A green patio in autumn isn't dirt, and that's why scrubbing it never seems to fix anything. What you're looking at is a living film of algae, and it arrives every September and October for reasons that have nothing to do with how tidy you are. Light levels drop, surfaces stop drying out between showers, and a single-celled organism that has been sitting there quietly all summer finally gets the conditions it wants.
We manufacture Moss-Go trade-strength patio cleaner in East Kilbride, so we get the phone calls in October. The most common one starts with "I pressure washed it in July and it's back already." Here's why that happens, and what actually changes the outcome.
Why does a patio turn green in autumn and not in summer?
Three conditions have to line up, and autumn lines all three up at once.
The first is moisture. Algae need a surface that stays damp. In July a slab that gets rained on at nine in the morning is bone dry by eleven. In October the same slab stays wet all day, and in a shaded spot it may not fully dry from one week to the next.
The second is light. Algae photosynthesise, but they're poor competitors in bright conditions and much happier in the low, diffuse light of a British autumn. That's why the green appears first on north-facing slabs, under a hedge, along a fence line, and last in the middle of an open sunny terrace.
The third is food. Organic debris breaking down on the surface feeds the film. Leaf fall, bird droppings, pollen residue, spent bedding compost from a nearby pot. It all breaks down into exactly what the algae want.
Nothing there is about cleanliness. A spotless patio in a shaded damp corner goes green faster than a neglected one in full sun.
Why doesn't pressure washing fix it?
Because pressure removes the visible film without removing what regrows it, and it damages the surface at the same time.
What you can see is only the top layer. Algae and lichen sit in the pores of the stone as well as on it, and spores survive in joints, chips and the bedding sand. A washer at 100 bar blasts the surface clean and leaves the reservoir intact. Two months of damp weather later, you're back where you started.
Then there's the collateral damage. Pressure washing sandstone and soft limestone erodes the face of the slab, and on block paving it fires out the kiln-dried joint sand. Once the joints are empty the blocks loosen, water sits in them, and weeds root in the gap. We've had customers spend a Saturday washing and then a fortnight re-sanding.
The honest summary is that pressure washing gives you an instant result and a shorter interval before the next one. It works, briefly, and it costs you surface.
What actually works on a green patio?
A chemical clean works differently. Rather than stripping the film off mechanically, it breaks down the organic matter in place and lets weather and a stiff brush do the rest.
Moss-Go is formulated with surfactants and micro-encapsulated salts rather than sodium hypochlorite, which matters more than it sounds. Hypochlorite bleaches organic growth white so the patio looks clean within hours, but it's strongly alkaline, it strips a thin layer off the stone and dissolves iron deposits, and the small cavities it leaves behind are ideal habitat for the next generation of moss. It also holds water that expands when it freezes. That's a cleaner that makes next year worse.
The practical method:
Sweep first with a stiff-bristled broom. Loose debris shields the surface and wastes product.
Dilute 1 litre of concentrate in 5 litres of water for general and moderate growth. For heavy algal staining you can go as strong as 50:50.
Apply with a sprayer held roughly 6 inches above the surface, or a watering can with a rose at about 12 inches. Work from one side to the other so nothing is missed.
Agitate black spot and ingrained areas with a stiff brush after application, not before.
Then leave it. The surface brightens over 24 to 48 hours as the growth breaks down, and a brush off after a week clears the residue.
A 5 litre container covers roughly 100 to 200 square metres at the general dilution, so most domestic patios are one container.
When should you apply it, and when shouldn't you?
Autumn and early spring are the right windows, which is convenient because that's when the problem shows up. Cooler weather also means less evaporation and less risk of runoff.
Apply on an overcast day. If the slabs are warm to the touch, hose them down and wait 30 minutes before you start, or the actives evaporate before they've done anything. Wear goggles and gloves, read the label, and keep pets and children off until the surface is dry.
Do not use it when heavy rain is due within a few hours. You'll wash it off before it has worked.
How long does a treatment last?
The label figure is up to 12 months, and on an open, reasonably sunny patio that's realistic. In a permanently shaded north-facing corner under a conifer, expect less. We would rather say that than have you disappointed in March.
This is the part most cleaning articles skip: treatment addresses the growth, not the conditions. If a surface stays damp and shaded, algae will return, because everything it needs is still there. Cutting back overhanging growth to let light and air in, clearing leaves promptly, and fixing a downpipe that discharges across the slabs will do more for the interval between cleans than any product will.
Moss returning on paths and in joints is a slightly different problem, and we've covered the treatment-versus-conditions question in more depth in moss on paths and patios. The full step-by-step method, including surface-specific notes, is in our how to clean your patio guide, and there's more on technique and what to avoid in cleaning a patio without wrecking the pointing.
Do you always need a cleaner?
No. A light green bloom on an open patio in a sunny spot will often weather off over winter, and on a small area a stiff brush and a bucket of hot water is enough. If you can shift it dry with a broom, you don't need us.
Where a cleaner earns its place is large areas, ingrained black spot, north-facing surfaces, and anywhere slip risk is a real concern. Wet algae on steps is genuinely dangerous, and that's the case where we'd say don't wait.
Clean it once, properly
Moss-Go trade-strength patio cleaner is made by us in East Kilbride, dilutes 1:5 for general use, covers 100 to 200 square metres per 5 litres, and needs no pressure washer or rinsing. See Moss-Go patio cleaner, with free next-day UK delivery.
Frequently asked questions
Why is my patio green in autumn but fine in summer?
Algae need a surface that stays damp and light levels that are low rather than bright. Autumn gives them both, plus a supply of decaying leaf debris to feed on.
Will a pressure washer get rid of a green patio for good?
No. It removes the visible film but leaves spores in the pores, joints and bedding sand, and it erodes soft stone and blasts out joint sand. The green returns once damp conditions do.
What dilution should I use on a badly stained patio?
1 litre of concentrate to 5 litres of water handles general growth. For heavy algal staining you can increase it up to a 50:50 ratio.
How soon will I see a difference?
The surface starts to brighten over 24 to 48 hours as the organic growth breaks down. Brush the residue off after about a week.
Is it safe around pets and children?
Once the treated surface is dry, yes. During application wear goggles and gloves and keep pets and children off the area, and always read the label first.
