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Mulling Over Mysterious Matter in Mulch

Many homeowners utilize organic mulches in their home landscape. However, after a while, if not maintained or refreshed, mysterious things can begin to appear in mulched areas.

Before we explore the mysterious, let’s talk about basic mulching techniques and practices.

There are a variety of organic mulch sources and colors available. The benefits of mulch include aesthetics, weed suppression, mitigation of water loss, and a visible cue to mowers to help avoid damaging the bases of trees with the mower deck.

In general, 2-3 inches of mulch is sufficient in the primary root zone of a plant, or within the planting bed area. Never “volcano mulch” around a tree or shrub. In fact, no mulch should be in contact with the trunk; it should be pulled back an inch or two to leave an air gap. (For more information on mulching around trees, see Purdue plant disease diagnostician John Bonkowski’s article, “Mounds upon Mounds of Mulch,” at

So, what is that mysterious matter in your mulch?

Depending on your situation, you may see one of three common things in mulch, or things that can result from unrefreshed mulch.

First, you may see slime molds. These are nasty-looking blobs of yellow stuff that literally look like dog vomit (apologies for being so graphic). In a day or two, it turns brown. Former Purdue plant disease diagnostician, Dr. Gail Ruhl (now retired), explained about this fungus in a recent article.

She said that slime molds are not parasitic and therefore do not cause disease. “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,” she said. “One particular slime mold, aptly named ‘dog vomit slime mold,’ Fuligo septica, is very common in landscapes and draws the most attention.”

After several days of dry weather slime molds will usually become less noticeable. “Although dried masses can be removed with a shovel, slime molds will likely reappear with the return of wet weather,” she said. “Managing moisture (e.g. irrigation/drainage) may help limit or discourage their emergence.” Aerating your mulch with a rake may also help.

Find Ruhl’s original article at:

A second mysterious thing that may be present in wood mulches is bird’s nest fungi. These fungi resemble miniature bird’s nests, only about ¼ to ½ inch wide, complete with tiny “eggs” inside. In fact, the “eggs” are structures containing spores that splash out of the “nest” with rain, then split open to release the spores when the “egg” dries. These fungi are not a problem, and provide an interesting nature lesson for you to share with children.

Finally, something that can be problematic is artillery fungi, also known as shotgun fungi. This fungus grows in old mulch, making the case for adding new mulch every year or two to cover the old mulch. The fruiting body of this mulch can orient itself toward bright or light-colored surfaces, such as house siding, parked vehicles, or the undersides of tree or shrub leaves. When spores are released, they exit with enough velocity to reach the second story of a building (hence the name). The sticky spore mass is very hard to remove, and looks like a tiny speck of tar. Some have described the spore mass as looking like fly specks or fly dung.

Later this fall or early winter, many homeowners will be mulching their vulnerable landscape plants for winter protection.

Former Purdue consumer horticulture specialist, Rosie Lerner, wrote that timing is critical when applying winter mulch. Applying it too early can smother the plant and encourage disease development. “Once the plants are completely dormant and temperatures are consistently below freezing, then the winter mulch can be applied,” she said. “In most cases, 2 to 4 inches of organic material, such as straw, pine needles, hay or bark chips, will provide adequate protection.”

Find Lerner’s original article at:


(Banner photo: Purdue University/Gail Ruhl)

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