You've noticed them appearing on your patio. Small, dark, roughly circular marks, some the size of a coin, some larger, scattered across the stone or concrete. They don't brush off. They don't shift with a scrub. The jet wash barely touches them. And every year there seem to be slightly more of them, slowly spreading across the surface.

Black spots on patios are one of the most misunderstood problems in outdoor surface maintenance. Most people assume they're a type of mould or algae, something that a strong cleaner or a high-pressure washer should deal with quickly. They're not. And that misunderstanding is why so many attempts to remove them fail, why they return so quickly after cleaning, and why the same spots seem impossible to shift despite repeated effort.

This guide explains exactly what patio black spots are, why they're fundamentally different from other types of biological staining, which surfaces are most affected, and the correct method for removing them permanently.

What Patio Black Spots Actually Are

In the vast majority of cases, patio black spots are black lichen, specifically, crustose lichen. And lichen is not what most people think it is.

Lichen is not a single organism. It's a stable, symbiotic partnership between two entirely different kingdoms of life: a fungus and either algae or cyanobacteria, living together in a single structure that functions as one. The fungal component provides the physical structure, anchoring the organism to the surface and protecting it from desiccation. The algal or cyanobacterial component performs photosynthesis, generating the nutrients that feed them both.

This partnership produces something considerably more resilient than either organism would be alone. Lichen can survive in conditions that would kill most other organisms: extreme cold, prolonged drought, direct UV exposure, and, critically for UK patios, the repeated wet-dry cycles of temperate British weather that the organism treats as normal habitat rather than as a stressor.

Crustose lichen, the type responsible for black patio spots, grows in a form that's physically fused to the surface beneath it. Unlike foliose (leafy) or fruticose (branching) lichen, which can be physically peeled or scraped away, crustose lichen cannot be separated from the stone. It grows into the surface rather than onto it, with root-like structures called hyphae, burrowing deep into the microscopic pores, cracks, and grain boundaries of the stone or concrete. These hyphae are what anchor it, and they're also what makes it resistant to pressure washing. You can remove everything visible above the surface and leave the hyphae completely intact, driving rapid regrowth from the same spot.

The black colouration comes from melanin, the same pigment that causes suntan in human skin, which the lichen produces as a defence against UV radiation. This melanin is what gives mature black lichen its characteristic dark, sometimes almost tar-like appearance, and it's what persists on the surface even when the surface looks clean after pressure washing.

Why Black Spots Are Different From Green Algae and Mould

This distinction matters practically because the products and methods that work effectively on green algae and surface mould don't work on black lichen, and using them on black spots wastes time and delays the correct treatment.

Green algae is a surface coloniser. It grows on top of the stone rather than into it, forming the slippery green or grey-green film that's the most common form of patio biological staining. It spreads quickly, looks dramatic when established, and responds relatively quickly to biocidal cleaners like GK Path & Patio Cleaner or GK Pro. The organism is relatively simple, doesn't produce a protective outer crust, and its root structures are shallow.

Surface mould and mildew are similarly surface-level. They produce the dark patches and greying that gives patios a generally dirty appearance. Biocidal cleaners kill them effectively and results are often visible within days.

Black lichen is a fundamentally different proposition. It has a hard, protective outer crust, a biofilm of melanin and fungal compounds, that physically resists penetration by thin liquid biocides. Beneath that crust, the thallus (body) of the lichen extends down into the stone. Standard biocides applied to the surface may not reach the living cells beneath the crust in sufficient concentration to kill the organism. This is why a patio that's been treated with a standard algae cleaner can look clean immediately after treatment, only to show the same black spots reappearing within weeks. The spots were suppressed or cosmetically lightened but not killed.

Identifying which type of staining you're dealing with before applying anything determines whether your treatment succeeds or fails.

How to tell the difference:

A green or grey-green film that covers large areas evenly and feels slippery underfoot is almost certainly green algae. It may darken over time but doesn't form distinct circular or irregular spots.

Black spots that are discrete, roughly circular, and feel hard or crusty to the touch, almost like dried tar or chewing gum stuck to the stone, are almost certainly crustose lichen. They resist scrubbing and feel physically different from the surrounding surface.

Dark mottled patches that look like staining rather than distinct spots, and that occur most heavily in damp or shaded areas, may be mould. These often respond more quickly to treatment than lichen.

If you're genuinely uncertain, treat with a standard biocidal patio cleaner first. If the surface clears and stays clean, it was algae or mould. If distinct dark spots remain, particularly any that look unchanged, those are almost certainly lichen and need targeted treatment.

