Black Spots on Siding Near Mulch: Artillery Fungus Explained

Tiny black dots speckling your siding just above a mulch bed are most likely spore masses from artillery fungus, a mulch-dwelling fungus that literally shoots its sticky spores toward light-colored surfaces — siding, cars, downspouts, and windows. The dots look like flecks of tar, stick like glue, and are notoriously difficult to remove once cured. Importantly, it isn't a mold growing on your house: the fungus lives in the wood mulch below, and the fix is changing the mulch, not treating the wall.

Most likely causes

  • Artillery fungus in wood mulch — pinhead tar-like dots that won't wipe off
  • Spider or insect droppings — similar specks that do wipe away with a damp cloth
  • Mulch splash and mildew — smudgy gray-black staining low on the wall after rain
  • Sooty mold under a tree or shrubs — a filmy black coating, not discrete dots

Compare the possible causes

Possible cause Key signs When it happens How likely
Artillery fungus (Sphaerobolus) Uniform black or dark brown dots about 1/10 inch across, peppered across siding within roughly 8 feet of a mulch bed, glued on so firmly a fingernail won't budge them Cool, wet stretches in spring and fall, when temperatures sit roughly in the 50s and 60s and mulch stays damp Very common
Spider and insect droppings Dark specks concentrated under light fixtures, eaves, corners, and webs — and they soften and wipe away with warm soapy water Summer and early fall, when insect and spider activity peaks Common
Mulch splash and surface mildew A smudgy, uneven band of gray-to-black staining along the bottom foot or two of siding, worst under roof drip lines and after storms Rainy seasons, and anywhere gutters overflow or downspouts splash Common
Sooty mold from insects overhead A thin, sooty black film on siding, windowsills, or anything parked under a tree or large shrubs, often paired with sticky residue Mid to late summer, following aphid or scale infestations in trees above Less common

Visual clues to check

  • Try the wipe test on one spot with warm soapy water: spider droppings and mildew come off, artillery fungus stays put like a fleck of dried tar
  • Map the spots: artillery fungus concentrates on the lower 6 to 8 feet of wall facing a mulch bed, thinning out with height and distance
  • Check what's beneath: hardwood or bark mulch that's a few years old and stays damp is the fungus's home base — stone or rubber beds nearly rule it out
  • Look at nearby light-colored surfaces: matching dots on a white car, downspout, or fence facing the same bed confirm spores are being launched from the mulch
  • Inspect the mulch closely after a cool rainy spell for the source: clusters of tiny cream-to-orange cup-shaped fruiting bodies, each smaller than a BB
  • Pick at a dot's edge with a plastic scraper: artillery fungus spots pop off in a hard little disk, often leaving a faint brown stain beneath

The causes in detail

Artillery fungus (Sphaerobolus)

Artillery fungus — sometimes called shotgun fungus — grows in decaying hardwood mulch and produces tiny cream-colored, cup-like fruiting bodies that build internal pressure and fire a sticky black spore packet several feet into the air, aiming at bright, reflective surfaces the way it would aim at sunlight in the wild. That's why light-colored siding, white cars, gutters, and window frames collect the dots while dark surfaces nearby stay clean. Most spots land within 6 to 8 feet of the ground and close to the mulch bed. Each dot cures like varnish within days, which is why scrubbing barely touches it; on vinyl and painted surfaces, aggressive removal often damages the finish more than the spot does.

Spider and insect droppings

Spiders that camp near porch lights leave dark droppings dribbled and speckled on the siding below their webs, and various insects leave similar marks. The pattern gives it away: clustered beneath webs and light fixtures at any height, rather than spread evenly across the wall's lower few feet near mulch. The wipe test is definitive — these come off with a sponge, while artillery fungus spots do not.

Mulch splash and surface mildew

Rain hitting mulch and bare soil bounces fine dark debris onto the wall, and constantly damp siding grows patchy mildew on top of that. The result is diffuse staining rather than crisp, identical dots. Most of it washes off with a garden hose and a soft brush with house-wash solution — a cleanup difficulty of one, compared to artillery fungus's ten.

Sooty mold from insects overhead

When aphids or scale insects feed in a tree, they rain down sticky honeydew, and sooty mold grows on the coating below — on leaves, patios, cars, and siding. It reads as a continuous dark film rather than individual glued-on dots, and it washes off once the sticky layer is dissolved with soapy water. The fix is managing the insects in the tree, not treating the wall.

