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Black spots on a concrete driveway can be algae or mildew, sticky artillery-fungus spores, oil or tire marks, fertilizer stains, or discoloration within the concrete itself. Their location and texture offer clues, but there is no single cleaner that reliably removes every kind. Identify the likely source first, test any treatment in an inconspicuous area, and use a product labeled for your concrete’s finish.
Identify the likely cause before cleaning
Start with where the spots appear, what they feel like, and what is nearby. These clues can narrow the possibilities, but they do not confirm the cause as a laboratory test would.
- Shady or persistently damp areas: Algae or mildew is more plausible where water sits, drainage is poor, or the concrete stays damp. Maryland Home & Garden Information Center notes that algae tends to grow on shady or damp surfaces and that nutrients on the surface can contribute. Maryland Home & Garden Information Center: Algae on concrete.
- Small, sticky dots near wood mulch: Artillery fungus is a possibility, especially near hardwood mulch. The University of Maine Cooperative Extension says: “When the mass hits a surface, a sticky coating causes it to adhere, producing a small black spot about 1/10 inches in diameter.” University of Maine Cooperative Extension: Artillery Fungus.
- Marks in tire paths or beneath parked vehicles: Consider hot-tire marks, oil, or grease. Tire discoloration can involve plasticizers migrating into some sealers; oil and tire marks need different treatment from biological growth. Concrete Network: Driveway Cleaning.
- Marks near lawn treatments, leaves, or other debris: Fertilizer can leave mineral or metal stains, and organic material can stain concrete. A general cleaner may not address these causes.
- Spots that resist surface cleaning: Some dark-gray spots may be embedded dirt clumps within the concrete rather than a surface deposit. Cleaning may not remove discoloration that is within the slab. Concrete South: Discoloration / Spotted Concrete Driveway.
Choose a treatment that matches the stain
| Likely stain | First treatment to consider | Important limits |
|---|---|---|
| Algae or mildew | Scrub with a surface-appropriate exterior cleaner, or use a mold-killing detergent or bleach-and-water solution as described by Concrete Network. | Follow the product label, protect nearby plants, and wear gloves if handling bleach. Never mix bleach with other household cleaners. Growth may return if damp or shaded conditions persist. |
| Fresh artillery-fungus material | Try soap and water with a stiff-bristle scrub brush while the spots are fresh. | Once dried and adhered, removal is difficult and may leave a mark. No cleaner is guaranteed to work. |
| Oil or grease | Use a concrete degreaser whose label covers the surface and stain. | Do not assume an algae cleaner or bleach will remove oil. Test first, especially on colored or sealed concrete. |
| Hot-tire marks | Use a concrete cleaner or degreaser labeled for tire marks and the driveway finish. | If the discoloration has penetrated a sealer, cleaning may not restore it; the affected sealer may need removal. |
| Fertilizer, mineral, or embedded discoloration | Identify the specific stain before selecting a treatment. | Acidic treatments can etch or change concrete. Discoloration within the slab may not be removable as surface dirt. |
Concrete Network discusses degreaser for oil and tire marks and cautions that acidic solutions can etch or alter concrete. A treatment suitable for bare gray concrete may not be suitable for colored or sealed concrete, so check the label and test a discreet patch before treating the whole driveway. Concrete Network’s driveway-cleaning guidance.
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How to remove algae or mildew
- Clear loose debris. Remove leaves and dirt so the cleaner can reach the surface.
- Check compatibility. Use only a cleaner whose label covers concrete and the intended application. If considering bleach, protect nearby plants, wear gloves, and never mix it with another household cleaner.
- Test a hidden patch. Apply the product as directed to an inconspicuous spot and check for unwanted color or surface changes before proceeding.
- Scrub and rinse as directed. A stiff brush can help loosen surface growth. Follow the cleaner’s label for application, dwell time, and rinsing rather than improvising a stronger mixture.
- Address the conditions that support regrowth. Improve drainage where practical, reduce standing water, and keep the area clear of debris. Maryland Extension notes that cleaning may need to recur when favorable damp or shady conditions continue.
One example of a labeled paving product is LTP Black Spot & Algae Remover. Its 2025 technical data sheet lists concrete among suitable paving and directs users to test an inconspicuous area. The sheet also indicates bleaching action and displays corrosive and environmental hazard pictograms. Read the current local label and safety directions; suitability for paving does not mean it is harmless or appropriate for every driveway. LTP Black Spot & Algae Remover Technical Data Sheet (2025).
What to do about artillery-fungus spots
Artillery fungus produces small, sticky spore masses that can adhere to hard surfaces. Hardwood mulch nearby is a useful clue: Delaware Extension associates artillery-fungus growth with hardwood mulch and recommends soap and water with a scrub brush for material before it dries. University of Delaware Cooperative Extension: Artillery Fungus and Other Things That Grow in Mulch.
For fresh spots, try gentle scrubbing with soapy water. For dried spots, set expectations carefully: Delaware Extension says they are difficult to remove after adhering, while UConn Extension reports that no commercial or homemade cleaner had proven effective for removing them from houses or cars and warns that scraping can leave a mark or stain. Those warnings do not establish that every driveway spot is permanent, but aggressive scraping or harsh chemicals can damage the concrete or its finish. UConn Extension: Mulch Molds.
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To reduce the chance of new spots, inspect nearby wood mulch. Delaware Extension describes bark or pine bark as less favorable to artillery fungus than hardwood mulch and discusses disturbing or removing infested mulch as management options. The extension guidance does not make these steps a guarantee against recurrence.
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If a stain remains after a compatible, correctly used treatment, stop escalating to stronger chemicals. The mark may be embedded in the slab, may have penetrated a sealer, or may have a different cause than first suspected. Acidic cleaners can etch or alter concrete, and scraping adhered artillery-fungus spots can leave visible damage. For a large or valuable sealed or colored area, ask a concrete-cleaning professional to identify the finish and stain before attempting more aggressive removal.
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