Yes, permanently in most cases, and it is the commonest regret on this shelf. The precaution is simple and it has to be taken before the water goes on.
What the staining is
Iron in solution, oxidising on and in a porous surface. On concrete, on sandstone, on a cast slab, it produces the orange-brown of rust and it goes into the pores rather than sitting on them. Rain does not lift it.
Non-porous surfaces — glazed tile, some engineering brick — fare better. Timber decking stains. Painted metal stains where the paint is chipped.
Where it actually happens
Granules on the path. A broadcast spreader throws to both sides, and the strip along a path edge gets the edge of the pattern. Every granule that lands on paving becomes a mark when it is watered in.
Wheeling the spreader across the patio with the hopper open, which leaves a trail.
Runoff. Water applied heavily on a lawn that slopes toward a hard surface carries dissolved product with it.
Drift, with liquid forms, onto a nearby wall or fence.
Boots and mower wheels, tracking wet product onto a drive an hour later.
What avoids it
Sweep the hard surfaces before watering in — the single most effective step, and it takes two minutes.
Close the spreader before crossing anything hard, and shut it at the end of each pass rather than at the end of the run.
Use an edge guard where the spreader has one, or a drop spreader rather than a broadcast one along boundaries.
Water in moderately rather than flooding, on a lawn that slopes toward paving.
What the label says
Rates, precautions, protective equipment and what the product may be applied to are all on the label, which is a legal document for a regulated product. This site prints what the maker states about coverage and points at the label rather than paraphrasing its instructions.
How this site handles it
The moss killer pages here state the active as the maker names it and the coverage as stated. Where a product is an iron-based granule, the page says so plainly, because that is the information that decides whether it goes near a patio at all.
Wherever this page and the maker’s own documentation disagree, the documentation wins. Every cutting width, cutting height, stated area, pack capacity, flow rate, head, sound level and application rate on this page comes from the maker’s own published figures, with the unit as printed and the source named. None of it is measured here, and no figure is converted into another convention, because a converted number is a different number. And the document this page hands back: the dose, the precautions and what a product may be applied to are on its label. That label is a legal text for a regulated product, and paraphrasing it would be rewriting it. This page reports the coverage and the analysis as the maker prints them, and points at the label for everything else.
The questions that keep coming in
Will the stains wash off?
Rain does not remove them. They are iron oxide bonded into a porous surface, and on concrete or natural stone they are effectively permanent without a chemical rust remover — which carries its own risk to the stone.
Is the liquid form safer around a patio?
Safer from granules bouncing, worse for drift and overspray. Each form has a different way of ending up where it should not be, and neither is forgiving once it is there.
Last reviewed 26 September 2026