Mix 1 tablespoon (15 mL) of potassium bicarbonate per gallon (3.8 L) of water. Add 1 teaspoon (5 mL) of liquid soap. Spray both sides of leaves. Apply every 7 to 10 days. For heavy powdery mildew, some growers use 2 tablespoons per gallon. Test on a small area first.
powdery mildew on cannabis buds during flowering treatment
Dosage
Standard rate: 1 tablespoon per gallon. Heavy rate: 2 tablespoons per gallon. Do not exceed 2 tablespoons per gallon. One gallon covers about 1,000 square feet of leaf surface. For small plants, mix 1 teaspoon per quart (0.95 L).
powdery mildew on cannabis buds during flowering treatment
How It Works
Potassium bicarbonate is a contact fungicide. It raises the pH on the leaf surface. This disrupts the cell walls of powdery mildew fungi. It does not kill spores inside plant tissue. It works best as a preventive spray or at the first sign of infection.
Application Steps
- Mix potassium bicarbonate and soap in water. Stir until dissolved.
- Pour into a spray bottle or tank sprayer.
- Spray leaves top and bottom until wet but not dripping.
- Repeat every 7 to 10 days. Stop 2 weeks before harvest.
Cautions
- Do not spray in direct sun or when temperatures exceed 80°F (26.7°C). Leaf burn can occur.
- Do not mix with other fungicides or pesticides unless the label says it is safe.
- Potassium bicarbonate adds potassium. It can affect nutrient balance in hydroponics.
- No federal label exists for cannabis. Rates come from greenhouse and horticultural use.
What to Know for Cannabis
Cannabis is not a federally legal crop. The EPA has not registered potassium bicarbonate for cannabis. However, potassium bicarbonate is exempt from EPA registration under FIFRA 25(b) for many uses. Growers use it off-label. Powdery mildew on cannabis often appears in indoor grows with poor air flow. Keep humidity below 60%. Remove infected leaves before spraying.