Test Your Knowledge - June

“Dog vomit” slime mold growing on mulch
“Dog vomit” slime mold growing on mulch
© Thomas F. Lytle

Yes, this garish yellow mass is called “dog vomit” slime mold. Slime molds are fungus-like organisms. This particular one is commonly found on mulch, typically on hardwood mulches that contain a high proportion of wood.

Slime molds often appear suddenly in moist, warm weather. They can be light pink, brown, yellow, orange, tan, and other colors. Slime molds are harmless to plants, despite their startling appearance. They feed on decaying organic matter, such as mulch, and also on bacteria and other microbes. New hardwood mulch is an excellent source of food for this particular slime mold. Once the mulch is older and more decayed, it is less likely to support slime mold growth.

Slime molds can move across the landscape, extending finger-like protrusions. Some types of slime molds grow so rapidly under very moist conditions that they were the inspiration for the movie “The Blob”. Occasionally a patch of slime mold will engulf or appear to climb the stems of plants. This is also not harmful, although it can cause some bleaching of foliage that is shaded by the mass.

Some slime molds occur on lawn grasses. From a distance, the grass will appear to have turned black, gray, or even pink overnight. On close inspection, grass blades will be covered with thousands of tiny bead-like specks of a slime mold. Slime mold on lawns usually occurs after rainfall or under frequent irrigation. Slime mold that returns daily on lawns often indicates that the lawn is being watered too often.

Pink slime mold on turf
Pink-colored slime mold on turf grass
Flickr: BugMan50

Another slime mold on turf
Slime mold on turf

There are no pesticide sprays for slime molds, and no need to control these odd but harmless organisms. When slime mold appears on mulch, it can be scooped up with a shovel and disposed of. Changing to another kind of mulch, such as pine bark or pine straw can prevent “dog vomit” slime mold by removing its preferred diet.

Slime mold on lawns can be rinsed off with a vigorous spray of water from a garden hose. Also determine if overly frequent watering could be causing slime mold to thrive.

For more information on slime molds, see HGIC 2354, Spanish Moss, Lichens & Slime Molds and HGIC 1604, Mulch.

Karen Russ
HGIC Horticulture Specialist

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This information is supplied with the understanding that no discrimination is intended and no endorsement by the Clemson University Cooperative Extension Service is implied. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.