How to Treat Mildew or Mold in Soil

Updated February 21, 2017

It is common to see mould and its partner mildew on plants and in soil. Mildew, a growth produced by moulds, can appear ashy, white or blue-green. Mold is a simple form of plant life belonging to the family of fungi. Both appear when conditions are moist to overly wet and when there is a lack of heat and air circulation. Molds are found in outdoor soils, usually in pots, but also in shady, overly crowded areas that retain moisture. Indoor moulds and mildew do not hurt plants but can be a problem for people with allergies.

Put a mask over your mouth and nose if you are sensitive to allergens. Spread the black tarp out in a sunny, open location. Scrape off the top half inch of soil in outdoor beds that have been infected with mould and mildew. This will remove the organisms in the soil that are making mould appear.

Spread the soil out on the tarp and let it dry in the sun for a day. Every two or three hours disturb and redistribute the soil so all the particles are exposed to the heat and light. The sun will kill the mould spores and in turn the mildew. Return the soil to the garden bed after it has "cooked" for a day.

Use a fungicide to spray any foliage in the bed that has mildew on it, following manufacturer's instructions. To prevent the problem from recurring, prune limbs on trees or shrubs that may be hindering sunlight. Allow a little sun into the garden bed and increase the air movement by removing overcrowded vegetation.

Look for the presence of aphids or scale insects, which secrete honeydew as a byproduct of their feeding, a sweet substance that will cause sooty mould. This will usually form on foliage, but it can drip onto soil. Get rid of the pests with a safe insecticidal soap, using as directed.

Put on your mask if you are affected by allergens. Scrape off the top surface of the soil where you see mould and dispose of it. Move the plant to a warmer area to dry out the soil. Do not water for a few days or until soil is dry. The presence of mould and mildew indoors indicates too much moisture, which is probably from over-watering.

Place a fan in the room where the plant is kept. An increase in air circulation will help keep the soil from staying so sodden and giving mould spores an opportunity to grow. Alternatively, you can put the plant outside for two or three days, if it is not too cold for the plant.

Check the plant for the presence of insects. Ants in the pot can indicate the presence of aphids. Ants and aphids are often together because aphids produce honeydew, which ants love. Honeydew in turn forms sooty mould. You should put the plant in the sink and rinse it off or spray the plant with an insecticidal soap. Some types of scale insects also produce honeydew.

Look the soil over carefully in a week. If mould and mildew are still present, you will have to repot. Before filling the pot with new soil, wash the pot thoroughly in a gallon of water with a tablespoon of bleach. This will kill any spores or disease the pot may still contain. Let the pot air dry completely, then fill with soil and repot the plant.

Things You'll Need

  • Mask
  • Black tarp
  • Shovel
  • Fungicide
  • Insecticidal soap
  • Pruners
  • Trowel
  • Fan
  • Potting soil
  • Water
  • Bleach
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About the Author

Bonnie Grant began writing professionally in 1990. She has been published on various websites, specializing in garden-related instructional articles. Grant recently earned a Bachelor of Arts in business management with a hospitality focus from South Seattle Community College.