Which Surfaces Are Most Affected and Why

Black lichen doesn't colonise all patio surfaces equally. The key factor is surface porosity, the degree to which the material offers microscopic pores, grain boundaries, and channels that the lichen's hyphae can penetrate and anchor into.

Indian sandstone is the most commonly affected patio surface in the UK. Sandstone is a sedimentary stone with a relatively open, granular structure and high porosity. Lichen hyphae penetrate easily and deeply, which is why black spots on Indian sandstone are frequently the most stubborn to remove and have the longest regrowth cycle. The warm, orange-buff colours of Indian sandstone also make black spots particularly visible.

York stone and other natural limestone are similarly porous and similarly prone. The calcium carbonate in limestone provides minerals that lichen consumes as part of its nutrition, which is one reason lichen causes surface pitting and etching in limestone over time, as the organism's organic acids dissolve the stone surface.

Concrete paving slabs are moderately susceptible. Concrete is less porous than sandstone but more porous than porcelain, and wet-cast concrete slabs with their relatively low cement content provide adequate anchorage for lichen growth over time.

Block paving is affected, particularly older or weathered blocks where the surface has opened up. Pressed block pavers with high cement content are more resistant, but not immune.

Porcelain tiles are the least susceptible of common patio surfaces. Porcelain is virtually non-porous. Its dense, vitrified structure provides almost no anchorage for lichen hyphae. Black staining on porcelain is far more likely to be algae or mould in the grout lines rather than lichen on the tile surface itself.

Granite is more resistant than sandstone or limestone but not immune. Its harder, denser crystalline structure makes penetration harder, so lichen tends to establish more slowly, but on north-facing or heavily shaded granite surfaces, it will still colonise over time.

The common thread: porosity plus moisture plus shade. The more water a surface retains between rain events, and the less UV exposure it receives, the faster lichen establishes and the deeper it penetrates.

Why Pressure Washing Doesn't Solve It

Pressure washing is the first response most homeowners try on patio black spots. It's also why so many people end up frustrated and searching for a better answer.

A pressure washer removes what's above the surface. The visible lichen thallus, the crusty, dark body of the organism that you can see, may be partially or fully removed by high-pressure water, particularly if the spots are relatively young and the hyphae haven't yet penetrated deeply. After washing, the surface looks clean. Sometimes it looks dramatically better.

Then, over the following months, the spots return. Same locations. Same pattern. Often slightly darker and harder than before.

What's happened is that the hyphae anchored into the stone were untouched by the water pressure. The lichen was decapitated, not killed. The living root structures below the surface simply regenerate the thallus above, and because the process repeats, the surface progressively becomes more porous and more susceptible with each pressure washing cycle.

Additionally, high-pressure water on sandstone and other soft stone surfaces erodes the stone itself. Each pressure wash opens the surface slightly, creating more pore space and micro-roughness that provides better anchorage for the next generation of lichen. It's a cycle that gets harder to break the longer it continues.

The correct approach requires chemistry, a product formulated to penetrate the lichen's protective outer crust and kill the organism at the cellular level, including the hyphae below the surface rather than mechanics.

Why Standard Biocides Often Fail on Black Lichen

Many standard patio biocides and algae removers are formulated primarily for algae and mould. They're effective at those targets, but the protective outer crust of crustose lichen presents a barrier that thin, low-viscosity biocide solutions may struggle to penetrate in the dwell time available.

Thin liquids applied to a vertical or textured surface run off quickly, reducing contact time with the lichen significantly. The same happens on horizontal surfaces in warm or windy conditions where evaporation is rapid. The biocide may kill the surface-level algae present around the lichen, which can make the overall surface look cleaner, but leave the lichen itself largely intact.

What's needed specifically for black lichen is a product with:

Extended dwell capability: the formula needs to remain in contact with the lichen for long enough to penetrate the outer crust and reach living cells beneath it. This may require hours, not minutes.

A thixotropic (gel-like) formulation: a slightly viscous formula that clings to the surface rather than running off, maintaining contact with the lichen throughout the dwell period rather than draining away within seconds of application.

Alkaline chemistry with wetting agents: the wetting agents reduce the surface tension of the outer lichen crust, improving penetration into the organism's structure, while the alkaline chemistry targets the biological bonds holding the lichen to the surface.

This is exactly what GK Black Spot Remover was formulated to provide.

