Every autumn the same thing happens. A patio that looked clean in August starts to darken at the edges in October, turns a slippery olive green by December, and by February the joints are sprouting cushions of moss. Fences, north-facing walls, decking and garden furniture follow the same path.
It isn't bad luck and it isn't a sign that your paving is poor quality. It's the UK winter doing exactly what it does best: keeping surfaces wet, cool and in shade for months at a time. This guide explains what the green stuff actually is, why it takes hold in winter rather than summer, and what the numbers look like where you live.
The short answer: green algae and moss need a surface that stays damp for days on end. Between October and February, UK surfaces get more rain, far less sun and much longer shadows, so they barely dry out. North-facing and shaded areas can go weeks without any direct sunlight at all.
What is the green stuff on my patio?
Most of the green film on patios, paths, fences and walls is green algae: microscopic single-celled plants that live in a thin film of water on the surface. A handful of hardy, drought-tolerant species do most of the work. They survive dry spells as dormant cells and spring back into growth as soon as the surface stays wet.
As it builds up, the film gets thicker and slimier, traps dust and organic debris, and starts to hold water itself. That layer of algae and grime is exactly the kind of damp, slightly nutrient-rich seedbed that moss spores need to germinate. This is why moss usually appears in the joints and edges of paving that has been green for a while.
Other things you might see alongside it:
- Moss: soft green cushions or mats, usually in joints, on the shaded side of walls, on roofs and along the bottom edge of fences.
- Lichen: flat grey, yellow or pale green crusty patches, often circular. Lichen is a partnership between a fungus and an alga and grows very slowly, so it tends to appear on surfaces that have been left undisturbed for years.
- Black spots: small, stubborn black dots on sandstone and concrete, which are also lichen. They don't brush off like algae.
- Orange or rust-coloured staining: on stone and render in damp, sheltered spots, this is often a type of algae rather than rust.
Why do surfaces go green in winter and not summer?
Algae actually grow faster when it's warm, so on paper summer should be worse. In practice, though, the limiting factor in a British garden isn't warmth. It's how long the surface stays wet. Four things change as the year turns.
1. Surfaces stop drying out
In summer, a shower on a patio evaporates within hours. In winter, low temperatures, weak sunshine and still, humid air mean evaporation almost stops. A patio that gets wet on a Tuesday can still be damp the following week, especially if it rains again in between, and across much of the UK it rains on around half of all winter days, or more.
2. The sun is lower and weaker
In December the midday sun in London is only about 15° above the horizon, and in Glasgow about 11°. Low-angle sunlight carries far less energy per square metre, so even a sunny winter day does little to dry a surface. The same low angle also makes shadows enormous (more on that below).
3. Leaves, soil and debris pile up
Autumn leaf fall is a major and often overlooked cause. Leaves sitting on paving hold moisture against it, block what little light there is, and as they rot they release nutrients that feed algae. Soil splashed up by rain and blown into joints does the same thing.
4. Algae and moss are built for the cold
Many surface algae and mosses keep photosynthesising at temperatures just a few degrees above freezing, as long as they're wet. Unlike most garden plants, mosses don't have roots or a waterproof skin. They absorb water directly through their leaves and can dry out almost completely, then revive within minutes when they get wet again. For moss, a British winter is growing season.
The UK autumn and winter in numbers
The tables below use the Met Office's official regional climate averages for 1991–2020, which is the current standard 30-year baseline. They show why the green comes earlier and heavier in some parts of the country than others.
Average temperature (°C)
| Region | Sep | Oct | Nov | Dec | Jan | Feb |
|---|---|---|---|---|---|---|
| South East & central S England | 14.7 | 11.3 | 7.7 | 5.3 | 4.9 | 5.0 |
| South West England & S Wales | 13.8 | 10.8 | 7.7 | 5.5 | 5.0 | 5.1 |
| Midlands | 13.8 | 10.4 | 6.9 | 4.5 | 4.2 | 4.5 |
| East & North East England | 13.2 | 9.9 | 6.4 | 3.9 | 3.7 | 4.1 |
| North West England & N Wales | 12.9 | 9.8 | 6.7 | 4.4 | 4.1 | 4.2 |
| Northern Ireland | 12.6 | 9.6 | 6.7 | 4.7 | 4.5 | 4.7 |
| West Scotland | 11.8 | 8.8 | 5.9 | 3.7 | 3.5 | 3.7 |
| East Scotland | 11.3 | 8.0 | 4.8 | 2.5 | 2.4 | 2.8 |
| North Scotland | 10.9 | 7.9 | 5.0 | 2.9 | 2.9 | 2.9 |
| UK average | 12.9 | 9.7 | 6.5 | 4.2 | 3.9 | 4.1 |
Temperatures stay mostly above freezing across the UK through winter. That's cold enough to stop surfaces drying, but not cold enough to stop algae and moss growing.
