Mario Bautista
Q • The siding and fence on one side of my house near a flower bed always gets covered in small black specks that I can’t wash off. What’s causing that, and what can I do to stop it?
A: If the flower bed is mulched with a shredded hardwood mulch, the probable cause of the black specks is a fungus commonly called artillery fungus.
This fungus lives on dead wood (it is not a threat to any plants you have), and it reproduces by shooting small, black, spore-containing structures into the air towards the light and/or light-colored surfaces like your fence and siding. These spore masses are sticky and difficult to remove from surfaces.
Little research has been conducted on ways to discourage artillery fungus from growing on mulch, but one study found that blending shredded mulch with compost helped reduce its growth. Raking your mulch to disturb it and allow it to dry more quickly may also help reduce the number of spores released. Finally, mulch material made from pine bark rather than shredded hardwood has been shown to harbor less artillery fungi.
Write to the Missouri Botanical Garden’s Center for Home Gardening at plantinformation@mobot.org or the Horticulture Answer Service, 4344 Shaw Blvd., St. Louis, MO 63110.
