CORVALLIS, Ore. — It appears overnight and looks like a horror-show blob that’s slithered its way into your garden.
The gross-looking substance known as slime mold shows up on mulch and lawns but is harmless to plants. Instead, it feeds on decaying matter, fungi or bacteria, said Neil Bell, a retired horticulturist with the Oregon State University Extension Service.
Slime mold may look like it’s growing on plants, but it’s only consuming organic mulch such as bark dust or compost.
While active, slime mold can “flow” across mulch for several feet, eating dead matter as it goes.
“It’s called slime mold and is not harming the grass or plants in the mulch,” he said. “It appears really suddenly, seemingly overnight. It occurs after rainfall in summer.”
Slime mold — a primitive organism placed in a family by itself — ranges from several inches to more than 2 feet in diameter and appears in colors from bright yellow or orange to white or shiny black. In its fresh state, it’s often called “dog vomit” for its appearance. As conditions change, it dries, turns brown and produces spores that float away. In the woods, other colors are common, Bell said.
Slime mold may look like it’s growing on plants, but it’s only consuming organic mulch such as bark dust or compost.
In lawns, Bell said, slime mold resembles grayish black beads. It may be visible one morning and vanish the next.
“People are surprised to see it and want to do something,” Bell said. “You can rake it out to speed drying. It’s okay to put it in the compost pile or mix it into the mulch. Or spray it off with a blast of water from the hose. It’s certainly not something you’d apply a pesticide to because it will disappear of its own accord.”
Mulch matters more than the mold
Though it might be unsettling to see this amoeba-like organism in your mulch, Bell said it’s not a reason to skip mulching altogether. Organic matter is essential for soil and plant health.
Bark mulch — often called bark dust — is commonly used to hold moisture, reduce evaporation, cool soil, control erosion, prevent compaction, improve structure and add nutrients. Weed suppression is one of its biggest benefits.
“Bark mulch is high in carbon and low in nitrogen,” Bell said, “so weed seeds don’t tend to grow on it. It’s particularly effective on annual weeds. One thing you can count on: if you leave soil bare, it won’t be bare for long. The weeds will find it.”
Over time, bark mulch breaks down and compacts. In a year or two, it disappears, consumed by billions of soil microbes, so it must be reapplied every one to two years to remain effective.
Choosing and using organic mulches
Composted yard debris is another common mulching material. Like bark dust, it’s widely available, inexpensive and easy to spread. It breaks down faster than bark mulch and adds nutrients more quickly.
“A compost like yard debris is a lot richer as a growing environment than bark mulch,” Bell said. “It’s not as effective as weed control as a result. It’s better used as a soil amendment.”
Bell recommends applying 2 to 4 inches of bark mulch in spring, then fluffing it in fall to refresh it and reduce compaction that can repel water.
To learn more about mulch, refer to the Extension publication Mulching woody ornamentals with organic materials.
Previously titled Slime mold looks ugly in the garden but does no harm
