Quick dose
Mix 1 tablespoon (about 14 g) of potassium bicarbonate in 1 gallon (3.8 L) of water. That is near 3.7 g per liter. Add a surfactant to break surface tension. Spray until both leaf surfaces are wet but not dripping. Repeat every 7 to 14 days while conditions favor powdery mildew.
Dose chart
- 1 quart (0.95 L): 1 teaspoon, about 4.7 g
- 1 gallon (3.8 L): 1 tablespoon, about 14 g
- 5 gallons (19 L): 5 tablespoons, about 70 g
- 100 gallons (378 L): 2.5 to 5 lb of an 85% potassium bicarbonate product, which is the MilStop label rate
Commercial potassium bicarbonate products run 3 to 6 g per liter at label rates. Home mixes at the top of that range raise the risk of leaf burn.
Surfactant
Plain water beads on cannabis leaves. A non-ionic spreader at 0.25% to 0.5% by volume, or 2 to 3 drops of mild dish soap per gallon, improves coverage. Soap at high rates strips leaf wax. Test any mix on 2 or 3 leaves and wait 24 hours before spraying the whole plant.
Milk Method for Powdery Mildew on Marijuana Explained
How to apply
Spray the underside of leaves first. Powdery mildew colonies start there and on new growth. Cover the top surface second. Stop before runoff drips into the medium. Potassium bicarbonate adds potassium, and repeated drenches shift the nutrient balance in coco or soil.
Spray in the evening or under low light. Leaf burn shows up when mixes dry in full sun or at air temps above 85°F (29°C).
Timing and interval
Potassium bicarbonate is a contact fungicide. It kills spores and surface mycelium it touches. It does not move through plant tissue. New growth that opens after a spray is unprotected. Reapply after rain, after overhead watering, and every 7 days when humidity stays above 60% and temps sit between 68°F and 77°F (20°C to 25°C).
What it does
The salt raises pH at the leaf surface and disrupts ion balance in fungal cells. Greenhouse trials show suppression of powdery mildew, not eradication. It works as one part of a rotation: potassium bicarbonate, sulfur, hydrogen peroxide, and biologicals such as Bacillus amyloliquefaciens.
Limits
- No systemic action. It cannot reach mildew inside stems or deep in buds.
- Residue. It leaves a white film. That film matters in flower.
- Rotate modes of action. Repeated use of one contact material selects for tolerance.
- Check state rules. Some states list potassium bicarbonate for cannabis; others do not. Confirm with your regulator or testing lab.
Flower stage
Stop spraying potassium bicarbonate 2 weeks before harvest at minimum. In late flower the film sits on trichomes and can show up in visual grading. A bud wash after harvest removes some residue but also removes some terpenes.
Do not confuse it with baking soda
Sodium bicarbonate works the same way on contact, but it leaves sodium. Sodium builds up in soil and coco and harms roots over time. Potassium bicarbonate leaves potassium, which plants use. The tradeoff is a higher potassium load on the medium.