Tiny black specks suddenly appear on white siding, window frames, railings, and even the family car, and most homeowners assume insect droppings or tree sap. The real culprit is often artillery fungus (Sphaerobolus stellatus), a wood-decay fungus that lives in mulch and fires sticky spore packets at nearby surfaces. It earns the nicknames shotgun fungus and cannonball fungus because of the way it launches those spores, which can land several feet away and dry into stubborn, tar-like dots. The stains are unsightly, but the fungus is not harmful to humans or pets, does not kill garden plants, and causes no structural damage. Fixing the problem means identifying the source, cleaning the affected surfaces, and changing the conditions that let the fungus thrive, a sequence that mirrors how mold remediation starts with finding the moisture source before any cleaning begins.
What Is Artillery Fungus and Where Does It Grow?
A Fungus With Several Nicknames
Sphaerobolus stellatus is a saprophytic fungus, which means it feeds on dead and decaying wood rather than living plants. Its fruiting bodies are tiny cups about a tenth of an inch across that look like bird nests or flakes of pepper scattered through mulch. Inside each cup sits a small, dark spore mass, and when the cup matures it turns inside out and fires that spore mass into the air. The genus name comes from Greek and means sphere thrower, a fair description of a fungus that can hurl its spores several feet. Researchers have recorded launches that send the sticky packets upward and outward far enough to reach second-story windows.
Where the Fungus Lives
Hardwood bark mulch is the favorite home of this fungus. Large, fresh bark nuggets from trees such as oak, hickory, and pine hold moisture and break down slowly, which gives the fungus a long feeding window. The spores also grow on rotting stumps, old wood chips, and weathered lumber. Problems typically show up three to six months after fresh mulch is spread, which is why the first specks often appear in late spring or early summer following a spring mulching. Mulch that is aged, composted, or ground into fine particles is far less hospitable.
How the Spores Are Launched
The fruiting bodies orient themselves toward light before firing, and the spore packet is coated in a sticky glue that makes it adhere to whatever it hits. Light-colored surfaces that absorb heat, such as white vinyl siding and pale cars, attract the most spores because the fungus aims at the brightest, warmest target. Wet, warm weather triggers the heaviest firing. Confirming the source follows the same discipline as hazard identification and risk assessment on a construction site: look for the conditions that produce the problem before you treat the symptom, and in this case the condition is mulch.
The Life Cycle in Brief
- Fresh hardwood mulch is spread in spring.
- The fungus colonizes the mulch over the following weeks.
- Fruiting bodies mature in three to six months, usually by late spring or early summer.
- Warm, wet weather triggers spore firing.
- Spores survive the winter on siding, windows, and cars.
- The mulch breaks down over two to three years, and the fungus fades.
| Scientific name | Sphaerobolus stellatus |
| Common names | Shotgun fungus, cannonball fungus |
| Food source | Decaying hardwood bark and wood chips |
| What it stains | Light siding, windows, cars, outdoor furniture |
| Risk to humans and pets | None |
| Risk to plants and structures | None |
How to Identify Artillery Fungus Stains
What the Stains Look Like
The spore packets dry into small black or dark brown dots, most under a tenth of an inch across. They often appear in clusters or short lines where a single fruiting body fired a burst, and they collect on the sunniest side of the house. Fresh spots are sticky and can smear; dried spots are hard and resist scrubbing. Scrape one with a fingernail and the raised dot pops off as a hard, round pellet, which is the protective shell of the spore packet. That hardness is the key difference between artillery fungus and most other stains.
Lookalikes and How to Tell Them Apart
- Insect droppings: softer and irregular, often clustered below eaves or windows where insects rest.
- Sooty mold: a black powder that grows in colonies on sticky honeydew, usually on leaves and stems, and wipes off as dust.
- Tree sap: amber and glossy when fresh, flows down trunks and branches instead of appearing as dots on siding.
- Fly specks: smaller and more scattered, without the hard pellet core.
A Quick Field Test
Press the edge of a credit card or a fingernail under one of the dots. If it pops away as a single hard, round pellet with a light ring underneath, it is an artillery fungus spore packet. If it smears or crumbles, look for another cause.
Add a siding and mulch inspection to the same seasonal walk-around you already do with a fire safety and prevention checklist; catching the first specks in spring makes removal far easier than dealing with a full summer of firing.
