Green patches on your paving, green edges along the joints or a slippery wooden deck: green growth doesn't just look untidy, it also makes your terrace slippery. Fortunately, with the right approach you can clean it off without damaging your paving, joints or timber. In this guide you'll read what green growth is, which method suits which surface, how to remove it in six steps and, most importantly, how to stop it coming back.
In brief: green growth thrives where it stays damp for long periods and in the shade. Always start by sweeping and scrubbing with soapy water and a brush. A pressure washer can be used on paving, on a low setting and not straight at the joints, but not on timber. If you use a green growth remover, follow the label exactly and don't let anything run into the drain or pond. After that, everything comes down to prevention: light, drainage and regular sweeping.
What is green growth and why does it come back?
Green growth is a thin layer of mainly algae, often together with moss in the joints. It grows in places that stay damp for a long time: in the shade of a fence or tree, on the north side of the house, under overhanging branches and where rainwater pools. In autumn and winter, when the sun is low and the terrace dries more slowly, you'll often see it appear fastest.
You'll recognise three types of green on the terrace:
- A green film: a smooth, thin layer on paving or timber. This is green growth in the strict sense, and it makes the surface slippery when wet.
- Moss: soft, green cushions, mainly in the joints and on porous stone.
- Green edges: growth along the joints and around spots where pots, rugs or furniture have stood in the same place for a long time.
Why does it come back? Cleaning removes the growth, but not the cause. As long as a spot stays wet, shaded and full of leaves or dirt, something will grow there again. That's why this guide, alongside removal, also covers prevention in some detail.
Growth on your furniture too?
This article is about the floor of your terrace. For green growth on tables, chairs and lounge sets, read cleaning garden furniture, green growth on plastic and green growth on a wooden garden table.
Three methods: brush, pressure washer or remover
There are broadly three ways to remove green growth from your terrace. You often combine them: first sweep and scrub, and only where that isn't enough, use a pressure washer or a product.
1. Scrubbing with soapy water and a brush
This is the safest method for every surface and often simply the best. You'll need: a stiff broom, a hard scrubbing brush for paving, a softer brush for timber and natural stone, a joint scraper, a bucket of lukewarm water with a mild detergent such as soft soap, and a garden hose for rinsing. A light film is usually gone in one go. It takes some elbow grease, but you won't damage anything.
2. Pressure washer, used with care
A pressure washer is quick on a large terrace, but it's also the method most likely to cause damage. On concrete and ceramic paving it can be used if you follow a few rules:
- Start on a low setting and keep a good distance from the paving.
- Spray at an angle across the paving, not straight at the joint, so the jointing sand stays put.
- A patio cleaner attachment (a round hood over the nozzle) splashes less and spreads the jet more evenly.
- Wear safety glasses and closed shoes.
On timber, composite and soft natural stone, it's better not to use a pressure washer. The jet roughens the surface, and on a rough surface new growth actually takes hold more easily.
3. Green growth remover
For large areas or stubborn growth, there are special green growth removers. Most are applied to a dry terrace with a watering can or pump sprayer, after which the product does its work. How long that takes, how often you need to repeat it and whether you need to rinse off depends on the product. A few general rules:
- Read the label and only use the product on the surfaces it lists, at the dosage stated.
- Not in the gutter, the drain or the pond: don't let the product run into the drain, the ditch or a pond, and keep it away from plants where the label says so.
- Watch the weather: many products need to stay dry for a while. Check the label to see whether you need to avoid rain.
- Keep children and pets away from the treated terrace for as long as the label advises.
- Wear gloves and store the product safely afterwards.
At the moment we don't have cleaning products or terrace brushes in our comparison. You can buy green growth removers, scrubbing brushes and joint scrapers at a DIY store or garden centre. Choose a product there that the label says is suitable for your surface.
The best remedy for green growth is a terrace that dries quickly. Any spot that stays wet a day longer than the rest after a rain shower will be the first to turn green again. So after a downpour, take a look at which paving dries last: that's where you start with prevention.
Approach per surface in one table
The method you choose depends on your surface. If you're not sure which stone or timber you have, check the invoice from the paving contractor or ask your supplier.
| Surface | Method | Watch out for |
|---|---|---|
| Concrete paving | Stiff brush, soapy water | Low-pressure jet only, not on joints |
| Ceramic tiles | Brush, soapy water, rinse | Spare the joints |
| Natural stone | Soft brush, pH-neutral | No vinegar or acid |
| Timber and decking | Brush with the grain | No harsh jet |
| Joints | Joint scraper, joint brush | Top up afterwards |
| Composite | Soft brush, soapy water | Follow the manufacturer's advice |
Want to give the whole terrace a thorough clean, not just the green patches? The full step-by-step plan for each surface, including composite, is in cleaning a terrace by surface.
