If your patio has small dark dots that won't scrub off and seem to come back after every clean, you're not dealing with dirt. You're dealing with lichen — a slow-growing organism that lives in the pores of natural stone and concrete paving. Pressure washing alone won't kill it. Here's what does.
What black spots actually are
Black spot is a type of lichen, a symbiotic life-form that bonds permanently to porous stone. Once established, it grows roots into the slab. A jet of water knocks the surface fluff off, but the spores stay alive in the pores and re-emerge in months.
Why pressure washing alone fails
High-pressure water without pre-treatment damages the surface of porous slabs — it removes the sealed top layer, opens the pores wider, and gives the lichen even more anchor points for the next bloom. We've seen Indian sandstone and limestone permanently scarred this way. The patio looks clean for a fortnight, then the spots come back darker.
What works
- Apply a specialist black-spot remover (chemical) and leave it to dwell for 2–4 hours so it kills the lichen at the root.
- Rinse off with low-pressure soft-wash — not a high-pressure jet — to clear the dead lichen without damaging the stone.
- Repeat on heavy infestations. One pass usually clears 80–90%; a second pass clears the rest.
- Finish with a sealer where appropriate (sandstone, limestone, porous concrete) to slow re-bloom.
Common myths
Vinegar and bleach are routinely suggested online. Both look like they work for a day or two — they bleach the surface lichen — but they don't penetrate the stone. Worse, both are acidic enough to dissolve mineral content in limestone and natural sandstone, leaving the surface more porous than before.
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