Removing Black Spot Lichen: Why Pressure Washing Isn't Enough

You spent the entire Saturday with the pressure washer. You blasted every inch of your Indian Sandstone patio. The green algae is gone. The dirt is gone. The moss is gone. But the Black Spots are still there.

In fact, now that the dirt is gone, they look even worse - like ink splatters across your beautiful stone. You might think your pressure washer isn't powerful enough. You might hold the lance 1cm away from the stone to try and blast them off (and damage your pointing in the process).

Stop. You cannot pressure wash black spots away. These aren't dirt marks; they are a living organism. To remove them, you need to switch from Physics (water pressure) to Chemistry (biocides).

Here is why the "Black Plague" of patios is so stubborn, and the only way to get rid of it.

1. The Biology: It’s Not a Stain, It’s a Plant

"Black Spot" is actually a Lichen - a symbiotic organism made of fungi and algae living together. Unlike moss, which sits on top of the stone, lichen feeds on the minerals inside the stone.

• The Roots (Hyphae): The lichen sends microscopic fungal filaments deep into the pores of the sandstone or concrete. They anchor themselves like tree roots in soil.

• The "Iceberg" Effect: The black spot you see on the surface is just the "flower." The main body of the organism is below the surface.

When you pressure wash, you shear the top off the black spot. But because the roots are still safe inside the pore, the spot remains visible, and it will grow back rapidly.

2. The Solution: Sodium Hypochlorite

To kill a fungus deep inside a rock, you need a chemical that penetrates and oxidises. You need Sodium Hypochlorite.

This is the active ingredient in household bleach, but for patios, we use an industrial strength (usually 14%–15% concentration, diluted down). In the trade, this is called "Soft Washing."

• How it Works: The liquid soaks into the stone pores. It kills the biological spore at the root and bleaches the black pigment, turning it clear/white.

• The Result: The spots disappear before your eyes without you needing to blast the stone.

3. The Process: The "Soft Wash" Technique

Do not just pour neat bleach on your expensive patio. You must follow a protocol to avoid damage to the garden.

Step 1: Clean the Surface

Pressure wash the patio first to remove the loose dirt, moss, and slime. You want the chemical to have direct contact with the black spots, not the mud covering them.

Step 2: Apply the Chemical

• Dilution: For heavy black spot, a 1:1 ratio (1 part Water to 1 part 15% Hypo) is standard.*

• Application: Use a plastic watering can or a low-pressure sprayer. Saturate the stone.

*Always check and follow instructions on the kit you have acquired.

Step 3: The Dwell Time (Critical)

This is where patience is required. You must leave the chemical on the stone for 30 to 60 minutes.

• Keep it Wet: If the chemical dries out, it stops working. If it’s a hot day, mist the patio lightly with water to keep the chemical active.

• Agitate: Use a stiff broom to scrub the liquid into the stubborn spots.

Step 4: The Rinse

After the dwell time, rinse the area thoroughly with a hose or pressure washer. You will see the black spots have vanished or turned a faint grey (which will bleach out further in the sun over the next 24 hours).

4. Protecting Your Garden

Sodium Hypochlorite kills lichen. It also kills grass, prize roses, and hedges. You must manage the runoff.

• The "Pre-Soak": Before you apply any chemical, soak your grass and flower beds with plain water until they are sodden. If the soil is full of water, it cannot absorb the bleach runoff.

• The "Post-Rinse": Immediately after rinsing the patio, flood the grass/beds with plain water again to dilute any chemical that splashed over.

5. What About "Patio Magic" (BAC)?

You will see products like "Patio Magic" (Benzalkonium Chloride) sold in garden centres.

• The Verdict: These are excellent preventatives and green algae killers. They are generally too weak to remove established Black Spot Lichen.

• The Rule: Use Hypo to cure the problem. Use BAC to maintain it once a year.

Conclusion

If you are staring at a leopard-print patio, put the pressure washer lance down. You are just eroding the stone. You need to kill the spores.

• Respect the chemical (Wear gloves and old clothes).

• Soak the grass.

• Let the chemistry do the work.