Why is there mold or fungus growing in my mulch?

Your mulch is growing fungus because it is supposed to. Wood and bark are food for fungi and slime molds, and when rain soaks the bed they push up mushrooms, foam, threads or tiny cups. Plant pathologists call them saprobes, organisms that live on dead plant material and do not infect plants or cause plant diseases.^[1] Most need nothing more than a rake. One, artillery fungus, is worth acting on.

Why it shows up when it does

Penn State Extension says of these mulch fungi that they are not harmful to landscape plants and carry no known health hazards unless eaten,^[2] and that they turn up from April through October, usually after rainy weather.^[2] The fungus lives in the mulch all year; the mushroom stage usually shows after long periods of rain.^[1] A wet spring or summer in Virginia, a sprinkler that runs too often, or mulch piled deep enough to stay damp makes it more likely.

What you are probably looking at

Mushrooms. Small brown caps or larger ones, often in clusters after rain. They are harmless to plants, but some mushroom species are poisonous,^[1] so remove them where children or pets play, and wear gloves if you remove them by hand.^[1] Expect them back: the fungal threads stay in the mulch, so mushrooms may come back in wet weather.^[1]

Stinkhorns. You will smell these first. Stinkhorns are foul-smelling mushrooms, often horn-shaped with slimy caps.^[1] Clemson Extension advises you to scoop them up as soon as they appear and to consider replacing a woody hardwood bark mulch with pine needles, pine bark or a composted mulch.^[4]

Yellow or orange foam. This is a slime mold, often Fuligo septica, the common slime mold that looks like dog vomit or scrambled eggs.^[3] It turns brown and powdery as it dries. Slime molds do not cause diseases,^[3] although they can block sunlight and yellow the leaves they grow over.^[3] The remedy is to break them up and dry them out, or rake them up and dispose of them.^[3] No pesticides are recommended.^[4]

Tiny nests. Bird's nest fungi look like tiny nests about a quarter inch across holding several tiny eggs.^[1] They are harmless, and their spores occasionally stick to surfaces but are easily removed and do not leave a stain.^[2]

White threads or a crusty mat. That is mycelium, the body of the fungus. When thick layers of dry mulch are applied or existing mulch dries, some fungi form a mat of mycelium that repels water.^[4] Plants under it can go dry even after rain. Clemson's fix is to break the matted layers apart with a rake or claw cultivator, and to water newly spread mulch.^[4]

Artillery fungus: the one to take seriously

If you see small black specks, like tar spots, on light siding, windows or a car parked beside a bed, look in the mulch for a tiny cream or orange-brown cup about a tenth of an inch across holding one black egg.^[2] That is artillery fungus. It aims toward bright surfaces such as light-colored houses and parked cars,^[2] and it shoots its black, sticky spore mass, which can be windblown as high as the second story of a house.^[2] Once stuck, a speck is very difficult to remove without damaging the surface, and if removed, it leaves a stain.^[2]

The fix is in the bed, not on the wall. The University of Kentucky advises you to remove the mulch containing the fungus along with a substantial portion of the surrounding mulch.^[1] Clemson suggests you use pure bark mulches, especially pine, or cover hardwood mulch that is already in place with pine needles.^[4] Penn State researchers found that blending 40 percent used mushroom compost into landscape mulch greatly suppresses artillery fungus.^[2]

Make the bed less inviting

Moisture drives all of this. Managing moisture, such as irrigation, or improving drainage may help limit these organisms.^[1] Water the plants deeply and less often rather than misting the mulch, since light watering may only wet the mulch layer while the soil stays dry.^[5]

Depth matters too. Maryland Extension's rule is that organic mulch should not exceed three inches,^[5] and a thinner layer dries out between rains. When you buy more, Virginia Tech recommends mulches that do not have a rotten egg or ammonia odor,^[6] which is a sign of badly composted material. For depth and placement, see how to mulch flower beds the right way, and for when to add or remove mulch, how often to replace mulch.

Stop and call a pro if

• Mushrooms are growing from the base of a tree or a shrub's stem. Mushrooms growing at the base of trees, or attached to living tissue, may indicate root and crown decay,^[1] which needs an arborist or your county Extension office.
• Artillery fungus spots cover siding or a car and you want them cleaned; the spore mass is very difficult to remove without damaging the surface.^[2]
• Plants under the mulch are wilting, yellowing or dying back, which points to a root or water problem rather than mulch fungi.
• Water stands in the bed after rain and the fix is grading or drainage.

Browse the lawn and landscaping answers, see mulch and bed services, or look up lawn and landscaping businesses around Blacksburg. To have affected mulch cleared and replaced, describe the job once and local businesses quote it.