How to Kill Mildew on Indoor Houseplant Soil

Mildew can form on the soil of any houseplant.

Molds and mildews are common in all environments, but are especially virulent in the summer months when nights are cool and humid and days are warm and dry. They can affect both indoor and outdoor plants and can sometimes grow on the soil surface. Watch your houseplant soil for patches of fuzzy white or gray powdery growth. Although it looks unattractive and can be worrisome, mildew is not harmful to your plant. However, it can cause respiratory problems in susceptible individuals.

Things You'll Need

• Dust mask
• Gloves
• Small trowel or spoon
• Bleach
• Small bucket or bowl
• Sponge or rag
• Potting soil
• Vinegar
• Sulfur
• Neem oil
• Sprayer

Instructions

1. Repot your plant. After you remove it from its pot, take it outdoors and thoroughly wash all potting soil from the roots. Scrub the pot using a solution of household bleach mixed about 50-50 with water. Use new, uncontaminated potting soil to replace the soil that had mildew.

2. Remove the top layer of potting soil if it’s not possible to repot the entire plant. Scoop out the top inch or so of soil with a spoon or trowel and dispose of it in a sealed bag. Replace this soil with new, uncontaminated potting soil.

3. Improve air circulation and reduce the humidity in the area where your plant resides. Allow the soil to become dry before you water your plant again. Providing additional light can help to prevent the mildew spores from growing; many kinds of houseplants will not suffer when you give them a little more light. Keep an eye on your plant, however, and be cautious about placing most plants in direct sun.

4. Spray a vinegar solution on the soil surface if the mildew returns. Mix 3 tbsp. of apple cider vinegar with 5-percent acidity with 1 gallon of water, place it in a hand sprayer and douse the soil in the morning. One folk remedy says that sprinkling ground cinnamon on the soil will help to control mildew.

5. Spray the soil with a natural fungicide, such as sulfur or neem oil. These products are sold at garden supply stores. Always follow label instructions for correct mixing and application.

Tips & Warnings

• The best way to keep mildew away from your houseplants and their soil is to provide better air circulation, let the soil become dry between waterings and keep the humidity lower. Moving your plant to an area with a bit more light can also help to prevent mildew from forming.

• Many molds and mildews exist, some of which are toxic to humans. To be on the safe side, wear rubber gloves and a dust mask when working with infested plants or soil.

References

• Colorado State University Extension; Powdery Mildews; B. Edmunds, et al.; May 2009
• University of Minnesota Extension; Powdery Mildew of Houseplants; Janna Beckerman; April 2003
• The Garden Helper: Causes and Cures for Mold and Mildew Growing on House Plant Soil
• North Dakota State University Extension; Questions on Misc. Houseplants; Ron Smith
• Golden Harvest Organics: Plant Diseases

Resources

• University of California, Davis: Active Ingredient — Neem Oil