Artillery fungus is a tiny wood-decay fungus that lives in landscape mulch and shoots sticky black spore masses onto nearby surfaces like house siding, cars, and windows. Scientifically known as Sphaerobolus stellatus, each fruiting body is only about 2.5 mm (roughly 1/10 inch) across, but it can launch its spore packets up to 6 meters (about 20 feet) from its base. The result is dozens or hundreds of small tar-like specks that bond so tightly to surfaces they’re nearly impossible to remove.
What It Looks Like
The fungus itself is easy to miss. The fruiting bodies grow directly on mulch, decaying bark, or old wood and resemble tiny cream-colored or orange-brown cups. They typically appear in spring and autumn when moisture and moderate temperatures coincide. Areas of mulch colonized by the fungus may look matted, gray, or slightly bleached.
What most people actually notice are the spore masses (called peridioles) that have already landed on a surface. These are dark brown or black specks about 2 mm across, roughly the size of a pinhead. They look like tiny dots of tar or dried paint splatter. Because the fungus aims toward light and reflective surfaces, they tend to cluster on light-colored siding, white garage doors, and parked cars.
How It Launches Its Spores
The “artillery” name is well earned. Inside each tiny cup, the fungus builds up water pressure through a process similar to how a plant cell swells. It breaks down stored glycogen into smaller sugar molecules, which draws water into the cells of an elastic inner membrane. As pressure builds, the membrane suddenly flips inside out, like one of those rubber toy poppers that jump off a table. This snap-buckling motion launches the spore packet vertically at an estimated peak velocity of 10 meters per second, or about 22 miles per hour.
The fungus is phototropic, meaning it orients its launch toward the brightest light source. In practice, this often means it fires toward sunlit house walls, reflective car paint, or light-colored fencing. The spore masses can travel up to 6 meters horizontally, which is why mulch beds placed close to a house are the most common source of problems.
Why the Spores Won’t Come Off
Each spore mass is coated in an extremely sticky substance produced by degraded cells within the fungus structure. On contact with siding, glass, or automotive paint, this adhesive bonds almost immediately and hardens over time. The specks resist scrubbing, pressure washing, and most household cleaners. On vinyl siding, aggressive scraping can remove the spore body but often leaves a stain behind. On car finishes, removal attempts risk scratching the clear coat.
There is no reliable, damage-free removal method. Some homeowners report partial success with careful scraping using a plastic putty knife on cool days (heat softens vinyl and makes staining worse), but the adhesive residue typically remains. Prevention is far more practical than cleanup.
Where and When It Grows
Artillery fungus thrives in moist, decomposing wood. Hardwood mulch is its preferred habitat, especially mulch that has been in place for two or more years and has begun to break down. The fungus is most active during cool, damp weather, which is why homeowners typically see new spore spots appearing in spring and fall rather than midsummer.
Excessive rainfall, overwatering foundation plantings, and shady beds that stay damp all create ideal conditions. The fungus is present across much of North America and is especially common in the Mid-Atlantic and Northeastern states, where spring and autumn moisture levels are high.
Mulch Choices That Reduce Risk
No mulch is permanently immune. Penn State Extension tested 27 different mulch types in the field and found that all of them eventually supported artillery fungus after being outdoors for several years. However, some performed significantly better than others.
Large pine bark nuggets were the most resistant mulch in Penn State’s trials. Cypress mulch also performed well, likely because it contains natural antifungal compounds that slow decay. Bark-based mulches in general provide a less favorable environment than shredded hardwood blends.
Colored (dyed) mulches offered only a slight, temporary advantage. The dye chemicals themselves didn’t inhibit the fungus in lab tests. Instead, the dyed mulch was marginally more water-repellent when fresh, keeping the surface drier. Once rain and sunlight faded the color, the fungus colonized it readily.
The most effective strategy for sites with a history of artillery fungus is blending fresh mushroom compost into landscape mulch at a rate of 40% or more. Research from Penn State found this mix can significantly suppress the fungus, likely because competing microorganisms in the compost outcompete Sphaerobolus for resources.
Practical Steps to Minimize Damage
Because removal is so difficult, the goal is to keep the fungus from firing at your house in the first place. A few approaches make a real difference:
- Increase the mulch-to-wall gap. Since spore masses travel up to 20 feet, moving mulch beds farther from siding helps, though complete protection requires a larger buffer than most landscapes allow.
- Replace old mulch regularly. Mulch that has been decomposing in place for two to three years is prime habitat. Removing and replacing it with fresh material, especially pine bark nuggets, resets the clock.
- Improve drainage. Reducing irrigation near the foundation and improving airflow in shady beds makes conditions less hospitable.
- Use stone or rubber mulch in high-risk zones. Inorganic mulches directly adjacent to the house eliminate the food source entirely, though they don’t offer the soil benefits of wood mulch.
- Add mushroom compost. Mixing spent mushroom compost into wood mulch at 40% or higher acts as a biological suppressant.
Is Artillery Fungus Harmful to People?
Artillery fungus poses no specific known toxicity to humans. It is not a pathogen, and its spore masses are not airborne in the way that mold spores are. The primary concern is cosmetic damage to property. In general, outdoor fungal spore exposure can aggravate asthma or allergies in people who are already sensitized to fungi, but this applies broadly to the thousands of fungal species present in any outdoor environment and is not a unique risk from artillery fungus. Handling colonized mulch with bare hands is fine, and the fungus does not damage plants or soil health.