Average rainfall and rain days
Each cell shows average monthly rainfall, then the average number of days with at least 1 mm of rain.
| Region | Sep | Oct | Nov | Dec | Jan | Feb |
|---|---|---|---|---|---|---|
| South East & central S England | 61 mm / 9 | 90 mm / 12 | 94 mm / 13 | 89 mm / 13 | 84 mm / 13 | 61 mm / 10 |
| South West England & S Wales | 92 mm / 12 | 140 mm / 16 | 146 mm / 17 | 152 mm / 17 | 138 mm / 16 | 104 mm / 13 |
| Midlands | 64 mm / 10 | 82 mm / 12 | 80 mm / 13 | 82 mm / 13 | 73 mm / 13 | 59 mm / 11 |
| East & North East England | 64 mm / 10 | 77 mm / 12 | 81 mm / 13 | 77 mm / 13 | 69 mm / 13 | 59 mm / 11 |
| North West England & N Wales | 110 mm / 13 | 140 mm / 16 | 142 mm / 17 | 155 mm / 17 | 133 mm / 16 | 110 mm / 14 |
| Northern Ireland | 88 mm / 14 | 114 mm / 16 | 122 mm / 18 | 121 mm / 17 | 115 mm / 17 | 92 mm / 15 |
| West Scotland | 142 mm / 16 | 194 mm / 18 | 194 mm / 19 | 206 mm / 19 | 205 mm / 19 | 163 mm / 16 |
| East Scotland | 89 mm / 12 | 130 mm / 16 | 125 mm / 16 | 122 mm / 15 | 125 mm / 16 | 97 mm / 13 |
| North Scotland | 137 mm / 16 | 180 mm / 20 | 177 mm / 20 | 192 mm / 20 | 201 mm / 20 | 160 mm / 18 |
| UK average | 91 mm / 12 | 122 mm / 15 | 123 mm / 16 | 127 mm / 16 | 122 mm / 15 | 96 mm / 13 |
In western and northern Scotland it rains on roughly two days in three from October to January, and in the North West, the South West and Northern Ireland on more than half of days. Even in the drier South East, it rains on around 13 days a month. There simply aren't enough dry days in a row for paving to dry out properly.
Average sunshine (hours per month)
| Region | Sep | Oct | Nov | Dec | Jan | Feb |
|---|---|---|---|---|---|---|
| South East & central S England | 155 | 113 | 70 | 54 | 60 | 80 |
| South West England & S Wales | 142 | 100 | 64 | 48 | 54 | 76 |
| Midlands | 136 | 100 | 62 | 50 | 53 | 76 |
| East & North East England | 135 | 99 | 65 | 53 | 56 | 81 |
| North West England & N Wales | 124 | 88 | 54 | 40 | 45 | 69 |
| Northern Ireland | 113 | 85 | 54 | 38 | 43 | 67 |
| West Scotland | 109 | 77 | 50 | 32 | 36 | 64 |
| East Scotland | 114 | 82 | 57 | 36 | 43 | 71 |
| North Scotland | 99 | 68 | 39 | 22 | 28 | 57 |
| UK average | 128 | 92 | 58 | 43 | 48 | 72 |
Compare September with December. Sunshine hours fall by roughly two-thirds across the UK, and in North Scotland a whole month of December averages around 22 hours of sun. That is the drying power that disappears in winter.
Sunrise and sunset times
Times are for the 15th of each month in 2026/27, in local clock time. British Summer Time ends on 25 October 2026, which is why sunset jumps an hour earlier between the October and November columns.
| City | 15 Sep | 15 Oct | 15 Nov | 15 Dec | 15 Jan | 15 Feb | Daylight on 21 Dec |
|---|---|---|---|---|---|---|---|
| London | 06:35–19:14 | 07:24–18:07 | 07:18–16:10 | 08:00–15:51 | 07:59–16:20 | 07:15–17:14 | 7h 49m |
| Cardiff | 06:47–19:27 | 07:36–18:19 | 07:30–16:23 | 08:12–16:03 | 08:12–16:32 | 07:27–17:27 | 7h 49m |
| Birmingham | 06:41–19:22 | 07:33–18:12 | 07:29–16:14 | 08:12–15:53 | 08:11–16:22 | 07:24–17:19 | 7h 38m |
| Manchester | 06:42–19:24 | 07:36–18:12 | 07:34–16:11 | 08:19–15:49 | 08:17–16:19 | 07:28–17:18 | 7h 27m |
| Newcastle | 06:39–19:23 | 07:36–18:07 | 07:38–16:02 | 08:25–15:37 | 08:22–16:09 | 07:29–17:12 | 7h 10m |
| Belfast | 06:56–19:40 | 07:52–18:25 | 07:54–16:21 | 08:40–15:57 | 08:37–16:28 | 07:46–17:30 | 7h 14m |
| Glasgow | 06:48–19:34 | 07:48–18:16 | 07:53–16:09 | 08:41–15:42 | 08:38–16:15 | 07:42–17:20 | 6h 58m |
| Edinburgh | 06:44–19:30 | 07:44–18:11 | 07:49–16:04 | 08:38–15:37 | 08:34–16:10 | 07:38–17:15 | 6h 57m |
| Inverness | 06:47–19:35 | 07:51–18:13 | 08:00–16:01 | 08:53–15:30 | 08:47–16:05 | 07:47–17:15 | 6h 35m |
In the far north of Scotland, the shortest day has less than seven hours between sunrise and sunset. With the sun barely clearing rooftops and trees for much of that time, many gardens get only an hour or two of direct light, if any.
