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Greenhouse Updates: March 21, 2022

Mold on the Surface of Growing Media
March 21, 2022

Last week, mold was observed on the surface of growing media in a greenhouse. During prolonged periods of overcast or rainy weather, growing media can be slow to dry out after watering. This creates an ideal environment for mold. Mold can grow on almost any organic material including peat-based media if the humidity is high. These molds are not pathogenic or harmful to plants or people in any way. They are saprophytic fungi which live on dead plant material such bark, peat, coir or compost. Although they are not harmful to the plants, they can be unsightly and a nuisance. In severe cases these molds can form a layer on the surface of the growing media and limit water penetration.

To control these molds, reduce irrigation during periods of overcast weather and increase airflow to lower the relative humidity in the greenhouse.  Growers who are concerned about appearance can remove the fungi manually.

These molds should not be confused with algae (green slime), which is a more serious problem. Algae creates an ideal environment for fungus gnats and shore flies. Fungus gnats not only feed on algae and other fungal growth in the growing medium, but also on plant roots, and can transfer plant pathogens through their mouthparts. Algae can also form an impermeable layer on the media surface making it difficult for water to properly wet the growing media.

Mold on media 1 (G. Njue) Mold on media 2 (J. Mussoni)








- Geoffrey Njue, UMass Extension Greenhouse Crops and Floriculture Program with Dr. Angela Madeiras, UMass Extension Plant Diagnostic Lab, and Jim Mussoni, Private IPM Scout.