Green growth on terrace paving
Concrete paving is porous: the growth doesn't just sit on top, it also works a little into the surface layer. That's why it often stays dull after a quick sweep. Wet the paving, apply soapy water and scrub with a stiff brush. Work in strips of a few slabs at a time, so the soapy water doesn't dry before you rinse. If a green film remains, you can use a pressure washer with care or a green growth remover that's suitable for concrete.
Ceramic tiles absorb hardly any moisture. The growth sits mainly on top and in the joints, so soapy water and a brush are usually enough. With ceramics, pay particular attention to the joints: they're the most vulnerable.
Are your tiles lighter after cleaning than the ones that always stood under the lounge set? That's normal. The difference often evens out after a while.
Green growth on natural stone
With natural stone, the type of stone you have matters. Limestone-based varieties such as Belgian bluestone, limestone, travertine and marble are sensitive to acid. Vinegar, citric acid and acidic cleaners can dull this stone or leave light marks. Granite is less sensitive, but even there, a mild product is the safe choice.
Use lukewarm water with a pH-neutral product and a soft to medium-hard brush, and rinse thoroughly. If you choose a green growth remover, check the label to see whether it's suitable for your type of stone, and test first in an inconspicuous spot. Don't use a wire brush, and be restrained with the pressure washer. You can read more about natural stone in cleaning natural stone.
Green growth on a timber deck or boardwalk
A timber boardwalk goes green quickly, especially in the shade. Green growth on wood is not only slippery, it also holds moisture against the timber. So tackle it in good time, but always gently:
- First clear the gaps between the boards. Leaves and dirt in the gaps hold moisture and stop the wood from drying out.
- Scrub with lukewarm water, a mild soapy solution and a brush, always in the direction of the grain.
- Rinse with the garden hose and leave the timber to rest for a few dry days.
- If you use a product, choose one that the label says is suitable for timber.
Keep the pressure washer off timber. The jet frays the wood fibres, the surface becomes rough and splinters, and new growth settles in those rough fibres more quickly. Incidentally, greyed timber isn't dirt but a natural process. If you want to protect the wood after cleaning, treating wooden garden furniture explains the difference between oil, stain and varnish; the principles also apply to a boardwalk.
Cleaning green joints
The growth lodges deepest in the joints, often together with moss and weeds. Soapy water over the paving won't remove it. Here's how to tackle the joints:
- Scraping: remove moss and weeds with a joint scraper or a narrow knife.
- Brushing: a joint brush with stiff bristles removes the rest. There are also electric joint brushes for large terraces.
- Sweeping and rinsing: sweep up the loosened material and rinse off.
- Topping up: fill empty joints with jointing sand or the jointing compound that goes with your paving. Empty joints hold water and are the first to turn green again.
If you use a pressure washer, it can blast the jointing sand out in a few seconds. So always check afterwards whether the joints need topping up.
Six-step plan: removing green growth in 6 steps
This plan works for any surface. Only adjust the brush and product to what you read in the table. Choose a dry, overcast day without frost.
Remove furniture, pots, cushions and the outdoor rug. That way you can reach everywhere and see straight away where the growth is thickest: often exactly under the things that have stood in the same place for a long time.
Sweep away leaves, sand and loose dirt with a stiff broom. Scrape moss and weeds out of the joints. Otherwise, wet dirt just gets smeared around.
First try your soapy water or product on a small area that won't be noticed. Let it dry and check that colour and sheen stay the same.
Scrub in strips with soapy water and the right brush. If you use a pressure washer or a green growth remover, follow the rules above and the product label.
Rinse away soapy water and loosened growth with the garden hose. Make sure rinse water containing products doesn't end up in a pond, the vegetable patch or the drain. If a product should be left on according to the label, skip this step.
Let the terrace dry completely. Top up empty joints with jointing sand or compound. Only then put your furniture, pots and rug back.
Preventing green growth
Cleaning is hard work, but prevention is mostly a matter of habits. The drier and lighter your terrace, the less chance green growth has.
