Black spot on terrace paving

BenCotto

Using the pressure washer I have removed most of the small black mould spots on my natural stone paving, but I was wondering if people have found the chemical wash solutions to be effective. Are there any recommended brands? When swilling away the residue, is it harmful to adjacent plants? 

Thanks.

gjautos

My father in law used chemicals on his patio. I can't remember the make, it did a good job but, his Indian sandstone slabs have been turning green ever since. This never happened when he only used a pressure washer.

fritterer

There's this: https://algonorganics.co.uk

Doghouse Riley

I've a crazy York stone patio which I laid thirty years ago. I sometimes use a Karcher jetwash on it and other times, if it's just a bit "green,"  "Wet n' Forget."

It's sandstone, so over time it has got some dark marks on it as it's slightly porous so it's unavoidable. They will never come out.

It's not too bad, so I just accept it.

This was it in March after I'd removed a dozen roses  in ceramic pots and jet-washed it.

Always a bit of a pain.

Nollie

Depends on the type of stone and the chemicals used. Using an acid chemical on an alkaline stone (such as sandstone and limestone) will seem effective to begin with, but the acid causes pitting, where the surface roughens and develops microscopic holes which then attract greater quantities of mould, algae and dirt, making it ever more difficult to clean. I suspect that’s what happened with your FIL’s patio @gjautos. It essentially dissolves and weakens the stone. Over-zealous pressure washing will ultimately have a similar outcome and is similarly counter-productive.

A very dilute solution of ammonia (alkaline) may work. More effective still, a specialised Lithofin product for natural stone. I have found the latter to be the only safe and effective product. HG market a similar one, it’s harmless but useless.

Whatever you use, probably best to mop up with clean water, changing the water regularly, rather than hose it into your plants.