Why Is My Patio So Dirty? Algae, Mould & Black Spots Explained
A patio is exposed to changing weather, dirt and everyday use throughout the year. Over time, mud, leaves and outdoor debris can collect across the surface, while damp and shaded conditions can encourage algae, mould, lichen and black spots to develop.
Some patio dirt is easy to remove. Loose debris and surface grime may only need sweeping or general patio cleaning. However, if your patio keeps going green, develops dark patches or remains covered in black marks after cleaning, the problem may be more than everyday dirt.
Knowing what is making your patio dirty can help you choose the right cleaning method. This guide explains how to identify surface dirt, mould, algae and patio black spot, what causes these common problems and how to keep your patio cleaner for longer.
What Causes Dirt to Build Up on a Patio?
Not every dirty patio has a mould, lichen or algae problem. Sometimes the surface is simply covered in everyday outdoor dirt.
Mud, soil, dust, leaves and other garden debris can gradually collect across paving slabs. Foot traffic can carry additional dirt onto the patio, particularly during wet weather, while fallen leaves and plant material can gather in paving joints, corners and beneath garden furniture.
Loose surface dirt can usually be removed with a stiff patio cleaning brush. Regular sweeping prevents debris from building up and makes it easier to see any marks beneath it.
For more noticeable grime, warm soapy water and a patio scrubbing brush may help. Work across the patio in manageable sections and pay particular attention to corners, paving joints and heavily used areas.
If the patio still looks green, dark or stained once the loose dirt has been removed, look more closely at the colour and texture of the remaining marks. Green staining may be algae, darker patches could be mould, and persistent black spots may be established lichen.
For a complete comparison of patio cleaning tools, pressure washers, homemade solutions and specialist patio cleaners, read our how to Clean a Patio Guide.
Why Does My Patio Keep Going Green?
If your patio keeps going green, algae is one of the most likely causes. It commonly appears as green staining, a thin film or larger green patches spreading across paving slabs. When wet, algae-covered paving can also become slippery underfoot.
Unlike mud, leaves or loose garden debris, algae is biological growth on the surface. This is why simply sweeping a green patio won't solve the problem. You may notice that some areas stay relatively clear while others repeatedly turn green, depending on the conditions around the paving.
If you're wondering how to clean green off a patio, it therefore helps to understand why the algae appeared in the first place before looking at algae removal.
What Causes Algae on a Patio?
Algae is closely associated with moisture. Paving that stays damp, receives limited sunlight or doesn't dry quickly can be more susceptible to green growth. Areas beneath trees, alongside walls and fences, or where water regularly collects may therefore turn green faster than more exposed sections of the same patio.
Leaves, soil and other garden debris can make the conditions more favourable by collecting on the surface and retaining moisture. Porous or textured paving can also hold moisture differently from smoother surfaces, which may help explain why algae appears more readily on some patios than others.
This is also why cleaning algae from a patio isn't always about how dirty the paving is. Two areas exposed to the same weather can look completely different if one stays shaded and damp for longer.
Why Does Algae Keep Coming Back?
You might remove algae from a patio only to notice the familiar green staining gradually returning. That's because cleaning the visible growth doesn't change the outdoor conditions around the paving.
If an area continues to remain damp or shaded, the conditions that encouraged algae in the first place are still present. Leaves and organic debris can also build up again, while areas with poor drainage may continue to hold water after rainfall.
This is why there is a difference between quickly removing visible algae and treating an affected surface with a product designed for outdoor organic growth. Frequent pressure washing can produce an immediate visual result, but it doesn't change the conditions around the patio.
Regular maintenance, keeping debris from building up and early algae removal can therefore make recurring green growth easier to manage. Damp conditions, shade, poor drainage and organic debris are all associated with recurring algae on paving.
What Causes Mould on a Patio?
Mould is a type of fungal growth that can develop on patios where the surface remains damp. It may appear as dark, grey or black patches across paving, particularly in sheltered areas that take longer to dry.
Interestingly, on materials such as concrete, mould isn't necessarily growing because it is feeding on the paving itself. Organic material such as dirt and debris can collect on the surface and provide a source on which mould can grow when moisture is also present.
This is one reason keeping a patio clear of fallen leaves, soil and other garden debris can be useful. These materials can accumulate on the paving while also helping moisture remain around the surface for longer. Areas that stay damp due to shade, poor drainage or a build-up of leaves and other organic debris can provide favourable conditions for mould to develop.
How Can You Tell If It's Mould?
Mould can be difficult to identify from colour alone. Dark or black marks aren't automatically mould, particularly on a patio where algae, dirt and lichen may all be present at the same time.
Look at the overall appearance instead. Mould may form dark patches or staining, while algae is more commonly associated with green surface growth and lichen can form more defined, persistent spots.
That distinction matters because a mould stain isn't the same as everyday patio grime. If the surrounding dirt is cleared but darker patches remain, you may be looking at biological growth rather than dirt sitting on the surface.
Why Does Patio Mould Keep Coming Back?
Recurring mould often points back to recurring moisture. A shaded corner, an area beneath garden furniture or paving where water regularly collects can take longer to dry, creating favourable conditions for mould to return.