How to keep your terrace clean for longer
- Sweep regularly, and more often in autumn: leaves hold moisture
- Trim back overhanging branches and shrubs for more light and wind
- Make sure rainwater can run away, and fix spots where puddles remain
- Keep the joints filled, so no water stands in them
- Stand pots on feet, so the area underneath can dry
- Lift the outdoor rug now and then, and never lay it on a wet terrace
- Store cushions dry instead of leaving them on the terrace
Things on the terrace: watch what happens underneath
Anything that stands in the same place on the floor for a long time holds moisture. Under flower pots, an outdoor rug or a stack of cushions, the surface dries poorly, and that's where you'll see the green edges first. That doesn't mean you have to do without these things, but do move them now and then or lift them up.
An outdoor rug makes the terrace cosy, but only lay it on a dry surface, and lift it after a wet spell so the rug and the paving underneath can dry. The outdoor rugs from Garden Impressions have the same care instructions: remove dirt with a soft brush or vacuum, wash with lukewarm water and mild soapy water, and let the rug dry completely before storing it.
Tilo outdoor rug 160x230 cm old sand
You'll find more colours and sizes in outdoor rugs. Styling tips can be found in outdoor rugs for the terrace.
Cushions that are left on the terrace or on the furniture turn green themselves and keep the spot underneath wet. A cushion box gives them a fixed place. Do note: for both boxes below, the product information states that they are not waterproof. So only put dry cushions in them, and place the box somewhere it dries well.
Cambridge cushion box 155x97xH82.5 cm
Prefer a bag to store in the shed, or a cover for your furniture? A protective cover is never completely waterproof; a cover support ensures no water stays sitting on the cover.
Coverit cushion bag 200 x 75
View bag →
Adjustable cover support
View support →All storage boxes
View range →You'll find all covers in protective covers. For the smartest way to store cushions, read storing garden cushions, and for which cover fits which piece of furniture, see covering garden furniture. Are you ready for new furniture after cleaning and not sure what you're looking for? The selection guide will help you on your way with a few questions.
Common mistakes
Pointing the pressure washer straight at the joints
A hard jet blasts the jointing sand out. Empty joints hold water, turn green first and let paving wobble. Spray at an angle on a low setting, and top up the joints afterwards.
Cleaning timber with a harsh jet
The wood fibres fray, the surface becomes rough and splinters. On a rough surface, the growth returns faster. Scrub with the grain instead.
Vinegar, salt or bleach from the kitchen cupboard
Vinegar attacks limestone-based stone and joints, and salt and bleach don't belong in the border or the drains. Use soapy water or a product that the label says is intended for green growth on your surface.
Using more product than the label says
More doesn't work any better, but it does burden your garden and the water. Stick to the dosage and don't let leftovers run into the gutter, the drain or the pond.
Only cleaning and leaving the cause in place
If the spot stays wet and shaded, it will be green again within a reasonable time. Tackle drainage, light and leaves too.
Checklist
Tick off what you've done:
- Chose a dry, overcast day without frost
- Removed furniture, pots, cushions and rug from the terrace
- Swept and scraped moss and weeds out of the joints
- Chose a method that suits the surface
- Tested the product in an inconspicuous spot and read the label
- Rinsed off without rinse water getting into a pond or drain
- Topped up empty joints
- Dealt with wet spots, branches and drainage
Frequently asked questions
What is the quickest way to remove green algae from tiles?
On concrete and ceramic tiles, scrubbing with a stiff brush and a bucket of soapy water is the quickest and safest method. A pressure washer covers large areas faster, but set it to a low setting, keep your distance and spray at an angle rather than straight onto the joints. Then top up empty joints, or the algae will return more quickly.
Can I use vinegar or salt against green algae?
We advise against it. Vinegar is acidic and can damage lime-containing natural stone and cement-based joints. Salt and vinegar also wash into the soil and are not good for plants in the borders next to the patio. Instead, use soapy water and a brush, or a product labelled for green algae on your particular surface.
How does a green algae cleaner work?
You apply a green algae cleaner to a dry patio, after which the product breaks down the algae. How long that takes, how often you need to repeat the treatment and whether you need to rinse off varies by product. So follow the label precisely, do not let the product run into the gutter, drain or a pond, and keep children and pets away for as long as the label advises.
How do I remove green algae from a wooden deck?
First sweep out the gaps between the boards so the wood can dry. Then scrub with lukewarm water, a mild soap and a brush, moving in the direction of the grain, and rinse off. Do not use a harsh jet: it raises the wood fibres, leaving the wood rougher and, as a result, more prone to green algae.
How do I prevent green algae from coming back?
Green algae grows where surfaces stay wet for long periods. Sweep regularly, remove fallen leaves promptly, make sure rainwater can drain away, and trim back overhanging branches so more light and wind reach the patio. Lift pots, rugs and other items occasionally so the surface beneath can dry out.
