Cannabis Bud Rot Prevention: Humidity and Airflow

Bud rot in cannabis is gray mold, Botrytis cinerea, a fungus that enters tight flower clusters and rots them from the inside. The spores need free water on plant tissue for six hours or more before they germinate. Cut that wet window and the fungus has no opening. Keep flower-room relative humidity between 45% and 55%, hold air temperature at 68 to 75 F, run constant air movement through the canopy, and open every cola with your hands once a day.

cannabis bud rot vs mold difference

Conditions That Start an Outbreak

• Relative humidity above 60% during weeks 4 through 9 of flower
• Temperatures between 59 and 75 F, the range where Botrytis grows fastest
• Still air and a canopy so dense that leaves stay wet after lights-off
• Overhead watering, splashing from trays, or a flood table that sits full
• Nitrogen-heavy feeding late in flower, which produces soft, leafy buds
• Reused growing media and unwashed scissors that carry spores between plants

Set the Room Before Flower Starts

Install a dehumidifier sized for the room volume, not the plant count. Add an exhaust fan that exchanges the full room volume in one to three minutes. Place oscillating fans above the canopy and near the floor. Aim them at the walls or across the tops of the plants so no fan blows straight into a cola. Keep night temperature within 10 F of day temperature, since a cold night pushes humidity to the dew point on flower surfaces. Water at the soil line in the first hours of lights-on. Empty drain trays within 30 minutes.

can-bud-rot-be-stopped-once-it-starts

Prune for Air, Not for Yield

Strip leaves and larf from the bottom third of each plant at day 14 and day 28 of flower. Remove any fan leaf that lies on top of a bud site. Space plants so a hand passes between neighboring canopies. In a screen-of-green setup, thin the layer under the net. Each cut leaf raises airflow and drops the micro-humidity inside the cola.

cannabis bud rot early signs and prevention

Scout Each Plant Daily

Look inside the bud, not at it. A single brown sugar leaf at the top of a cola is the first sign. Pull the cola apart with clean hands and check for gray fuzz, a soft brown patch, or a musty smell. Botrytis hides in the center of dense flower, so surface inspection misses it. Mark any suspect plant with a tag and check it twice a day.

Remove Rot the Same Day

Cut 2 inches below the infected area with sterilized scissors. Slip a bag over the bud before you cut so spores do not fall onto lower flowers. Seal the bag, remove it from the room, and discard it in the trash. Do not compost it and do not smoke it. Wipe the scissors with 70% isopropyl alcohol between plants. Check the two plants nearest the infected one. If rot returns on the same plant within three days, harvest that plant early or remove it from the room.

Harvest, Dry, and Cure

Take plants at peak ripeness. A plant left standing past its window loses resistance. Dry at 60 F and 60% relative humidity for 10 to 14 days with indirect airflow. Inspect each branch during trim and cut out any brown tissue. Cure in jars at 58% to 62% relative humidity and burp the jars twice daily for the first week. Moldy flower cannot be salvaged by drying or heating.

What Is Not Known

No fungicide is labeled for cannabis flower in the United States. Any product claim for bud rot control on cannabis is unverified.