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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Black spots on a concrete driveway usually come from one of five sources: living growth in damp or shaded areas, sticky spore masses from artillery fungus near wood mulch, oil or tire marks, mineral or fertilizer stains, or dark clumps trapped inside the slab. Each responds to a different treatment, and a cleaner that works on one can do nothing for another or damage the surface. Start by looking at where the spots sit and how they behave when wet or touched.
Identify the spot by where it sits and how it behaves
The clues below point toward a likely cause. They are diagnostic guides, not laboratory identifications, so treat the first treatment as a test.
| What you see | Most likely cause | Where to go next |
|---|---|---|
| Dark growth or patches in shaded areas, where the surface stays damp or water pools; often slick when wet | Algae or mildew | Organic growth section below |
| Small, round, black dots that feel sticky, appearing on concrete near wood mulch | Artillery fungus spore masses | Artillery fungus section below |
| Dark streaks in vehicle paths, under parked cars, or along tire tracks | Oil, grease, or tire-related marks | Vehicle marks section below |
| Stains linked to fertilizer, metals, minerals, or decaying leaves | Mineral, fertilizer, or organic leaf stains | Vehicle and chemical marks section below |
| Rounded dark-gray spots that do not respond to cleaning | Dirt clumps embedded in the concrete | Embedded spots section below |
Test before you treat
Every cleaner in this article can change the look of concrete if used wrongly. Apply the product to a small, inconspicuous area first and check the result after it dries. Acidic solutions can etch or alter concrete, so avoid them unless a test patch shows no harm and the product label covers concrete.
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Organic growth: algae and mildew
University of Maryland Extension, in an answer on algae on concrete dated January 22, 2019, says algae tends to form where surfaces are shady or damp or where water sits. Surface nutrients also contribute, and cleaning may need to be repeated as long as those conditions continue. Read the Maryland Extension answer.
How to remove algae or mildew
- Sweep off leaves and loose debris, then wet the surface and any nearby plants.
- Choose a product. The Concrete Network driveway cleaning guide, updated April 2, 2020, suggests a mold-killing detergent or a bleach-and-water solution for mold and mildew. Read the Concrete Network guide. Mix bleach at the strength the label specifies.
- Wear gloves when handling bleach, and never mix bleach with other household cleaners.
- Apply the cleaner, let it dwell for the label time, then scrub with a stiff brush.
- Rinse thoroughly, cover or rinse nearby plants as the guide advises, and let the surface dry before judging the result.
A surface-specific exterior cleaner is a reasonable alternative only if its label names concrete and the application you are using.
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Artillery fungus near mulch
The University of Maine Cooperative Extension artillery fungus page describes these spots as sticky spore masses from Sphaerobolus species. The page states: “When the mass hits a surface, a sticky coating causes it to adhere, producing a small black spot about 1/10 inches in diameter.” The page is undated, and the size is a description rather than a study finding. Read the University of Maine page.
The University of Delaware Cooperative Extension fact sheet, dated April 9, 2019, links artillery fungus growth to hardwood mulch. It says soap and water with a scrub brush can be effective if you act before the material dries, and that removal becomes difficult once the spots adhere. Read the University of Delaware fact sheet.
Fresh spots
- Mix a solution of dish soap and water, and wet the spots.
- Scrub with a stiff-bristle outdoor brush.
- Rinse and check. Repeat before the spots harden.
Dried spots
Dried spots are the hard case. A UConn Extension article on mulch molds, dated July 10, 2013, states that no commercial or homemade cleaner it tested had been proven effective for removing these spots from houses or cars. It also notes that scraping can leave a mark or stain. Read the UConn Extension article. Expect that some dried spots may stay, and avoid aggressive scraping that could etch the surface.
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Stop the spots from coming back
- Inspect wood mulch beds near the driveway for the fungus.
- Delaware Extension says the fungus is associated with hardwood mulch and that bark or pine bark is less favorable for it.
- Disturbing or removing infested mulch is a possible management step, according to the same fact sheet.
Vehicle, fertilizer, and leaf marks
The Concrete Network guide separates these marks into several types. Each needs a different approach.
Oil and grease
For oil and grease, use a concrete degreaser rather than a general household cleaner. Test a small area first, then scrub and rinse.
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Tire marks
Dark marks left by hot tires are often linked to plasticizer migrating from tires into certain sealers. Degreasing may help, but the guide notes that sealer may need to be removed if the discoloration has penetrated it. Test first, and do not expect a degreaser alone to clear a sealer-level stain.
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Fertilizer and metal or mineral stains
Fertilizer and metal or mineral stains look different from growth and do not respond to the same products. Identify the source before choosing a treatment, and avoid acid unless a test patch shows it is safe for your concrete.
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Leaf stains
Organic leaf stains are typically tied to damp, decaying debris. Clear leaves promptly and treat the area with a cleaner labeled for concrete.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Dark clumps embedded in the slab
A ready-mix technical article from Concrete South, undated, describes some rounded dark-gray spots as clumps of dirt embedded in the concrete rather than material rising through it. This is one possible explanation for spotted concrete, not a universal diagnosis. If repeated cleaning has no effect, the mark may be inside the slab rather than on its surface. Read the Concrete South article.
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Products: check that the label covers your concrete
LTP’s 2025 technical data sheet for its Black Spot & Algae Remover describes the product as suitable for paving, including concrete, and directs users to test an inconspicuous area first. The sheet also says the product has bleaching action and displays corrosive and environmental hazard pictograms, so it is not a harmless general-purpose cleaner. Read the LTP data sheet. Availability and labeling vary by market, so confirm the current label on the container you buy before use.
Quick Recap
When nothing works
- If a test patch changes color or texture, stop and reassess the cause.
- If a spot persists after repeated correct cleaning, it may be embedded in the slab or stained beyond surface cleaning.
- If the surface is sealed or colored, check whether the cleaner and any stripping step are compatible with the coating.
Keep the driveway clear of recurring causes
- Reduce persistent dampness and standing water, since algae depends on it.
- Clear debris and leaves regularly.
- Inspect nearby wood mulch if small sticky dots keep appearing.
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