How to Remove Black Spots From Your Patio: Step by Step

What You'll Need:

• GK Black Spot Remover 5Ltr
• A stiff-bristled brush (nylon, not wire)
• A garden sprayer or watering can for application
• Protective gloves and eye protection
• A garden hose or low-setting pressure washer for rinsing

If the patio also has a general background of green algae or mould (common alongside black spots), treat the whole surface with GK Path & Patio Cleaner after the black spot treatment has completed. Black Spot Remover is formulated specifically for lichen; the broader algae and mould coverage is where the patio cleaner earns its place.

Step 1: Prepare the Surface

Clear the patio of furniture, plant pots, and debris. Sweep to remove loose material. If significant moss is sitting loose on the surface, brush it away, but don't scrub hard at the black spots at this stage.

Lightly pre-dampen the affected stone areas with a garden hose before application. Do not soak. You want the surface just moist, not wet. Pre-dampening reduces absorption of the product into the dry stone and helps keep it concentrated on the lichen at the surface where it needs to work.

Step 2: Apply GK Black Spot Remover: Ready to Use, No Dilution

GK Black Spot Remover is applied neat, no dilution required. Apply generously to all affected areas using a garden sprayer, watering can, or brush. The slightly viscous, gel-like consistency means it clings to the stone surface rather than running immediately into the joints. Ensure every black spot is fully saturated with product.

Coverage is approximately 5m² per litre depending on surface porosity. One 5-litre bottle covers around 25m² which is sufficient for a typical domestic patio.

Step 3: Dwell Time: This Is the Most Important Step

Allow the product to dwell on the surface for as long as possible. This is where GK Black Spot Remover's thixotropic formulation is the key advantage over thin liquid products. It maintains contact with the lichen throughout the dwell period rather than evaporating or draining within minutes.

Recently formed or lighter spots: minimum one hour dwell, ideally two to three hours.

Established or deeply embedded lichen: dwell overnight up to 24 hours. Do not rinse prematurely.

During the dwell period, periodically agitate the treated areas with a stiff brush. The agitation serves two purposes: it works the product into the lichen's surface structure, and it thins the thixotropic formula slightly, improving penetration. Reapply product to any areas that appear to have dried during an extended dwell period.

Do not allow the surface to dry out completely during dwell time. On warm days, reapply product to maintain a wet surface.

Step 4: Agitate and Rinse

After the full dwell time, scrub the treated areas firmly with a stiff-bristled brush before rinsing. This agitation helps break the bond between the killed lichen and the stone surface, making the rinse step more effective.

Rinse thoroughly with a garden hose. For best results, a pressure washer on a moderate setting used at this point, after chemical treatment, as a rinse tool rather than as the primary cleaning method, will remove dead material more efficiently. Work from the top of the patio downwards so that rinse water carries residue off the surface cleanly.

Step 5: Assess and Repeat If Needed

After rinsing and allowing the patio to dry, assess the result. On lighter or more recently formed black spots, a single treatment often produces a near-complete result.

For deeply embedded, long-established lichen, particularly on porous sandstone where hyphae may have penetrated several millimetres into the stone, a second application after the first has dried may be needed to address any remaining spots. This is not unusual with severely affected patios. Allow the surface to dry fully between applications.

Some residual discolouration may remain in the stone after the lichen has been killed as the melanin pigment deposited in the stone by the lichen can persist as a faint shadow even once the organism is dead. Over subsequent weeks, UV exposure and rainfall progressively fade this residual staining. The full aesthetic result continues to improve over the weeks following treatment.

Patios With Both Black Spots and Green Growth

It's very common for a patio to present both black lichen spots and a general background of green algae or surface mould, particularly on older, unprotected stone that hasn't been treated for several years.

The most efficient approach is to treat in sequence:

First: Apply GK Black Spot Remover to all affected areas. Allow full dwell time (overnight where possible), agitate, and rinse.

Then: Once the surface has dried, apply GK Path & Patio Cleaner across the whole patio surface at the appropriate dilution for your contamination level (2:1 for heavy soiling, 4:1 for lighter). Leave to work with weather cycles. No rinsing required.

This two-stage approach addresses both the difficult lichen and the background biological staining in a single cleaning session, with each product doing the work it's specifically formulated for.

After Cleaning: Sealing to Prevent Regrowth

Lichen will return to an untreated, porous patio surface as the spores are always present in the air, and a clean porous surface is simply available habitat for them to colonise. The question is how quickly, and the most effective way to slow it down significantly is to seal the cleaned surface.