Why north-facing and shaded areas go green first
This is where shadow length comes in, and it explains why one side of a garden turns green while the other stays clean.
The table below shows how long a shadow a standard 1.8 m (6 ft) fence casts at midday, when the sun is at its highest point of the day. At any other time of day the shadow is longer still.
| City | 15 Sep | 15 Oct | 15 Nov | 15 Dec | 21 Dec | 15 Jan | 15 Feb |
|---|---|---|---|---|---|---|---|
| London | 2.0 m | 3.1 m | 4.9 m | 6.6 m | 6.7 m | 5.7 m | 3.7 m |
| Cardiff | 2.0 m | 3.1 m | 4.9 m | 6.6 m | 6.7 m | 5.7 m | 3.7 m |
| Birmingham | 2.1 m | 3.3 m | 5.2 m | 7.1 m | 7.1 m | 6.1 m | 3.9 m |
| Manchester | 2.2 m | 3.4 m | 5.5 m | 7.6 m | 7.7 m | 6.5 m | 4.1 m |
| Newcastle | 2.3 m | 3.6 m | 6.1 m | 8.6 m | 8.7 m | 7.2 m | 4.4 m |
| Belfast | 2.3 m | 3.6 m | 5.9 m | 8.3 m | 8.4 m | 7.0 m | 4.3 m |
| Glasgow | 2.4 m | 3.8 m | 6.4 m | 9.3 m | 9.5 m | 7.7 m | 4.6 m |
| Edinburgh | 2.4 m | 3.8 m | 6.5 m | 9.4 m | 9.5 m | 7.8 m | 4.6 m |
| Inverness | 2.5 m | 4.1 m | 7.2 m | 10.9 m | 11.1 m | 8.9 m | 5.0 m |
In mid-December in Glasgow, a 6 ft fence casts a shadow over 9 metres long at midday. Any patio, path or lawn within that distance on the north side of the fence gets no direct sun at all for weeks around the winter solstice. In Inverness the shadow is over 11 metres. Even in London it's nearly 7 metres, which is deeper than most back gardens' paved areas.
For context, here is how high the sun gets at midday:
| City | 15 Sep | 15 Oct | 15 Nov | 21 Dec | 15 Feb |
|---|---|---|---|---|---|
| London | 41° | 30° | 20° | 15° | 26° |
| Manchester | 39° | 28° | 18° | 13° | 24° |
| Glasgow | 37° | 26° | 16° | 11° | 22° |
| Inverness | 36° | 24° | 14° | 9° | 20° |
The practical upshot:
- North-facing patios, paths and the north side of fences, walls and sheds sit in permanent winter shade. They go green first, go greenest, and stay green longest.
- Paving between two buildings or under trees gets almost no sun from October to February.
- South-facing surfaces get what little winter sun there is and usually stay much cleaner, even in the same garden.
To check your own garden, measure the height of your fence, wall or house on the south side of a paved area and multiply it by the "shadow ratio" for your latitude. Around the shortest day that ratio is about 3.7 in London and Cardiff, 4.3 in Manchester, 4.7 to 4.8 in Belfast and Newcastle, 5.3 in Glasgow and Edinburgh, and 6.2 in Inverness. If the distance to the far edge of the patio is less than that figure, that part of the patio gets no midday sun in December.
How moss takes hold, step by step
Moss rarely colonises clean, dry paving. It follows a predictable sequence:
- The surface stays wet. Shade, poor drainage or a lack of fall on the paving keep water sitting on it for days.
- Algae form a film. Within weeks of consistent dampness, a green film develops. Porous materials such as sandstone, concrete and old brick get there fastest because they hold water in their pores.
- Debris collects. The sticky algal film traps dust, soil, pollen and leaf fragments, especially in joints and along edges.
- Moss spores land and germinate. Spores are everywhere in the air. Once they land in a damp, sheltered, slightly nutrient-rich joint, they germinate.