How to Remove Artillery Fungus Stains
Artillery fungus is one of several household nuisances that arrive on a predictable schedule, much like cluster flies in the attic, which follow an equally predictable identification, treatment, and prevention cycle. Removal is tedious because the glue that holds the spores is designed to stick, so work from the gentlest method up.
Cleaning Siding and Hard Surfaces
Start with a plastic scraper or putty knife, which lifts dried pellets without scratching most siding, and follow with warm soapy water and a soft brush. For vinyl siding, a pressure washer with a wide fan tip and low pressure works, but test an inconspicuous corner first, because high pressure can crack or etch the panels. Oxalic-acid-based wood cleaners lift stains from painted wood, and a trisodium phosphate solution handles stubborn residue on masonry. Rinse thoroughly after any chemical treatment.
Cleaning Cars, Windows, and Outdoor Furniture
On car paint, wash promptly with soap and water, then use a clay bar to pull off remaining pellets; never scrape paint with a blade. On glass, a razor scraper held flat removes the dots easily. On outdoor furniture and railings, soapy water and a soft brush usually suffice. Old spores that have baked in the sun for months may leave permanent discoloration, especially on porous surfaces, which is why prompt cleaning matters.
When Stains Will Not Come Off
If spores have etched light-colored siding or baked into the surface through a long season, cleaning may not fully restore the finish. Repainting is the practical fix for siding, and replacement is reserved for badly stained panels. Prevention matters more than any removal method, because it stops the problem at the source.
Pressure Washing Done Right
- Use the widest fan nozzle, typically a 40-degree tip, and keep the pressure low.
- Hold the tip 12 to 18 inches from the surface and keep the stream moving.
- Apply cleaner from the bottom up so it does not streak.
- Rinse from the top down.
- Never aim the stream upward under the siding laps, where water can get behind the cladding.
Preventing Artillery Fungus From Coming Back
Choose Mulch That Does Not Feed the Fungus
The single most effective prevention step is to stop using fresh hardwood bark mulch within striking distance of the house. Extension services recommend keeping high-risk mulch at least several feet away from siding, and many homeowners remove bark mulch entirely from beds adjacent to the foundation. Safer options include compost, aged wood chips, pine straw, stone, and rubber mulch.
- Hardwood bark mulch: highest risk, especially fresh, large-particle bark.
- Aged or composted wood chips: moderate risk, safer once well broken down.
- Pine straw and pine bark fines: low risk.
- Stone and gravel: no risk, permanent.
- Rubber mulch: no risk, stays in place.
Maintain the Mulch You Already Have
Existing bark mulch can be made less hospitable. Rake it regularly to break up the surface and let it dry, keep the layer under 3 inches deep, and top it with compost rather than fresh bark. If you see the tiny pepper-flake cups, remove that mulch and replace it with a low-risk material. Turn the pile after heavy rain, because the fungus needs sustained moisture to fruit.
Work Safely Around the House
Removal and prevention work involves ladders, sprayers, and sometimes pressure washers, and the job goes safer when you apply the same mindset as construction site safety management: identify the hazards, assess the risk, and plan the sequence before you start. Set the ladder on firm ground, keep three points of contact, wear eye protection when spraying chemicals, and never work a wand from the top rungs.
A Seasonal Prevention Checklist
- Inspect siding, windows, and cars each spring for new specks.
- Check the age and type of mulch in beds near the house.
- Keep mulch 6 to 12 inches away from siding and foundation walls.
- Rake and turn mulch after wet spells.
- Replace suspect mulch with compost, stone, or pine straw.
A Long-Term Approach to Mulch and Exterior Care
When to Call a Professional
Large areas of stained siding, delicate surfaces such as stucco or cedar shingles, and multi-story walls all argue for professional cleaning. Soft-washing crews use low pressure and detergents that remove spores without driving water behind the cladding. Get two or three estimates and ask specifically how they handle mulch-related staining, because a crew that scrubs the siding without addressing the mulch leaves the same problem for next year.
Treat the Yard as One System
The same principle that drives spider control and prevention for residential construction applies to mulch management: keep the exterior maintained, remove the harborage, and nuisance populations drop on their own. Artillery fungus, spiders, and other pests all respond to the same basic housekeeping, which is why whole-yard prevention plans outperform spot treatments.
A mulch and siding plan belongs in the same annual maintenance file as the termite control methods homeowners rely on for identification, treatment, and prevention. Both are about catching small problems early, and both reward a consistent seasonal routine over emergency fixes. Check the mulch before you buy it, check the siding after every storm, and the specks that send neighbors to the car wash will stay off your house.