Causes of Mildew on Wooden Fences

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    • High relative humidity is a leading cause of mold and mildew growth. When humidity levels exceed 60 percent, the risk is especially high, according to the CDC. The excess water vapor penetrates the porous wood, allowing mold spores to germinate. As the spores begin to breed, mildew forms on the surface of the fence. While you cannot control outdoor humidity, you can reduce your risk of developing mold and mildew by painting your fence with a fungicidal paint or sealant.

    Lack of Sunlight

    • If your fence resides in a heavily shaded area, or in a location seldom affected by direct sunlight, mildew has a greater chance of forming than in an area with ample sunlight. Sunlight evaporates the natural moisture that occurs as a result of rainstorms, fog and morning condensation. If you lack sunlight, the moisture can remain trapped for long periods of time, giving mold spores ample time to germinate.


    • If you have plants and vines climbing or covering sections of your fence, you may develop mildew for multiple reasons. First, plants can block sunlight, which once again allows moisture to linger. Second, plants can inhibit air circulation, which is needed to draw moisture away from porous surfaces. Third, plants can develop fungi of their own, such as powdery mildew and downy mildew. Some strains of these fungi can spread to wood surfaces.

    Insufficient Sealing

    • Even if you live in a dry environment with no barriers to sunlight, moisture can still remain trapped deep within the surface of a wood fence. If your fence has not been sealed in several years, or if you sealed it with a minimally effective sealant such as a wax sealant, moisture can penetrate deep into the wood where no sunlight or air circulation can hope to reach it. Mildew can form in these types of environments.

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