What pressure washing actually removes
A pressure wash physically strips visible moss, algae and general grime off the surface. It looks completely clean afterwards. But pressure washing doesn't kill the organism at root level - microscopic spores and roots can remain within the surface, joints and crevices even after a thorough clean.
Why it grows back so fast
Because those spores were never killed, just displaced, regrowth after pressure washing alone is often faster than if the surface had been left untreated in the first place - there's more open surface and moisture for the spores to re-establish in.
What a biocide treatment actually does
A biocide is a chemical treatment applied to the surface that targets the moss, algae and fungus at root level rather than just removing what's visible, killing spores so they can't simply regrow within a few weeks.
How it's applied
Biocide treatment is typically applied by brush or low-pressure sprayer, with the surface thoroughly wetted, rather than blasted in with a pressure washer. It's a separate step to the wash itself, usually done after the surface has been cleaned.
What this means for your job
If your patio or driveway has recurring moss or green staining, a biocide/moss pre-treatment on top of the normal wash (see our pricing page) is what actually slows regrowth, not a harder pressure wash. Natural stone in particular is treated this way as standard, since high pressure alone can damage the stone anyway.
Sources: GoodGrow UK, "How to Kill Moss, Mould & Algae"; Streak Free Exterior Cleaning, "Biocide Application".