Artillery mold is a tiny wood-decaying fungus that launches sticky black spore masses onto nearby surfaces, leaving spots that look like specks of tar on your siding, car, or windows. The fungus itself is barely visible in your mulch, but the dark spots it shoots onto light-colored surfaces are the unmistakable calling card that brings most people searching for answers.
What Artillery Mold Looks Like
The spots artillery mold leaves on surfaces are tiny, only 1 to 2 millimeters in diameter, slightly raised or rounded. They start brown and darken with age until they look black. If you scrape one open, the center is off-white, finely granular, and gummy. People frequently mistake these spots for tar splatter, scale insects, or insect droppings.
In the mulch itself, the fungus is easy to overlook. The fruiting bodies are roughly 2 millimeters across, small enough to sit on the tip of a pencil. When mature, each one splits open into a tiny cup-shaped structure with a round spore mass (called a glebal mass) sitting in liquid at the bottom. That cup is the launch pad.
Artillery mold is a close relative of bird’s nest fungus, another common mulch dweller. The key difference: bird’s nest fungus has larger, more obvious cup-shaped structures with multiple egg-like spore packets inside, and it doesn’t shoot those packets onto your house.
How It Launches Spores
The fungus earns its name. As water and nutrients accumulate in the fruiting body, pressure builds until the cup suddenly inverts, catapulting the sticky spore mass toward the nearest light source. These projectiles can travel several feet, which is more than enough to reach siding, vehicles, or windows near a mulched bed.
This light-seeking behavior explains why artillery mold is especially problematic on the north side of buildings. Those shaded areas stay moist longer, creating ideal growing conditions in the mulch, while the fungus aims its spores toward reflected light bouncing off pale siding. Light-colored surfaces get hit hardest, both because the fungus targets brightness and because the dark spots show up most visibly against white or cream backgrounds.
Once a spore mass hits a surface, a sticky outer coating bonds it in place almost immediately. That adhesion is strong enough that removing the spot often damages the surface underneath, leaving a brown stain even if you manage to pry it off.
Where It Grows and Why
Artillery mold thrives in landscape mulch made from shredded recycled wood. These products are rich in cellulose, which the fungus breaks down easily. The finer the wood particles are ground, the worse the problem tends to be. Mulch with particle sizes below three-eighths of an inch in diameter creates the most favorable conditions. Fresh woody products like ground wood pallets are particularly troublesome unless they’ve been composted first.
Moisture is the critical factor. The fungus needs consistently damp conditions to fruit and fire, which is why shaded beds and north-facing foundation plantings are the usual trouble spots. Mulch that stays wet and is made of finely ground softwood creates a near-perfect habitat.
In natural settings, away from houses, the spore masses typically launch upward toward sunlight and land on grass or other vegetation. Grazing animals like deer, rabbits, and cattle eat the spore-coated plants, and the fungus survives passage through the digestive system. It then colonizes the animal’s dung, completing a dispersal cycle that has nothing to do with your vinyl siding but explains why the fungus is so widespread in the environment.
Is Artillery Mold Harmful?
Artillery mold poses no known health risks to people, pets, or plants. It is not toxic and doesn’t cause infections. The problem is entirely cosmetic, but that cosmetic damage can be serious enough to prompt homeowners to file insurance claims for siding replacement. The spots are so difficult to remove that replacing the affected material is sometimes the only clean fix.
Mulch Choices That Reduce Risk
No mulch is permanently immune. Researchers at Penn State tested 27 different mulch types in field conditions and found that all of them eventually supported artillery fungus after being outside for several years. But some performed significantly better than others.
Large pine bark nuggets were the most resistant mulch tested. They stay hard and dry, two conditions the fungus dislikes. Cypress mulch also performed well, likely because it contains natural anti-fungal compounds that slow decay. The common thread is that bark from mature softwood trees is made mostly of lignin, wax, and protected cellulose, all of which resist the rapid decomposition that artillery mold depends on.
Colored mulches showed only a slight, temporary advantage. The dyes themselves didn’t inhibit the fungus at the concentrations used, but the coloring process made the mulch slightly more water-repellent, keeping it drier at first. As rain and sun faded the color, the artillery fungus moved right in.
Stone mulch eliminates the problem entirely since there’s no organic material to colonize, but it offers none of the plant-health benefits of wood mulch.
Preventing Outbreaks
Your best strategy combines mulch selection with moisture management. Choose coarse bark nuggets over finely ground wood. If you do use recycled wood mulch, look for products with particle sizes above three-quarters of an inch, and don’t apply it deeper than two inches. Thicker layers hold more moisture and create a better environment for the fungus.
Avoid “sour” mulch, which has a pH as low as 2.5 to 4.8. Poorly stored mulch that smells like vinegar or alcohol has undergone anaerobic fermentation. This acidic environment not only promotes fungal growth but can be toxic to your plants.
If composting fresh wood products before use is an option, that extra step breaks down much of the easily digestible cellulose before the mulch ever reaches your beds, giving artillery mold less to feed on. Topping off existing mulch with a fresh layer each year can also help by burying older, colonized material, though the fungus will eventually work through new layers too.
Removing Spots From Surfaces
This is the frustrating part. There is no easy removal method. The adhesive coating on each spore mass bonds so tightly that it is nearly impossible to remove without damaging the surface underneath. Power washing alone rarely works, and scrubbing with dish soap produces minimal results.
Some homeowners have had success with a combination of physical scraping and chemical softening. A degreasing product applied full strength, combined with persistent scrubbing, can work on hard surfaces like vinyl siding, though it requires significant effort. The spots have the consistency and stubbornness of dried tar. Others have found that softening the spots with vinegar and baking soda, then rubbing by hand, loosens them from some surfaces.
On car paint, the stakes are higher because aggressive scrubbing risks scratching the clear coat. A clay bar designed for automotive detailing is sometimes effective at lifting the spots without damaging paint, but results vary depending on how long the spots have been in place. The longer they sit, the more firmly they bond and the darker the stain they leave behind.
For vinyl siding that’s heavily spotted, many homeowners ultimately replace the affected panels. The brown stains left after removal are permanent on most light-colored surfaces, and no cleaning product reliably eliminates them.