- Moss cushions spread. Moss holds several times its own weight in water, which keeps the joint and surrounding paving wet for longer and helps more algae and moss establish around it.
Once moss is in the joints, it also loosens jointing mortar and sand over time, which lets weeds in. That is why a patio that is left green for a couple of winters often looks much worse than one that is dealt with each autumn.
What causes green algae on fences and walls?
Fences and walls follow the same rules as paving, with a few extras:
- Timber holds water. Untreated or weathered softwood absorbs moisture and stays damp inside long after the surface feels dry. The bottom rail and gravel boards, which get splashback from the ground, go green first.
- Overhanging plants and climbers keep fences shaded and humid and drip water onto them after every shower.
- Render and masonry near the ground wick up moisture, and walls behind dense planting never get air moving across them.
- Cleaner air. Since the 1980s, air pollution in the UK has fallen sharply. That is good news in almost every way, but it has also made outdoor surfaces friendlier to algae, moss and lichen, which older pollution used to suppress.
Is green algae on a patio dangerous?
The main risk is slipping. A wet film of algae on paving, decking or steps can be as slippery as ice, and it's worst in exactly the months when it's also dark early. Keep paths and steps clear through winter, especially where older people or children walk.
It is not the same thing as the "blue-green algae" that you may see warnings about at lakes and reservoirs in summer. Those are toxic blooms of cyanobacteria in still, nutrient-rich water. The green film on your patio is a different organism living in a very different place, so it doesn't carry that risk. It's still sensible to wash hands after handling it, and to rinse any cleaning products off before pets walk on the area.
How to stop your patio going green
You can't change the weather, but you can change how long your surfaces stay wet.
- Clean in early autumn, not spring. Clearing algae in September or October removes the starter film before the wet season, so there's much less to build on over winter.
- Keep it clear through winter. Sweep leaves, soil and debris off paving at least weekly from October to December, and pay particular attention to the joints.
- Let the light and air in. Cut back overhanging branches and shrubs, and thin out climbers on fences near paving. Even a modest increase in light and air movement speeds up drying.
- Fix standing water. Paving should have a gentle fall so rainwater runs off, and gutters and downpipes shouldn't be spilling onto it. Puddles that sit for a day after rain are a green patch in the making.
- Maintain the joints. Well-filled, sound joints leave nowhere for debris and moss spores to settle.
- Treat when it's dry. If you use a patio or fence cleaner, apply it during a dry spell and follow the label on timing and dilution. Many treatments work over several weeks as the weather does the rest.
- Protect timber. A good preservative or stain on fences and decking reduces how much water the wood absorbs.
Getting rid of green algae and moss that's already there
For established growth, a stiff brush and a suitable cleaner are usually more effective, and kinder to the surface, than blasting it with a pressure washer. A few points to keep in mind:
- Pressure washers remove algae quickly, but on sandstone, limestone and softer concrete they can erode the surface and strip out jointing. That leaves a rougher, more porous finish that goes green even faster next winter. If you use one, keep to a low setting with a wide fan nozzle, held well back.
- Scrape moss out of joints before cleaning, and re-point or refill any joints that are left open.
- Bleach will lighten algae but can harm nearby plants and soil life, can discolour some stone, and doesn't prevent regrowth.
- Vinegar is a common home remedy for moss, but it's acidic and can etch limestone, marble and some concrete, so test it on a hidden area first.
For more on paving materials, falls and cleaning, see our guide to patios, paths and paving. If moss is spreading into your grass too, the causes are much the same, and the lawns section covers how to deal with it.
Common questions
Why is only one side of my patio green?
That side is almost certainly in shade for most of the winter, usually because a fence, wall, house or tree stands to its south. Use the shadow table above to check. In December a 6 ft fence shades a strip between roughly 6.7 and 11 metres deep at midday, depending on how far north you are.
When is the best time to clean a green patio?
Early autumn is best, before the wet season starts, with a follow-up on a dry spell in late winter or early spring. Cleaning in summer looks satisfying, but the green returns quickly once October arrives.
Why does my patio go green so quickly after cleaning?
Usually because the conditions haven't changed: the area is still shaded, still collects debris or still holds water. Pressure washing can make it worse by roughening the surface. Tackling shade, debris and drainage makes much more difference than cleaning more often.
Does moss damage paving?
Not directly, but moss holds water against the surface and in joints. That speeds up frost damage to porous stone and loosens jointing over time, which then lets weeds in.
Is moss on a patio a sign of poor drainage?
Often, yes, or of shade, or both. Moss spreading across the flat surface of paving rather than just in the joints usually means water is sitting on it for long periods.
Climate figures are 1991–2020 averages calculated from the Met Office UK and regional series. Sunrise, sunset, sun height and shadow lengths are calculated for each city for 2026/27, with shadows measured at solar noon on flat ground.