What is the sticky black film on my deck, patio and car under a tree, and how do I clean it off?

What is the sticky black film on surfaces under a tree?

The sticky black film on a deck, patio, car or patio table under a tree is usually honeydew with sooty mold growing on it. Honeydew is the sugary liquid waste of insects that feed on tree sap, and it lands on everything below the branches. Sooty mold is a black fungus that grows on that sugar. The honeydew is the tacky part that grabs your shoes, and the sooty mold is the dark color that builds up over the following days and weeks.

The pattern on the ground tells you the source. The film is heaviest under the busiest limbs and fades out near the edge of the canopy, so railings, furniture tops, grill lids and car roofs parked in that zone collect the most while surfaces in the open stay clean. Hard black dots the size of a pinhead clustered near a mulch bed are a different problem, explained in our guide to artillery fungus specks.

What is honeydew, and which insects drop it?

Honeydew is the excess sugary sap that sap-feeding insects excrete while they feed. Insects with piercing, sucking mouthparts draw more sap from a tree than they can use, and the surplus falls from the leaves and bark onto whatever sits below. Aphids, scale insects and spotted lanternflies all produce honeydew, so a sticky deck does not by itself mean lanternflies are in the tree.

Spotted lanternfly stands out for the volume of honeydew it produces. Penn State Extension notes that on sunny days honeydew can be seen falling from infested trees like a light rain. The sugar also draws ants, wasps, bees, flies and yellow jackets, according to Penn State and the Michigan Department of Agriculture and Rural Development (MDARD). Those stinging visitors are a practical reason to clean a sticky deck instead of waiting on the weather.

Has spotted lanternfly been confirmed in Macomb and Oakland counties?

Spotted lanternfly has been confirmed in both Macomb and Oakland counties. As of September 2026, MDARD lists confirmed populations in Berrien, Jackson, Lenawee, Macomb, Monroe, Oakland, Washtenaw and Wayne counties, and the insect was first detected in Michigan in 2022. MDARD states that spotted lanternfly poses no risk to human or animal health, though it can be a nuisance in outdoor areas.

Spotted lanternfly prefers tree-of-heaven, an invasive tree that Penn State Extension reports along woodland edges, roadsides, railways and fencerows, and it also feeds on grapevines, black walnut, river birch, willow, sumac and red maple. Adults are about an inch long, with gray-brown spotted forewings and red hindwings that show when they open. People in the eight counties above do not need to report sightings, because MDARD already knows the insect is there. Anywhere else in Michigan, MDARD asks for a photo and a report through the Eyes in the Field tool. Egg masses look like gray, putty-like smears of old chewing gum on trunks, vehicles and outdoor furniture, so check those before hauling anything out of the area.

What is sooty mold, and does it damage the surfaces it grows on?

Sooty mold is a group of dark fungi that feed on honeydew instead of on the surface underneath. Because it lives on the sugar, sooty mold appears only where honeydew has landed and stayed. Penn State Extension states that sooty mold does not directly harm the surfaces it grows on, but that it stains tree trunks, decks, patios and vehicles beneath infested trees and that those stains can be very difficult to remove.

Sooty mold gets harder to remove the longer it sits. Fresh honeydew is a sugary liquid, and warm, soapy water lifts it easily. Weeks later, the black growth has worked into the open grain of a wood deck, the pores of a concrete patio and the texture of composite boards, and dried honeydew underneath has collected dust into a tacky crust. On plants, the film shades the leaves and cuts down the light they use. A deck cleaned within days of the drip is a soap-and-rinse job, while one left for a full season is a stain removal.

How do you clean honeydew and sooty mold off a deck, patio and furniture?

Honeydew and sooty mold come off a deck, patio or furniture with warm, soapy water and a soft brush, followed by a thorough rinse, done in an order that keeps the drip from landing on clean boards. University of California IPM notes that sooty mold can be washed off plants and surfaces with a strong stream of water or with soap and water. The steps below put that advice into an order for a deck or patio.

1. Pick a dry window. Clean after the heaviest drip has passed, or tarp the area so new honeydew does not land on wet, clean boards.
2. Move or cover what you can. Cushions, grills and planters come out of the drip zone first.
3. Pre-wet the surface and nearby plants. Water loosens dried honeydew and protects foliage from any cleaner that drifts.
4. Wash with warm, soapy water and a soft brush. Work small sections and keep them wet so dissolved sugar does not dry back in place.
5. Treat the black stain. Where sooty mold has set into wood or concrete, a cleaner suited to that material needs several minutes of dwell time before the rinse.
6. Rinse at low pressure. High pressure raises fuzzy fibers on wood decks and leaves wand marks on concrete; a gentle rinse carries the residue away without that damage.

Concrete patios clean evenly with a surface cleaner, the round tool that spins jets under a shroud at a fixed height. Macomb Power Washing Solutions prices deck cleaning from $199, about $1 to $2 per square foot, and concrete cleaning from $149.

How do you clean it off siding, windows and cars?

Honeydew and sooty mold come off siding, windows and cars with the same gentle approach: soap, soft contact and a low-pressure rinse. Siding under an infested tree gets a soft wash, a cleaning solution applied at low pressure and given time to dwell, so the black film releases without water being driven up behind vinyl laps, a risk our vinyl siding pressure guide explains. Sap and pollen on the same walls are covered in our guide to tree sap, pollen and tannin stains. Our house washing starts from $349.

Windows smear if sticky film is wiped dry, so wet the glass first, wash it with soapy water and squeegee it off. Cars parked under a dripping tree need car-wash soap and a soft mitt, worked in the shade so the soap does not dry on the paint, and no scraping or household degreasers. The simplest fix for a car is parking it outside the canopy until the season ends.

When in the year does honeydew build up in Michigan?

Honeydew from spotted lanternfly builds up in Michigan from late spring into fall, the months MDARD lists for feeding nymphs and adults. MDARD lists nymphs as most visible from May to September and adults from August to October. Penn State Extension notes that in the fall adults feed more from trunks and branches, and that in Pennsylvania adults stay active until hard freezes. Aphids and scale insects add their own honeydew through the growing season.

The adult drip overlaps the fall deck and house washing season. A cleaning booked late in the season, after the adults are gone, removes the year's film once instead of chasing new drips every week.

How do you keep the mess from coming back?

Keeping honeydew and sooty mold from coming back starts with the insects and the tree, then moves to the surfaces. For the insects, follow MDARD's spotted lanternfly guidance, which covers squishing, traps, insecticidal soaps and oils, and contacting a certified pesticide applicator about a large infestation. Washing cleans the surfaces but does nothing to the insects, so the two jobs run side by side. Because tree-of-heaven is the preferred host, an arborist can advise on one growing near the house. Penn State notes that cutting alone does not control it, because the tree answers with root suckers.

On the surface side, move furniture, grills and cars out of the drip zone from August through October, and cover what cannot move. Rinse fresh honeydew off the deck every week or two during the peak so sooty mold has no time to take hold, and schedule the deeper cleaning after the season ends. For a home on a wooded lot, our wooded-lot maintenance calendar shows where a fall cleaning fits.