Causes of Mold in Flower Pots

At one point or another, many gardeners are faced with mold or fungi growing in their flower pots. While it is not typically damaging to plants, mold in flower pots is unsightly and indicates several problems that will eventually harm the roots of your plants. Knowing the causes of mold in flower pots is essential to successfully treat the problem, as well as prevent future infestations.

Excess moisture is the most common cause of mold in flower pots and it is the most preventable. In many instances, excess moisture is the result of unintentional overwatering. The accumulated moisture creates an ideal environment for mold and fungus, and before long a fine fuzz of mold will appear on the surface of the soil. Repotting the plant in fresh soil will eliminate a mold outbreak caused by excess moisture but it can be maintained only by providing no more than 1 inch of water per week and allowing the soil to dry out in the top inch between waterings.

Mold outbreaks due to excess moisture are often caused by overwatering, but sometimes poor drainage is the culprit. Dense, heavy potting soil that fails to adequately distribute moisture or a flower pot without drainage holes at the bottom will both contribute to poor drainage and provide a hospitable environment for mold growth. Switching to a light potting mix with added perlite will improve the drainage and the need for frequent watering but it will help only if the plants are potted in a container with one or two drainage holes at the base.

Many gardeners attempt to cut expenses by reusing soil or digging soil from their garden to use in flower pots. However, potting with previously used or natural garden soil increases the likelihood of mold outbreaks since both harbor a variety of spores. Repotting the plant in fresh soil will eliminate a current mold infestation, but it is best to avoid the problem in the first place by always using fresh, sterilized soil. If fresh soil is unavailable, it is possible to sterilize soil at home by placing it in an oven set to 180 to 200 degrees F for 30 minutes or by placing it in a microwave oven on high for 90 seconds.

Reusing flower pots is a common practice and one that typically has a positive outcome. However, flower pots with residual dirt inside provide a hospitable environment for mold spores, particularly if they have been stored for a long period. The spores will grow into mold as soon as they are exposed to the moisture of routine watering. Mold that occurs as a result of dirty flower pots is easily avoidable by thoroughly washing the pots in hot, soapy water after each use and allowing them to air dry in the sun.