As with algae, simply removing what you can see doesn't change those surrounding conditions. Keeping debris from accumulating and addressing persistent moisture can therefore form part of longer-term mould treatment, while established staining may require a suitable mould stain remover.
What Are the Black Spots on My Patio?
If you've noticed small, stubborn black spots on patio stone, they may not be ordinary dirt. Persistent patio black spots are commonly associated with lichen, a type of biological growth that can become firmly attached to paving.
Black spots can initially appear as scattered dark marks, before becoming more noticeable across the paving. Unlike mud or loose surface grime, these marks can remain even when the rest of the patio looks clean. This is one of the reasons homeowners can mistake them for staining rather than biological growth.
Natural stone can be particularly susceptible. Lichen can occur on concrete paving too, but porous sedimentary stone such as sandstone provides a surface on which it can become particularly well established.
What Is Lichen?
Lichen is different from both mould and algae. It is formed by organisms living together: a fungus alongside a photosynthetic partner, usually an alga or bacterium. The fungal component also helps anchor the lichen to the surface on which it is growing.
Lichen isn't always black either. It can form different coloured growths, but on paving the dark spots commonly referred to as patio black spot are frequently associated with lichen.
Why Are Lichen and Black Spots So Stubborn?
This is where black spots become particularly different from everyday patio dirt.
The fungal part of lichen can produce tiny structures called hyphae, which extend into the surface it is growing on and help anchor the lichen in place. On porous materials such as sandstone, these structures can interact with pores and irregularities within the paving.
That's why black marks on a patio can remain after surrounding dirt and lighter surface growth have disappeared. Established lichen isn't simply resting on top of the slab.
It also explains why a patio black spot cleaner or lichen remover is a different proposition from simply washing loose dirt from paving. We'll look at the treatment options later in this guide.
How Can You Tell What's Growing on Your Patio?
More than one type of contamination can appear on the same patio. Looking at the colour, texture and pattern of the marks can help you work out what may be affecting the paving and what type of treatment could be suitable.
| Contaminant | What It Looks Like | What Can Encourage It | How to Remove It |
|---|---|---|---|
| Surface Dirt | Mud, dust, leaves and loose debris | Weather, foot traffic, soil and garden debris | Sweep away loose debris and use a suitable patio cleaning method |
| Algae | Green staining, a thin green film or slippery patches | Damp conditions, shade and areas that stay wet for longer | Use a suitable algae remover designed for outdoor surfaces like Wet & Forget |
| Mould | Dark, grey or black patches and staining | Moisture, organic debris, shade and areas slow to dry | Treat with a suitable outdoor mould remover or mould and algae remover like Wet & Forget |
| Lichen / Patio Black Spot | Persistent black spots or grey, white or yellow crust-like patches | Outdoor exposure and surfaces where lichen can become established | Use a suitable lichen remover or patio black spot cleaner like Wet & Forget |
The appearance of outdoor growth can vary depending on the paving and how established it has become. Lichen, for example, is not always black and can appear in several colours and forms, while algae is more commonly associated with green surface growth.
What's the Best Way to Get Rid of Mould, Algae and Black Spots?
General patio cleaning can remove dirt and visible surface growth, but mould, algae, lichen and black spots can return if the underlying contamination remains. A pressure washer, for example, can produce an immediate visual result, but it may not tackle growth within the paving and can disturb jointing material or damage more delicate surfaces if used incorrectly.
Wet & Forget takes a different approach. The formula treats mould, lichen, algae and black spot without intensive scrubbing or pressure washing, working gradually with natural weathering to clean the surface. It also has a residual effect that helps prevent premature recontamination and recurring organic growth.
There are three application options depending on the job:
- Wet & Forget Concentrate: dilute 1:5 and apply with a garden sprayer, ideal when you want greater control across larger areas.
- Wet & Forget Rapid Application: attaches directly to a garden hose for quick application across larger areas and harder-to-reach surfaces.
- Wet & Forget Ready To Use: pre-mixed with a trigger sprayer for smaller areas, isolated patches and quick touch-ups.
For a comparison of pressure washing, patio brushes and other patio cleaning methods, read our How to Clean a Patio Guide. Or, to find the right Wet & Forget application for your outdoor space, visit our Wet & Forget Product Comparison Guide.
Can Mould, Algae and Lichen Affect Other Outdoor Surfaces?
Patios aren't the only places where mould, algae and lichen can develop. The same outdoor growth can appear across driveways, block paving, concrete, tarmac, brickwork, natural stone, exterior walls, fences, decking and roofs.
How you tackle the problem matters, particularly across different materials. Pressure washing can quickly remove visible dirt and growth, but high pressure may disturb jointing material or damage more delicate surfaces if used incorrectly. It also may not tackle contamination remaining within paving, allowing mould, algae, lichen and black spots to return over time.
Wet & Forget offers a gentler approach. Its bleach-free formula treats mould, lichen, algae and black spots without pressure washing, intensive scrubbing or rinsing.
It can also be used across a wide range of surfaces, including natural stone, paving, driveways, tarmac, brickwork, decking, fences, roofs and artificial grass, making it a great all-rounder for tackling outdoor growth around the home and garden.
