Potato Water: The Free Terrace Trick for Removing Moss and Algae Without Bleach

Harriet M. Ashford • August 22, 2026 22:10

A free kitchen trick is all you need.

Plenty of homeowners still reach automatically for harsh cleaners when moss and algae take over the patio. But bleach not only harms the environment - it can also damage the surface itself. There is, however, a straightforward mixture from the kitchen that loosens the green film without scrubbing, without toxic additives, and for virtually nothing.

Why bleach on a terrace is a bad idea

Once winter has passed, many people step outside and realise with a jolt what has happened: the terrace has turned green, looks patchy, and can be dangerously slippery in places. Damp conditions, little sunshine and surfaces that don’t dry quickly create perfect conditions for moss, algae and lichens. These areas are particularly prone:

Timber decking and boards
Concrete slabs and block paving
Terraces made from natural stone or ceramic
Balconies with minimal fall and poor drainage

For convenience, a container of chlorine-based cleaner often ends up in the trolley - the idea being: pour once, job done. In reality, the downsides frequently outweigh the benefits.

"Bleach attacks jointing, can leach stone and wood, discolour surfaces, and harms plants, soil life and small animals in the garden."

In some countries, strict rules now apply to using these products outdoors. The clear direction is away from aggressive biocides and towards gentler methods that are kinder to materials.

The underestimated danger of moss and algae

A green coating is not just an aesthetic issue - it changes the surface itself:

Moss holds water and keeps the terrace damp for longer.
Algae create slick films, increasing the risk of falls.
Lichens grow deep into pores and tiny cracks.
Moisture can contribute to frost damage and weaken the material.

On top of that, dirt, leaves, soil and dust collect in the joints. This becomes a kind of nutrient bed where fresh spores thrive. If you leave it, you’ll face the same nuisance year after year - usually worse than before.

Before the miracle cure: a thorough basic clean

Before using the zero-cost trick, the terrace needs a mechanical deep clean. Chemicals do not replace this groundwork. Useful kit includes:

a stiff scrubbing brush or outdoor broom
gloves and, if needed, safety glasses
a bucket of warm water with a mild cleaner

Start by removing loose dirt and leaves, then use the brush to lift anything that’s stuck on. Professionals recommend applying the cleaner first, letting it work for a short while, and not scrubbing immediately.

"Most solutions need at least 20 minutes’ contact time to properly loosen moss and algae."

If you have a pressure washer, take care: too much pressure can blast jointing out of paving, roughen timber unnecessarily, and damage natural stone. Keep the pressure moderate and maintain plenty of distance.

The zero-cost mix: potato water as a moss killer

The actual trick comes straight from the kitchen and costs nothing extra: the cooking water from potatoes. Instead of tipping it down the sink, you can use it as an effective moss remover.

How the free mix works

Potato water contains a lot of starch, mainly made up of two components: amylose and amylopectin. These are what create the cleaning effect:

The boiling water (around 100°C) delivers a heat shock.
The cell structures of moss and lichen split open.
As it cools, the starch forms a fine film on the surface.
That film covers the plants’ “breathing openings” and deprives them of air.

"The heat shock kills the coating, and the starch film suffocates what remains - without any aggressive chemicals."

Step-by-step instructions for use

Boil potatoes as normal in water, but do not add salt.
Once cooked, drain the water straight away and use it while it is still boiling hot.
With the terrace dry, slowly pour the potato water over the affected areas.
Wear gloves and sturdy footwear, and avoid splashes.
Leave it to work for at least 24 to 48 hours; do not rinse it off.
After the waiting time, remove the moss cushions (now usually brown or black) with a brush or broom.

The coating typically comes away in flakes and can be swept up easily or rinsed away afterwards. On many terraces, one application per season is enough.

Alternative home remedies for green film

If you’re not cooking potatoes, there are other tried-and-tested options that are less aggressive than bleach:

Bicarbonate of soda or baking powder: dissolve in water, apply, leave to work, then scrub.
Liquid soap or soft soap (black soap cleaner): lifts grease and dirt and is gentle on most surfaces.
White household vinegar: its acidity attacks algae, but it should be used sparingly and not right next to sensitive plants.

Important: always test your chosen cleaner in an inconspicuous spot first to check the material can tolerate it. Natural stone, for example, often reacts badly to acidic products.

How to keep moss away in the long term

A free miracle remedy is useful, but it works far better if you also follow a few basics. The key is to remove what moss loves: constant moisture and nutrients.

Measures that make the results last longer

Ensure rainwater drains away properly; regularly check falls and channels.
Keep joints free of soil, leaves and seeds.
After autumn storms and heavy rainfall, give the surface a quick sweep rather than leaving it for months.
Don’t leave furniture sitting permanently on damp patches, so air can circulate.

After a thorough clean, a suitable hydrophobic treatment can be worthwhile. These products make the surface more water-repellent without fully sealing it. Water beads and runs off more easily, the terrace dries faster, and moss finds it harder to take hold.

What to bear in mind for timber, stone and tiles

Not every terrace surface can handle the same approach. This quick overview helps prevent damage:

Material	Suitable approach	Take care with
Timber	Softer brush use, potato water, mild soap solutions	Pressure washing, strongly acidic cleaners, bleach
Concrete / paving	Brush, potato water, bicarbonate solution, moderate pressure washing	Water left standing too long, excessive pressure, aggressive chlorine products
Natural stone	Gentle brushing, potato water, specialist stone cleaners	Vinegar and other acids, strongly alkaline products
Ceramic tiles	Brush, potato water, soap solutions	Abrasive brushes, scouring agents, undiluted “heavy” chemicals

Risks and limits of the home remedy

As handy as potato water is, it should not be used without a little thought. On very smooth tiles, the starch film can make the surface temporarily slippery - especially if rain hits it before you brush it off. Choose a dry day and allow it to work undisturbed.

If you have a pond or wildlife pool right next to the terrace, avoid letting large quantities of the liquid run straight into it. Small amounts soaking into the ground are usually not an issue, but bigger surges can briefly upset the balance in the water.

Why this kitchen trick is a great fit for spring cleaning

In spring, potatoes are often on the menu anyway - which means cooking water is produced regularly. If you collect it on purpose, you can treat the whole terrace gradually: one section at a time, without rushing, without extra shopping, and without additional cost.

Especially with rising prices, the mix of home remedies, mechanical cleaning and a bit of planning is doubly appealing: it’s kinder to your budget and reduces the use of problematic substances in the garden. As a side benefit, you pay closer attention to your outdoor surfaces, spot early signs of green growth returning, and can act in time.