Artillery Fungus on Siding: The Stain You Can’t Remove

Artillery Fungus on Siding: The One Stain Even Professionals Can’t Fully Remove

We’re going to start this one differently than most of our other guides, because this topic deserves it: we’re not going to promise we can make this problem disappear completely, because in most cases, nobody can. If you’ve got small, hard, black or dark brown dots stuck to your siding near a mulch bed in Jacksonville, and no amount of scrubbing, pressure washing, or store-bought cleaner has touched them, you’re probably not dealing with dirt, mold, or algae. You’re dealing with artillery fungus, and it’s the single most stubborn exterior staining problem we encounter in this business.

We’d rather tell you that upfront than sell you a “guaranteed removal” service that doesn’t exist for this particular problem.

Call (904) 848-6644 to Talk Through Your Specific Situation

What Artillery Fungus Actually Is

Artillery fungus — scientific name Sphaerobolus stellatus, also called shotgun fungus or cannonball fungus — is a small fungus that grows in decomposing wood mulch, particularly hardwood and wood-chip mulch. It’s a common organism across the eastern United States, and Jacksonville’s warm, humid climate and heavy use of mulch in landscaping beds make it a regular visitor to homes throughout the area.

Here’s the part that surprises most homeowners: the fungus doesn’t grow on your siding. It launches itself there. Each fruiting body is tiny — typically under a tenth of an inch — and cup-shaped, holding a single dark spore mass called a peridiole. When conditions are right, the fungus forcibly ejects that spore mass toward the brightest nearby light source, which is why it’s classified as phototropic. Spores have been documented traveling up to 20 feet horizontally and over 6 feet vertically, which is exactly why homeowners find these spots not just at ground level, but partway up a two-story home.

This is also why light-colored siding gets hit hardest. The fungus is aiming for reflected light, and white or light-colored vinyl and fiber cement reflect the most — which unfortunately describes a huge share of Jacksonville homes.

If you’re dealing with a broader mix of dirt, mold, and pollen buildup alongside suspected fungus spotting, our house washing (soft wash) service handles the surrounding grime safely, giving you a clear baseline to see what’s actually fungus-related.

Why It Looks Like Tar, Dirt, or Insect Waste (And Why That Confuses People)

Homeowners almost never identify this correctly on their own the first time. The spots are small, dark, and speckled — they genuinely do resemble mud splatter, soot, or even insect droppings at a glance. That mistaken identity is part of why it goes untreated for so long: people assume a good rinse will handle “mud splatter,” and by the time they realize it isn’t budging, the spore has often had weeks or months to cure and bond to the surface.

Quick ways to tell it’s artillery fungus and not something else:

• The spots are raised and feel gritty or sandpaper-like, not flat like dirt or a water stain
• They’re concentrated on the lower few feet of siding, closest to a mulch bed, though scattered spots can appear much higher
• They show up more heavily on light-colored, sun-reflective surfaces
• They appeared during spring or fall — this fungus produces fruiting bodies mainly when temperatures sit in a specific moderate range, and doesn’t fruit above roughly 78°F, so Jacksonville’s outbreaks tend to cluster in cooler, wetter stretches of the year rather than peak summer
• Standard mold and mildew cleaner does nothing to it, because it’s not a mold or algae problem at all

Why Pressure Washing Alone Doesn’t Remove It — And Can Make It Worse

This is the single most important thing to understand, and it’s the reason we’re writing this instead of just selling a service: high pressure does not remove artillery fungus, and in some cases it makes the problem more visible and harder to deal with.

Here’s why. The spore mass adheres with an adhesive strength that professionals in this industry regularly compare to superglue. It isn’t sitting on top of the siding the way dirt or pollen does — it’s mechanically fused to the surface. Blasting it with high PSI doesn’t dislodge it; it just knocks loose the surrounding dirt and grime that was partially hiding it, which is why homeowners sometimes report that spots look more obvious immediately after a house wash, not less.

What actually happens when removal is attempted:

• Scraping (plastic scraper, credit card edge, or similar) can pop off the outer cap of the spore mass, but it very often leaves behind a brown or dark stain that doesn’t come out
• Steel wool or abrasive scrubbing can lighten spots but risks removing paint or finish on the siding underneath — trading one problem for another
• High pressure directed at the spots rarely removes the fused spore and instead risks the same siding damage we warn against on every soft wash job: stripped paint, forced water intrusion, or panel damage on vinyl
• Even specialized surfactant treatments with proper dwell time, which are the closest thing to a “best practice” here, typically get spots to fade or partially lift rather than disappear completely, especially once the adhesive has had weeks to fully cure

If a company tells you they can 100% guarantee complete removal of established artillery fungus spots, that’s a claim worth being skeptical of. The honest, documented reality — backed by pest and plant diagnostic researchers, not just pressure washing companies — is that prevention and speed matter far more than any cleaning method after the fact.

The Real Fix Isn’t on Your Siding — It’s in Your Mulch Bed

Because the fungus lives and reproduces in the mulch, not on your house, the actual long-term solution is addressing the source, not just the symptom.

What genuinely helps:

• Switch mulch types. Hardwood and wood-chip mulch are the primary hosts. Pine bark, cedar, cypress, and stone or gravel mulch are far more resistant to this fungus.
• Keep mulch beds at least a few feet from siding, especially on the side of the house that gets the least direct sun — shaded, moist conditions are exactly what the fungus favors.
• Turn and aerate mulch periodically. Letting it dry out and exposing buried material to light discourages fruiting.
• Replace old mulch entirely if you’ve had a repeat problem — established fungal colonies in old mulch are the most common reason the issue returns year after year in the same spot.
• Act fast on fresh mulch. New mulch that hasn’t yet developed fungal activity gives you a window to prevent the problem before it starts, rather than treating spots after they’ve cured on your siding.

What We Can Realistically Do

We’re not going to tell you we can make every spot vanish, because that wouldn’t be true. What we can do:

• Correctly identify the problem the first time, instead of treating it like standard mold or algae and wasting a treatment cycle on the wrong approach

• Soft wash the surrounding siding properly, which handles the actual dirt, mold, and pollen buildup and gives you the clearest possible baseline to see what’s fungus-related versus what’s just grime
• Attempt gentle, surface-safe treatment on fresh or lightly established spots, where success rates are meaningfully better than on spots that have had months to cure
• Give you a straight answer on what’s realistically fixable versus what will likely leave a faint shadow, so you’re not stuck guessing or repeatedly paying for a service that can’t deliver what it’s promising
• Talk through mulch and landscaping changes that prevent this from becoming a recurring problem on the same section of your home

Frequently Asked Questions — Artillery Fungus on Siding

Related Reading & Services

• House Washing (Soft Wash) — the correct method for the mold, algae, and pollen buildup surrounding fungus spots
• Window Seals, Electrical Outlets & Pressure Washers: What Actually Goes Wrong When You DIY — before you try scraping or high-pressure spot-treating this yourself
• Full FAQ Page — more honest answers on what we can and can’t fix