Is Moldy Mulch Harmful to Vegetable Gardens?


Organic mulch provides an effective way to suppress weeds and nourish soil in vegetable gardens. Sooner or later, however, mold or fungi will grow in that organic matter. Some molds and fungi are harmless to vegetable gardens, according to Nature Hills Nursery, and even stimulate beneficial fungi. Some molds and fungi, however, are nuisances that harm plants, according to Harry A. J. Hoitink of the Department of Plant Pathology at Ohio State University.

Why Mulch

Garden mulch keeps down weeds, conserves moisture, discourages pests, controls erosion and moderates soil temperature, according to Iowa State University Extension.

Organic vs. Inorganic

Organic mulches, such as shredded leaves, pine needles, grass clippings and straw, eventually decompose, adding vital soil nutrients and increasing drainage. Inorganic mulches, such as black plastic and aluminum foil, last a long time and can be reused season after season.

Mulch Mold

Mulch material, temperature, pH and moisture determine whether your mulch mold is naughty or nice, according to Ohio State University. Shotgun or artillery fungus produces tiny spores that form black spots on plants and causes damage.

Slime Mold

Slime Mold (no longer classified as a fungus, but gardeners often mistake it for one) spreads in yellow or orange patches throughout a garden and can cover or shade plants.


Ohio State University advises gardeners to dig nuisance mold into surface soil and soak with water, or to heap mulch into a pile and soak with water. This will cause the mulch to heat and destroy the fungi.


  • Natare Hills Nursery
  • Iowa State University Extension
  • Control of Nuisance and Detrimental Molds (Fungi) in Mulches and Composts, Ohio State University

Who Can Help

  • Plant & Pest Digital Library, Purdue University
Keywords: mulch mold, mulch fungi, garden mold

About this Author

Lisa Kaplan Gordon's articles have appeared in "USA Today," "Washingtonian Magazine," "Redbook," "US Weekly" and in major newspapers throughout the United States. Gordon is a three-time winner of the National Headliners Award for Consistently Excellent Features Writing. She is creator of POWER WRITING, a corporate writing seminar, and is a private writing coach in McLean, Va.

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