Wooden fence turning black or mouldy: what's causing it

Why one section of fence blackens and the rest doesn't

Wood goes black or green where moisture sits on the surface for long periods without drying out. A fence run under trees, next to a hedge, or on the north side of a house gets far less sun and airflow than the rest of the garden, so rain and dew linger on the timber for hours or days longer. That damp surface is exactly what algae, lichen and mould need to establish themselves.

The same fence built from the same timber, treated at the same time, can look completely different section to section purely because of where the sun reaches and where air can move. A gap in a hedge, an overhanging branch, or a neighbouring building's shadow is often the whole explanation.

The other common cause is a missed treatment cycle. Stain and oil wear off faster in shaded, damp spots than in open, sunny ones, so if the whole fence was treated on the same schedule, the shaded run runs out of protection first and starts to show it.

Telling surface mould from real damage underneath

Before treating anything, check whether the blackening is only on the surface or whether the wood underneath has actually started to break down. Press a screwdriver or similar into the timber at the affected spot, particularly near the ground and around post bases. Sound wood resists; it doesn't sink in easily.

If the surface wipes or scrubs to reveal normal-coloured wood underneath, and the timber still feels solid and dry when you press it, you're dealing with a surface condition. If the screwdriver sinks in with little resistance, the wood feels soft, spongy or crumbly, or a section flexes more than the rest of the fence, that's a different problem and worth checking properly rather than just cleaning and repainting over it.

It's also worth looking at whether the blackening follows the grain in patches (typical of mould and algae) or whether there's a soft, darker, wet-looking patch concentrated at ground level, which points more towards rot starting at the base of a post or board.

Cleaning off surface mould and black growth

For a fence that's structurally sound but visibly black or green, cleaning comes before any re-treatment. A stiff brush with warm water and a mild detergent will shift a good amount of surface growth on its own, worked along the grain rather than across it.

For more stubborn patches, a diluted solution made for algae and mould removal, left to sit for the time stated on the bottle and then scrubbed and rinsed, usually finishes the job. A pressure washer can help but needs a wide fan nozzle and some distance from the wood, since a narrow jet held too close will gouge softwood fencing and leave it fuzzy.

Let the fence dry fully before treating it again. Depending on the weather, that can take anywhere from a day to the better part of a week, and treating over damp wood is one of the more common reasons a new coat fails early.

• Brush with warm water and mild detergent for light surface growth
• Use a proper algae and mould remover for heavier patches, following the dilution instructions
• Keep a pressure washer wide-angled and at a safe distance from the timber
• Rinse thoroughly and let the fence dry completely before any treatment
• Check for soft or spongy patches while cleaning, not just after

Choosing a new protective coat for a shaded fence run

A shaded, damp section of fence benefits from a different product, or at least a different maintenance rhythm, than a sunny run of the same fence. A stain or oil with a fungicide or algaecide additive, sold for exactly this purpose, will resist regrowth for longer than a plain stain in a spot that stays damp.

Open, breathable oils tend to suit shaded timber better than thick, film-forming varnishes, since a film that traps moisture underneath it is part of what caused the blackening in the first place. Reapplying more often in that one section, even if the rest of the fence is on a longer cycle, is a reasonable trade-off rather than a failure of the product.

If overhanging branches or a dense hedge are the root cause, cutting them back to let in more light and air will do more for that section long-term than any coating choice. Sometimes the fix isn't in the tin at all.

When it's worth calling in help

If checking with a screwdriver shows soft or crumbling wood, especially near ground level or around a post, that's rot rather than surface mould, and cleaning and restaining won't address it. At that point it's worth having someone look at whether individual boards or posts need replacing before the section is treated again.

For fence repair, replacing boards or posts, and general fence installation, our rate starts from 45 € per hour, with a fixed quote given for larger jobs once we've seen a description or photos. For smaller jobs like cleaning, treating and general upkeep around the section in question, our odd jobs rate from 39 € per hour usually covers it, with a minimum of one hour.

If you're not sure which category your fence falls into, sending a few photos of the affected area, including a close-up near the ground, is enough for us to tell you whether it's a cleaning job or a repair job before any work is booked.