Why Stone Turns Green

Walk out on a damp morning and that handsome grey patio can look distinctly emerald. The culprit is almost always one of three things, and they are worth telling apart before you reach for a brush. Algae is the thin, slippery green film that appears on any stone that stays damp — it is at its most treacherous on north-facing steps and shaded paths. Moss is softer and cushion-like, gathering in joints and hollows where moisture lingers. Lichen is the crusty or leafy growth in greys, greens, yellows and oranges, clinging tightly to the surface rather than sitting on top of it.

The British climate is generous to all three. Shade from a hedge, a leaking gutter, clay soil that holds water and a patio that never quite dries out — that is all the encouragement they need. And before you declare war, it is worth saying that a little lichen on old flagstones is part of their character. On ancient or listed paving it can be genuinely valuable, so think twice before stripping it away.

Know Your Stone First

Natural stone is not one material. Sandstone, limestone, slate, granite and York stone all behave differently, and the wrong approach can leave permanent marks. Limestone and many sandstones are acid-sensitive, so vinegar, citric acid and proprietary brick cleaners will etch and dull them. Slate and granite are far tougher, but still dislike wire brushes.

Always test on a hidden corner and leave it for a day before committing. Soft, riven or weathered surfaces scratch easily, and once the surface is opened up it holds dirt and moisture more readily than before. The rule is simple: the gentler the stone, the gentler the method.

The Gentle Toolkit

• A soft to medium nylon brush on a handle, plus a smaller stiff hand brush for joints.
• A wooden or plastic scraper for lifting moss and loose lichen.
• A pump sprayer or watering can for applying liquid evenly.
• Sodium percarbonate — an oxygen-based cleaner that breaks down into water and oxygen, far kinder to plants than bleach.
• A few drops of washing-up liquid as a wetting agent, so the solution clings rather than running off.
• Plastic sheeting and a supply of clean water for rinsing.

Leave the pressure washer in the shed, at least on full power. It blasts out jointing sand, erodes soft stone and drives water into the very places you want dry.

A Method That Works

Choose a mild, still, overcast day — somewhere between 10 and 18°C, with no frost and no rain forecast for twenty-four hours. Water surrounding plants thoroughly first, then cover them with sheeting.

• Wet the paving so the cleaner does not dry out on the surface.
• Mix roughly two tablespoons of sodium percarbonate per five litres of warm water, with a drop of washing-up liquid.
• Apply evenly and leave for fifteen to twenty minutes. Do not let it dry — mist it again if it starts to.
• Scrub in small sections with the nylon brush, working along the joints rather than across them.
• Rinse thoroughly with clean water and repeat on stubborn patches.

For lichen, be patient. Scrape what lifts easily, treat lightly, and accept that some will remain. Aggressive scrubbing does more damage to the stone than the lichen ever will.

Protecting Plants, Ponds and Pollinators

Run-off is the real risk. Even gentle cleaners can harm what they touch, so work in small sections, rinse generously and dilute any overspill with plenty of clean water. Never let run-off reach a pond — oxygen-based products still affect aquatic life, and a single bucketful can upset a small wildlife pond. Spray on a still day, and avoid working when bees and other insects are most active; early morning or evening is kinder.

Remember that moss and lichen are habitat. Leave the patches along wall bases, under hedges and in the corners nobody walks on. A garden with a little green in the margins is a healthier one.

Keeping It Clean Without Chemicals

Prevention is mostly about light and air. Trim back shrubs that shade the patio, clear gutters and fix dripping downpipes, and improve drainage where water pools. Brush the surface with a stiff broom every week or two through the damp months — it takes minutes and stops algae establishing.

Re-sand the joints when they hollow out, so water runs off rather than sitting in the gaps. One light clean each spring, using the mildest method that does the job, will keep natural stone looking its best for decades. And a faint green haze on a north-facing step? That is simply a British garden doing what British gardens do.