What to Know About Slime Mold on Grass

Patch of dog vomit slime mold.

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Some of the worst-looking things that you will ever come across in your landscape turn out really not to be that bad for your plants at all. For example, when large milkweed bugs (Oncopeltus fasciatus) swarm onto your garden plants, you may think that a plague of biblical proportions is being visited upon you, but these bugs do very little actual harm. Likewise, powdery mildew looks awful on plants, but it is not the killer that some other fungi are. Perhaps the best example of these bark-is-worse-than-their-bite organisms is dog vomit slime mold (Fuligo septica).

What Slime Mold Is

"Slime mold" is an umbrella term. It does not refer to one particular organism. In fact, there are hundreds of different types, and not all of them are even closely related to each other. Dog vomit slime mold, with its bright yellow color, is one of the more spectacular of them. Slime mold can appear on numerous different objects in the landscape, including on compost, on mulch, and on logs, but our focus is on slime mold that appears on your lawn grass.

Visually, slime mold reminds people of a fungus. This is one reason why, despite technically not being classified as a fungus, it is often referred to as a "fungus-like organism." While dog vomit slime mold belongs to the Fuligo genus, other common types of slime mold belong to:

  • The Physarum genus
  • The Mucilago genus

This fungus-like organism is a saprophyte. A saprophytic organism obtains food from dead organisms and decaying organic matter. Many mushrooms (which are the fruiting bodies of fungi) are also saprophytes; another trait slime mold shares with them is reproduction via spores.

Can Slime Mold Cause Any Harm to Lawn Grass?

While homeowners dislike having slime mold on their grass, the issue is mainly about lawn aesthetics. Slime mold has little effect on lawn health. At worst, a very large patch of slime mold that covers an area of your lawn for a long time can temporarily deprive the affected area of sunlight, rob the grass there of its ability to conduct photosynthesis, and thereby cause the grass to turn yellow for a short time.

Signs That You Have Slime Mold on Your Lawn

Some landscaping problems are difficult to detect. For example, you have to look for signs to detect root rot in a timely manner, because it develops underground, out of sight. This is not true of slime mold, which becomes an issue only when its fruiting bodies appear.

The fruiting bodies appear in plain sight. When you have dog vomit slime mold on your lawn, you know it right away: You really can't miss its bright yellow fruiting bodies. Other types of slime mold may not show up as clearly because their fruiting bodies are a duller color (gray, for example), but they are still visible. Indeed, since slime mold is an aesthetic issue, it is, by definition, easy to spot.

Why Slime Mold Appears in Your Landscape

Wood-based mulches often serve as a Trojan Horse that allows slime mold spores to infiltrate the landscape. Once the spores are present, all they need are three ingredients to be present to produce slime mold: warm weather, moisture, and dead organisms or decaying organic matter for the slime mold to feed on.

Does Slime Mold Have Any Benefits?

If you are looking for a silver lining to putting up with having slime mold on your lawn, you are in luck. Because slime mold obtains food from dead organisms and decaying organic matter, it may help you get rid of some bacteria and pathogens in the landscape that could otherwise cause problems for your plants.

How to Remove Slime Mold

Physically removing the unsightly part of the slime mold (the bizarre yellow mass, in the case of dog vomit slime mold) is as easy as can be. Just don't confuse "removing" here with "eradicating." To physically remove slime mold from your grass, just scoop it off with a shovel and dispose of it in the trash.

But eradicating slime mold is another matter altogether. That's because the fruiting bodies leave spores behind, which will generate future slime mold. Nor are chemical treatments effective in keeping slime mold from coming back. It's better to tolerate slime mold than to apply an ineffective chemical that may have unintended consequences in the landscape.

How to Prevent Slime Mold

You can approach slime mold prevention in a few different ways. Approaches can target minimizing the spore population, moisture, or decaying organic matter for the slime mold to feed on.

You can minimize the spore population by switching from a wood-based mulch to inorganic one, such as crushed stone. However, stone mulch is not superior to organic mulch in all ways. Moreover, spores can easily blow onto your landscape from other people's properties.

Minimizing moisture is also a challenge, since landscapes and water practically go hand-in-hand. But at least ensure that your yard has good drainage, so that excess moisture doesn't hang around.

Finally, you can minimize decaying organic matter in the yard by dethatching your lawn and bagging grass clippings whenever you mow your grass.