Patio Green Algae: Why It Comes Back Every Winter (and How to Prevent It)
The short answer is patios go green in winter because damp + shade + organic debris create perfect conditions for algae to re-establish. To slow it down, keep the surface drier (sweep leaves, improve drainage), clean correctly, and consider an after-treatment or a suitable sealant for porous paving.
But lets cover it in more depth, and more importantly a quick guide on how to get rid of it as slippery timber decking can be a real hazard.
Why patios turn green every winter
If your patio looks fine in summer and then turns green again by late autumn or winter, you’re not alone, especially in West Sussex. The pattern is usually a mix of damp, shade, and organic build-up.
You’ll usually notice the first green patches in the most shaded outdoor corners of the garden, where humidity stays high and drying time is low due to limited sunlight exposure. Algae is one of several surface organisms you’ll see outdoors (alongside moss and lichen) and they all thrive when surfaces stay damp and shaded.
Green film on paving is commonly algae (and sometimes mildew/mould-like growth in persistently damp corners). Once it establishes in microscopic pores and surface texture, it returns faster each year unless you change the conditions that support it.
In our local environment, West Sussex weather (mild temperature swings and frequent damp conditions) creates the perfect conditions for regrowth on shaded paving.
The three main causes
1) Shade + moisture = ideal conditions
North-facing patios, areas under trees, and zones that don’t dry quickly are prime territory. Areas beside walls and fencing, or under overhanging plants and garden furniture, often stay damp for longer because they get less airflow and sunlight.
2) Organic “food” builds up
Leaf litter, soil, and fine debris sit on the surface and in joints. Over winter, that becomes a thin organic layer that algae can use to re-colonise. Debris also gathers in corners around decking, along edges, and where patios meet driveways, feeding regrowth through winter.
3) Porous surfaces hold on to growth
Sandstone, concrete flags, and textured paving often hold moisture and staining. Even after a good clean, some growth can reappear if aftercare is skipped.
In our experience around West Sussex and Surrey, the patios that regrow fastest are usually north-facing, under trees, or next to borders where leaves and soil sit in joints through autumn. If you fix that (debris and drying time), you usually fix most of the problem.
Pressure washers with a range of fittings will include a dome to stop spray-back
Cleaning: what works (and what often backfires)
A good clean is usually a combination of:
pre-wet → detergent/soap where appropriate → scrub/brush detailing → rinse
and (where suitable) a controlled pressure wash for the heavier grime
Common DIY mistakes to avoid
Overusing pressure and blasting the joints (this removes jointing sand and can make regrowth worse).
Leaving residues (detergent/soap film can attract dirt, so rinse properly).
Reaching for harsh household chemicals (especially bleach) without understanding the surface or runoff risks.
Important safety note on bleach: Do not mix bleach (sodium hypochlorite) with ammonia-based cleaners as this can create irritating gases (chloramines).
If you use any treatment, follow the label, protect plants, and avoid uncontrolled runoff.
If you are cleaning yourself:
test any detergent on an inconspicuous area first
keep pressure conservative and avoid blasting joints
rinse thoroughly so residues don’t become a sticky film that attracts new dirt
If you’re using a pressure washer, keep the spray moving and avoid forcing water into joints, edges, or against walls where it can drive dirt deeper or loosen sand.
Mini decision table: best approach by surface type
| Patio surface | Common winter issue | Best approach | Cautions |
|---|---|---|---|
| Sandstone / natural stone | Green film + deeper staining | Gentle clean + after-treatment; consider sealing | Avoid aggressive pressure that can roughen stone |
| Concrete flags / textured paving | Algae in texture + joints | Controlled clean + brush detail; improve drainage | Don’t blast joints; rinse thoroughly |
| Porcelain | Slippery film (often in shade) | Lower-pressure clean + rinse; prevention focus | Avoid harsh chemicals; keep it debris-free |
Prevention: how to stop it coming back so quickly
You don’t need perfection, just a few practical changes that reduce regrowth speed. Think of this as light maintenance rather than a one-off clean: small outdoor habits reduce how quickly algae returns.
1) Keep it drier
Clear leaves and debris regularly (a quick sweep helps more than most people realise).
Improve drainage if water sits.
Trim back plants that keep the area shaded and damp.
2) Treat it after cleaning (where appropriate)
A post-clean treatment can slow regrowth by tackling remaining organic growth. Many “green stain remover” type products are designed to delay return when used as directed.
3) Consider sealing (big impact on porous patios)
A suitable paving sealant can reduce how easily algae establishes by sealing the surface and limiting moisture/contaminant penetration. This is particularly useful on certain stone and concrete surfaces (always check suitability first).
A simple seasonal routine (low effort, high payoff)
Early spring: clean + rinse + optional treatment. Clean patio and check nearby driveways for the same green film in shaded areas.
Mid-summer: quick sweep and spot-clean shaded corners
Autumn: clear leaves weekly, especially after windy days
Winter: Winter: keep it debris-free; move garden furniture occasionally so damp doesn’t sit underneath, and clear edges where exposure is lowest (near walls/fencing).
When to get a professional clean
If the patio is slippery, heavily stained, or the algae returns within weeks, it’s usually faster (and safer for the surface) to have it cleaned properly and then set up the right aftercare routine.
If you’re in West Sussex or Surrey and want your patio restored properly, see our patio cleaning and pressure washing service and request a free quote.
Patio algae FAQs
Is green algae dangerous?
It can become slippery, especially on shaded stone, so treat it as a slip risk.
Will pressure washing remove algae permanently?
It removes the current growth, but prevention (debris control + aftercare + optional sealing) is what stops rapid return.
Should I use bleach?
Be cautious. It can harm plants and may not be suitable for all paving/joints. Use specialist products as directed and avoid mixing chemicals.