The right way to remove lichen from concrete without it coming straight back
Lichen is the flat, crusty growth that looks like grey-green paint splatter. It is not dirt, and treating it like dirt is why it keeps coming back.
Why pressure alone fails
Lichen is a fungus and an alga living together as one organism. It anchors into the concrete surface with root-like structures called rhizines.
Pressure removes the visible body. The rhizines stay in the pore structure of the concrete, and they regrow. Six to twelve weeks is typical.
Worse, blasting spreads spores across the rest of the slab. People clean one patch and find three new ones the next season.
What actually works
Pre-treat with a biocide and let it dwell. Not a quick spray and rinse — it needs contact time to penetrate and kill the organism through.
Then pressure clean with a rotary surface cleaner to lift the dead growth evenly.
Then a second light biocide application as a residual, which slows re-establishment considerably.
Skipping step one is the single most common mistake, and it is the reason cheap driveway cleans do not last.
The lighter patch problem
Established lichen protects the concrete underneath it from UV and weathering. Remove it, and the concrete under it is a different shade from the surrounding slab.
This is not damage and it evens out over six to twelve months of weathering. But it is startling if nobody warned you.
On badly affected driveways we sometimes recommend cleaning the entire slab rather than spot-treating, purely so the finish is even.
Stopping it coming back
Cut back overhanging branches. Lichen needs shade and moisture, and reducing either slows it dramatically.
Fix drainage that keeps a section of slab permanently damp.
Sealing helps — a sealed surface gives rhizines much less to anchor into.
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