For herbaceous ornamentals, University of Wisconsin–Madison Extension’s homemade powdery mildew spray is 1½ tablespoons baking soda, 3 tablespoons lightweight paraffin-based horticultural oil, and 1 gallon of water. It has been shown to be effective, but it is not a guaranteed cure or a universal recipe for every plant. Test it on a small area before treating more, and avoid spraying in conditions that can injure foliage.
Homemade powdery mildew spray recipe
Use the proportions published by University of Wisconsin–Madison Division of Extension for herbaceous ornamentals:
- 1½ tablespoons baking soda (sodium bicarbonate)
- 3 tablespoons lightweight, paraffin-based horticultural oil
- 1 gallon water
Mix the ingredients in the water and apply only after testing a small number of leaves or plants. Check the treated area for damage before expanding use. The source describes this mixture as having been shown effective, but does not provide a percentage efficacy figure or establish that it is safe for all ornamentals, edible crops, or houseplants.
Check the plant and conditions before spraying
Identify likely powdery mildew
Powdery mildew commonly appears as white, powder-like growth on upper or lower leaf surfaces and stems, as if sprinkled with talcum powder or powdered sugar. Different powdery mildew fungi tend to be host-specific, so a similar-looking infection on two plant types may involve different fungi.
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Consider whether treatment is needed
A mild infection on an ornamental may be mostly cosmetic. University of Minnesota Extension recommends tolerating low levels that do not affect growth or flowering; severe disease can cause substantial leaf loss on susceptible plants. Phlox, Monarda, and zinnia are among plants Wisconsin Extension identifies as very susceptible.
Screen for spray injury risks
Oils can injure plants when applied above 90°F, to water-stressed plants, or within two weeks of a sulfur application, according to UC IPM. Test first even when conditions seem suitable: a plant’s sensitivity and disease severity matter, and bicarbonates can also damage foliage.
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What baking soda can—and cannot—do
Baking soda is sodium bicarbonate. Bicarbonates act mainly by raising leaf-surface pH, making conditions less favorable for disease. However, University of Florida IFAS Extension says bicarbonate fungicides do not offer a high level of disease control. Sodium can build up with repeated baking-soda applications and damage foliage; UF/IFAS says sodium bicarbonate may damage leaves at solutions of 1% or more or after repeated use.
Do not assume a stronger homemade mixture will work better. Potassium bicarbonate is generally more effective and less phytotoxic than sodium bicarbonate, but it can still injure plants. UC IPM warns that potassium bicarbonate products can cause severe phytotoxicity on plants with advanced powdery mildew. Bicarbonates are water-soluble, so formulated products may need reapplication after rainfall as directed by their labels.
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How the main treatment options differ
| Option | Role described by extension guidance | Main caution |
|---|---|---|
| Homemade baking soda and horticultural oil | Wisconsin Extension describes the mixture above as having been shown effective on herbaceous ornamentals; no efficacy percentage is stated. | Test a small area. It is not established as suitable for every plant or use. |
| Formulated horticultural oil | UC IPM describes oils as working best as eradicants against powdery mildew. | Avoid use above 90°F, on water-stressed plants, or within two weeks of sulfur treatment. |
| Formulated potassium bicarbonate | Has some eradicant activity; bicarbonates overall provide limited control. | Can injure infected plants, especially when disease is advanced. Follow the product label. |
| Sulfur | UC IPM describes sulfur as preventive. | Do not apply oil within two weeks of a sulfur application. |
These descriptions do not establish that any product is safe for every species. For a formulated fungicide or horticultural oil, follow its current label for eligible plants, conditions, mixing, and application.
Prevention can reduce repeat infections
- Choose resistant varieties when available.
- Space plants adequately and prune or thin growth to improve airflow.
- Avoid excess nitrogen fertilizer, which can encourage susceptible growth.
- Remove infected plant debris at the end of the season.
- Tolerate low-level infections if they are not affecting growth or flowering.
Powdery mildew affects more than 10,000 plants, according to Minnesota Extension’s 2026 review, so a prevention approach suited to one ornamental may not protect others in the garden.
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Why other household spray recipes are not included
Wisconsin Extension’s home-remedies resource discusses milk, vinegar, hydrogen peroxide, compost tea, and garlic as remedies its webinar debunks; it highlights baking soda plus oil as a DIY treatment that shows some promise. The resource does not provide comparative trial results establishing those other household mixtures as treatments.
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