- Two fans plus full-room exhaust, no dead corners.
- Defoliate lower growth and blocked bud sites.
- Water at the base, early in the cycle.
- Scout every cola daily from week 4 onward.
- Sterilize tools between plants.
- Bag and bin every infected bud immediately.