Cannabis Bud Rot Prevention With a Dehumidifier

A dehumidifier prevents cannabis bud rot by holding relative humidity at 40% to 50% during flowering. Botrytis cinerea spores need free water on plant tissue and humidity above 85% for 6 hours or more. Keep the grow room below 50% RH to stop spore germination.

cannabis bud rot prevention using dehumidifier

How Botrytis Infects Buds

Botrytis cinerea is a fungus. Spores land on buds. They need water. A wet leaf or a dense flower holds moisture. At 85% RH the spore germinates. At 59°F to 72°F it grows. A gray patch appears in 24 to 48 hours. The patch spreads to the stem. The cola dies.

Cannabis Bud Rot Prevention Tips for Indoor Growers: Airflow, Dew Point, and Canopy Control

Target Humidity by Stage

• Vegetative stage: 60% to 70% RH.
• Early flower: 50% to 55% RH.
• Mid to late flower: 40% to 50% RH.
• Last two weeks: 35% to 40% RH.

Night RH rises 10 to 20 points when temperature drops. Set the dehumidifier to hold the same RH day and night.

how to increase air circulation to prevent bud rot

VPD Is the Real Number

Relative humidity misses leaf temperature. Vapor pressure deficit (VPD) measures the drying force. In late flower, target 1.2 to 1.6 kPa. At 1.0 kPa or lower, water sits on buds. A dehumidifier raises VPD. A hygrometer at canopy level gives the reading. Wall sensors read low.

Sizing a Dehumidifier

• Tent, 16 sq ft: 30-pint compressor unit.
• Room, 100 sq ft: 50-pint unit.
• Room, 200 sq ft: 70-pint unit.
• Room, 400 sq ft: two 70-pint units.

Compressor models work above 65°F. Desiccant models work below 65°F. Desiccant models add less heat. Add 10 pints of capacity for every 100 watts of grow light.

Placement and Drainage

Put the dehumidifier above the canopy. Place it in the middle of the room. Keep 12 inches from walls. Use a drain hose. A 30-pint unit fills a 1-gallon bucket in 8 hours at 80% RH. A hose to a floor drain prevents shutdowns. Run the unit 24 hours per day during flower.

What a Dehumidifier Cannot Fix

A dehumidifier does not replace airflow. Botrytis spores settle on still buds. Add two oscillating fans per 4x4 area. Defoliate the lower third and the center of each plant. Space plants 12 inches apart. Remove dead leaves. A dehumidifier does not kill spores. It stops germination.

Sanitation After an Outbreak

Cut infected buds with a bag over the flower. Seal the bag. Do not compost. Wipe scissors with 70% isopropyl alcohol between cuts. Spray the room with 10% bleach or 3% hydrogen peroxide. Discard the grow medium. Spores live in dead tissue for months. A HEPA filter at 0.3 microns traps spores.