When to worry

  • Spots are multiplying each spring and fall — the mulch colony is expanding, and every season adds a new layer of nearly permanent dots
  • The dots are hitting a car regularly parked near the bed, where cured spores can damage paint during removal
  • You're preparing to sell or repaint: heavy accumulation may mean repainting or replacing siding sections, so stopping the source first matters
  • Spots keep appearing far from any mulch — that pattern doesn't fit artillery fungus, so it's worth ruling out an insect issue or a roof/gutter problem instead

What to do now

  1. Confirm the source before fighting the spots: find the fruiting bodies in the mulch during cool, damp weather, or match the spray pattern to the nearest bed
  2. Attack fresh spots fast — within the first days, before they cure, warm soapy water and a soft brush can still lift them; once hardened, removal without finish damage is unlikely
  3. Fix the mulch, not the wall: replace hardwood mulch within 10 to 15 feet of the house with stone, gravel, rubber mulch, or large pine bark nuggets, which resist the fungus far better than shredded hardwood
  4. If you keep wood mulch, bury the problem — turning the bed and topping it with a fresh 1-inch layer each year keeps the fungus's launching pads covered and dry
  5. Keep beds on the dry side: rake occasionally so mulch doesn't stay soggy, and redirect downspouts that keep one bed constantly wet
  6. For heavy accumulation on siding you can't live with, get quotes from an exterior cleaning professional and ask specifically about artillery fungus experience — and test any method on a hidden spot first

What not to do

  • Don't power-wash at high pressure hoping to blast the dots off — cured spores usually stay put while the pressure gouges vinyl, strips paint, and drives water behind siding
  • Don't scrub with razor blades, wire brushes, or harsh solvents; the scar left behind is often uglier and more permanent than the spot
  • Don't spray fungicide on the mulch or the wall — no product is effective against artillery fungus in landscape beds, so it's money and chemicals wasted
  • Don't pile fresh hardwood mulch on top of an infested bed and call it done without turning it; the fungus happily recolonizes the new layer as it decays
  • Don't assume the spots are mold on your house and treat the siding — the organism lives in the mulch, and treating the wall does nothing

Frequently asked questions

Can black artillery fungus spots be removed from siding?

Realistically, only if you catch them fresh — in the first few days, soapy water and a soft brush can lift them. Once the spore masses cure, they bond like dried varnish, and most removal methods damage vinyl or paint before they remove the spot. Many homeowners stop the source, then live with the existing dots or repaint.

Which mulch should I use to stop artillery fungus?

Near the house, inorganic wins: stone, gravel, or rubber mulch can't host the fungus at all. If you want an organic look, large pine bark nuggets resist it much better than shredded hardwood, which is the fungus's favorite food as it decays. Save the shredded hardwood for beds well away from siding and parked cars.

Is artillery fungus harmful to my house or my family?

It doesn't rot siding, harm the structure, or pose a meaningful health risk — the harm is stubborn cosmetic staining. The fungus feeds on the decaying mulch, not on your home. The family-safety note is general mulch-fungus common sense: nothing growing in a mulch bed should be eaten by kids or pets.

Why does it only hit my white siding and white car?

The fungus aims its spore packets at bright light. In nature that points the spores skyward toward gaps in the canopy, but next to your house the brightest target is a reflective light-colored surface — white siding, a white car, a bright downspout. Dark surfaces in the same spray zone often show few or no spots.

Will the fungus go away on its own?

Not while its food and moisture last. It fruits each cool, wet season — typically spring and fall — as long as damp, decaying hardwood mulch sits nearby, and each cycle adds spots. Colonies do decline as mulch fully breaks down, but replacing or covering the mulch ends it years sooner.

Related yard clues

Mushrooms & Growths Slime Mold in Mulch: The Gross-Looking Blob That's Harmless A foamy yellow, tan, or orange blob spreading across your mulch is almost certainly a slime mold — most often the species nicknamed 'dog vomit slime mold.' Despite the revolting look, it's completely harmless to plants, pets, and people, it usually rides in on fresh hardwood mulch, and it needs no treatment: rake it out, hose it apart, or just let it dry up on its own. Read the guide → Mushrooms & Growths Orange Fungus in Mulch: Stinkhorns, Slime, and Siding Spots Orange growths in mulch are almost always fungi feeding on the wood itself — most often stinkhorns (the smelly orange fingers), orange peel fungus, or a slime mold. They're harmless to plants and people who leave them alone. The one mulch fungus that costs money is artillery fungus, which shoots tar-like black dots onto siding and cars. Read the guide → Mushrooms & Growths White Mushrooms in Your Lawn: Why They Appear, What to Do White mushrooms popping up in a lawn are the fruiting bodies of fungi that are already living in your soil, usually feeding on decomposing roots, old stumps, or thatch. They surge after rain or heavy watering, and in most cases they're a sign of biologically healthy soil — not a lawn disease. The main job is keeping kids and pets from eating them. Read the guide → Mushrooms & Growths Yellow Mushrooms in Potted Plants: Harmless or a Problem? Bright yellow mushrooms popping up in a potted plant are almost certainly Leucocoprinus birnbaumii, the flowerpot parasol — a tropical fungus that rides into your home in potting soil and fruits when conditions turn warm and moist. It won't hurt your plant at all; it lives on decaying organic matter in the mix, not on roots. The only real concern is that the mushrooms are toxic if eaten, so households with small children or pets that nibble should remove them. Read the guide →

Still not sure? Compare with these clues

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