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What Are Small Brown Dots on My Vinyl Siding?

James Young

Brown or black dots spattered on vinyl exterior siding could have several causes. One of the most common culprits grows in wood chip mulch often used as a weed control in flowerbeds and around trees and shrubs. Seasonal insect swarms could leave disfiguring spots on vinyl, and problems with irrigation or gutters cause spatters on foundations and lower walls. Whatever the cause, swift cleanup prevents permanent stains.

Artillery Fungus

Wood chip mulch could harbor the cause of dots on siding or cars.

Small brown or black spots on siding could be the spore masses or peridioles of the artillery fungus. Sphaerobolus stellatus, or sphere thrower, colonizes wood or bark chip mulch and launches sticky capsules of spores at the strongest source of light. This common fungus could bombard the bright vinyl walls of a house repeatedly. The fruiting bodies, which measure only 2 mm across, remain active for two to three weeks. Spores travel up to 20 feet and reach heights of 6 feet. The peridioles dry hard enough to require removal by scraping or power washing. Dried peridioles leave long-lasting stains and could damage paint on homes and cars.

Summer Insects

Masses of dots higher on walls could be the dried fecal spatters left behind by swarming insects. Flies attracted to the warmth and light of vinyl siding regurgitate partly digested food from their mouths and defecate wherever they land. Flies leave lighter-colored specks from the front and darker specks from the rear. Swarms of night insects attracted to light-colored siding and security lights could also leave fecal stains. Spiders weaving webs in infested areas concentrate greasy fecal pellets in small areas. Insects or arachnids probably caused any specks deposited higher than 6 feet on walls.

Water Spatters

Brown specks along foundations and the lower parts of walls could result from defective gutter systems along the eaves as well as from poorly adjusted sprinklers. Gutters leaking at joints or overflowing because of clogged downspouts cause mud spatters when drips or streams strike bare ground. Leaking gutters could erode lawns and cause moisture problems in basements as well. Automatic irrigation systems cause similar problems if aimed too close to the house. Keeping gutters clean and repairing faulty joints eliminates spatters and other water problems. Reducing the arc of sprinkler systems and laying down straw mulch on bare ground near foundations provides water for the lawn and allows turf to regrow.

Clean-up and Prevention

Wood chip mulches cause most artillery fungus problems. Bark mulches provide less nourishment for the fungus than wood chips. Stirring the mulch discourages fungal growth, and new layers of mulch cover the fruiting bodies. Using wood chip mulch in areas more than 20 feet from homes and cars keeps the fungus at a safe distance. Other mulching materials such as gravel or stone chips provide attractive fungus-free cover nearer the home. Spore masses don't easily wash away and even when scraped off leave obvious stains. Power washing cleans most other residue off walls quickly. Using a detergent wash followed by power washing could speed up stain removal.