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Mold in House Plants

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The idea of mold spores lurking in and around your indoor garden can be creepy, but that's what they do. That white substance on the soil of your prized rubber plant may just be fertilizer salts left by incomplete watering, or it may be a fungus that can affect a child's allergies. Learn to recognize mold on houseplants and keep it from setting up colonies where you live and breathe.


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Molds are organisms that grow by distributing spores which grow and create colonies. Mold is also known as mildew or fungus. Mold spores are found everywhere in the atmosphere but they will root and grow only where moisture and a source of food, like decaying organic matter, is present. Mold’s place in nature is to aid the process of decomposition of plant and animal materials.


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Most molds on houseplants pose hazards for sensitive individuals only when they spread to surrounding surfaces, according to Dr. Thad Godish, professor of Natural Resources and Environmental Management at Ball State University. They do, however, pose a nuisance because they can affect plants and can ruin the appearance of plants and soil.


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Houseplant mold comes from mold spores that travel in soil, on plants, and in the air. They only bloom and grow, however, when they are treated to a warm, moist environment such as exists in a centrally heated home where the gardener waters her host plants frequently but not deeply. Mold grows on decaying leaves, declining plants and moist soil.


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The common white or yellowish molds that grow on soils are often saprophytic soil fungus or other phylloplane species. The dusty white or gray coating often found on African violet or begonia leaves is a fungus called powdery mildew. A third type of mold, botrytis gray mold, is a common disease of greenhouse floral crops and may come into the house on gift plants like poinsettias, cineraria or cyclamen.


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Deny mold a friendly environment in the indoor garden. Use sterile growing media and replace mold-infested media. Water plants deeply and infrequently; shallow watering encourages growth near the soil’s surface, not at root level. The Ohio State University recommends keeping indoor humidity below 85 percent, maintaining good air circulation around plants, and using copper-based fungicides to control botrytis mold. Cornell University’s Plant Disease Diagnostic Clinic suggests removing and destroying affected plant tissue and treating powdery mildew with fungicides containing potassium bicarbonate, Bacillus subtilis or neem oil.


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Never reuse potting soil indoors. Water only when soil becomes dry; never let soil stay soggy. Use pots that are not too large to minimize the amount of water needed for moisture. Allow water to drain completely, perhaps in a tub or shower, before returning plants to their homes. Use only chemicals labeled for indoor use and follow directions carefully.

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