A penetrating stone sealer reduces the porosity of the patio surface, filling the microscopic pores and grain boundaries that lichen hyphae anchor into. An unsealed sandstone patio may show returning black spots within two to three years. A sealed patio, with the pores blocked, gives lichen spores far less to anchor to, significantly extending the time before retreatment is needed.

GK Invisible Stone Sealer is suitable for all natural stone types including Indian sandstone, limestone, granite, and slate, as well as concrete and block paving. It's water-based and dries completely invisible with no surface sheen, no colour alteration, and the breathable formula allows moisture vapour to escape from the stone rather than trapping it beneath a surface film.

Apply to a clean, completely dry surface (allow at least 48 hours after rinsing before sealing). Apply two to three wet-on-wet coats, applying each coat while the previous one is still wet, for maximum penetration into porous stone. One 3-litre bottle covers approximately 40m².

Important: never seal over live lichen or algae. The sealer will trap the growth beneath it, prevent correct penetration, and create a far more difficult problem to deal with subsequently. Always complete the biological treatment and ensure the surface is clean before sealing.

Surface-Specific Notes

Indian sandstone: The most challenging surface for black spot removal due to its high porosity and relatively soft structure. Allow maximum dwell time (overnight) and be prepared for two applications on heavily affected patios. Seal promptly after cleaning as sandstone benefits more from sealing than almost any other patio surface.

Concrete paving slabs: Generally respond well to GK Black Spot Remover. The denser structure of concrete, relative to sandstone, means hyphae typically don't penetrate as deeply. One treatment is often sufficient on concrete.

Block paving: Apply product carefully to the stone surface rather than the joints. The jointing sand in block paving joints doesn't need treatment. Focus coverage on the block faces where lichen establishes.

Natural limestone: Lichen on limestone produces organic acids that etch into the calcium carbonate surface over time, which can cause permanent pitting if left for many years. Treat promptly and seal after treatment to reduce ongoing acid etching risk from future colonisation.

Porcelain: True lichen on porcelain tile faces is rare due to the non-porous surface. Dark spots in porcelain grout lines are typically algae or mould and respond to GK Path & Patio Cleaner rather than Black Spot Remover. If discrete spots appear on the tile face itself (unusual), test GK Black Spot Remover on a small area before full application.

Frequently Asked Questions

Are black spots on my patio harmful?

Yes, in two ways. First, lichen produces a biofilm that becomes extremely slippery when wet so black spots are a genuine slip hazard on external paving, particularly in autumn and winter. Second, over time the organic acids produced by lichen etch into the mineral structure of stone, causing surface pitting and weakening, particularly on limestone and softer sandstone. Treatment is both a maintenance and a safety issue.

Why do black spots keep coming back after I pressure wash?

Because pressure washing removes the visible lichen above the surface but leaves the hyphae (root structures) embedded in the stone alive and intact. The organism regrows from these roots. Only a chemical treatment that kills the hyphae below the surface breaks the regrowth cycle.

Can I use bleach on black spots?

Bleach can partially lighten the appearance of black lichen by destroying the melanin pigment that gives it its dark colour. This makes the surface look cleaner without the lichen actually being killed. The spots return quickly, often faster than before, and on natural stone, bleach carries a risk of causing surface etching or discolouration. It's a cosmetic fix, not a treatment.

How long does GK Black Spot Remover take to work?

For lighter or more recently established spots, visible improvement is often seen after one treatment with a full dwell time. For deeply embedded or long-established lichen, two applications may be needed, and some faint residual staining may continue fading over subsequent weeks as UV and weather cycles complete the process.

Does sealing a patio prevent black spots?

Sealing significantly reduces the rate at which black spots return by filling the surface pores that lichen hyphae anchor into. It doesn't make a patio permanently immune, spores will always be present, but it meaningfully extends the interval between required treatments and makes future cleaning considerably easier.

What's the white crusty growth alongside the black spots on my patio?

White crusty growth, distinct from the white powder of efflorescence, is often crustose lichen of a different species, typically a grey or white-forming variety. GK Black Spot Remover is effective on white and grey lichen as well as black lichen. If the white deposit is powdery and appears to be coming from within the stone rather than growing on it, that's efflorescence, a salt deposit that requires GK EFF OFF Efflorescence Remover rather than a lichen treatment.

Products for Black Spot and Patio Cleaning

Available from GK Products UK with free delivery on orders over £60:

 

Final Note

Not sure whether what you're dealing with is black lichen, algae, mould, or something else? Send a photo to the GK team at sales@gkproductsuk.com or call 07557 111 297, we're happy to identify the problem and recommend the right treatment.