As autumn approaches and September brings shorter days, wetter weather, and a little less sunshine, homeowners may start noticing green algae and moss appearing on patios, decking, fences, and other outdoor surfaces.
According to outdoor living expert Colin Potts, owner of Artificial Grass Direct, the green appearance is not necessarily a sign that a surface is failing. Instead, it is often a warning that an area is staying damp for too long.
Algae and moss thrive in moist conditions, and growth can be particularly rapid in shaded areas with poor air circulation. These growths can make surfaces dangerously slippery and could cause damage to wooden surfaces if left untreated.
“September is when many homeowners start noticing their outdoor surfaces changing colour. The combination of rain, fallen leaves, less sunlight, and cooler temperatures creates much better conditions for algae and moss to establish,” Potts explains.
“The mistake is simply treating the green growth without looking at why it is there in the first place. If an area is constantly shaded, holding water, or covered in damp leaves, you can clean it today and find the green growth has returned a few weeks later.”
Tackling algae on hard surfaces
Paving, patios, decking, and other hard surfaces are particularly susceptible as the weather becomes wetter. Green films can develop on hard surfaces, while moss can appear between paving slabs or along the edges. While these growths do not necessarily damage the material itself, they create safety hazards in areas where people regularly walk.
The first step is to remove existing growth using a stiff broom, brush, or a pressure washer where appropriate. However, where drainage is poor, adding large amounts of water can exacerbate damp problems.
“Look at what happens to the water after it rains,” says Potts. “If water is sitting on the surface, drains are blocked, or leaves are collecting in the same spot, you’re creating exactly the conditions that algae and moss like.”
To prevent regrowth, homeowners should clear drains and gullies, remove accumulated leaves and debris, check that water can run away, and cut back overhanging vegetation that creates excessive shade.
Protecting timber structures
Wooden surfaces like decking, fencing, cladding, and sheds need slightly different treatment, as prolonged exposure to moisture can contribute to timber deterioration over time. September offers a good opportunity to inspect and treat wooden structures while dry weather remains.
Potts, recommends
- Clean away green growth and dirt first. Do this rather than applying a treatment over a dirty surface.
- Allow timber to dry properly. Ensure it dries fully before applying any protective product.
- Use a suitable exterior wood preservative. Select an oil or protective treatment appropriate for the particular timber and existing finish.
- Pay particular attention to vulnerable spots. Focus on the base of fences, sheds, and cladding, where water and organic debris can collect.
- Keep leaves and soil away. Do not allow debris to sit against timber for long periods.
- Check for structural issues. Look for leaking roofs, damaged boards, or areas where water is repeatedly pooling.
“The aim isn’t to make every piece of timber completely waterproof,” Potts, explains. “It’s about keeping it maintained and preventing moisture from becoming a permanent feature of the surface. If you’re treating decking, fencing, or cladding, always make sure the treatment is designed for that particular material and follow the manufacturer’s instructions. A good protective treatment is useful, but it can’t compensate for poor drainage or timber that is constantly wet.”
Care for artificial grass
Artificial grass also requires attention during autumn. While synthetic fibres do not naturally attract algae, organic debris such as leaves and soil can build up on the surface. If the drainage underneath or around the grass is poor, persistent moisture creates an environment where unwanted growth and odours become more likely.
“With artificial grass, you don’t want to start throwing treatments at the surface every time it looks a little green,” Potts notes. “Start with the basics: remove the debris, brush the fibres, and make sure the drainage is doing its job.”
Addressing root causes before winter
According to Potts, the biggest mistake homeowners make in September is focusing entirely on removing visible green growth rather than addressing the underlying environmental conditions.
To prevent recurring issues, homeowners should evaluate their gardens against these key checks
- Is the area shaded all day? Overhanging trees, hedges, and structures can prevent surfaces from drying.
- Is water pooling? Look for areas where rainwater sits for hours after other parts of the garden have dried.
- Are drains blocked? Leaves and debris can stop water escaping quickly.
- Is organic material being left on the surface? Wet leaves, soil, and other debris can keep surfaces damp for longer.
- Has the timber been maintained? Fences, decking, cladding, and sheds should be checked and treated where appropriate before the wetter months arrive.
September is an ideal time to deal with green growth because homeowners can still take advantage of periods of dry weather to clean surfaces, improve drainage, and carry out maintenance.
“You don’t necessarily need an expensive solution,” says Potts. “In many cases, a stiff brush, clearing the leaves, fixing drainage, and giving timber the right protection can make a huge difference. The important thing is to deal with the cause rather than just the symptom. If your garden stays damp throughout autumn, green growth is going to keep coming back unless you change the conditions that are allowing it to thrive.”