What's Growing On My Mulch?
Tom Creswell, Plant and Pest Diagnostic Lab Director, Purdue University
When rains are frequent, moisture dependent slime molds often make an appearance on mulch. Slime molds develop rapidly and change color and texture over time; usually going from shiny and brightly colored, to powdery and gray or brown as they mature and get ready to release powdery spores which blow to new locations.
Slime molds get their nutrients from bacteria and small bits of organic matter which is why it is common to see them growing on mulch. Although slime molds may grow up onto nearby plants, like turf or vegetable in the garden, they do not harm plants. After several days of dry weather slime molds will usually be less noticeable. If you want to speed this process, you may rake the mulch or affected areas of the lawn to promote air drying. However, slime molds will fade in time without any action on your part, and will usually return with the rain.
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For more in depth reading on slime molds visit these articles: