There is nothing worse to see than a beautiful patio turning green. We look at how to prevent it, and how to remove algae from paving slabs.

Steps down to sandstone paving turning green with algae
The early stages of algae growth is already turning this sandstone green.

Why do paving slabs go green?

Natural stone paving turning green can be quite a common occurrence, especially in north-facing gardens.  The green is caused by algae, which flourishes in damp, shady conditions. Because natural stone is porous, holding water during and after wet weather, it can act like a magnet to algae growth, which happily breeds on a damp, permeable surface.

How to keep a patio from turning green

Fear not. There are measures you can take to reduce and remove algae growth.

Close up of sandstone paving with  green patches of algae
Sandstone paving turning green with algae.

Choose a hard paving

What makes certain types of paving hard is its physical density.  All natural stone paving is permeable, but harder stones are much less porous.  Therefore they will be much more resistant to algae growth and to staining in general. 

When we talk to a customer about paving, we always ask about the conditions in the garden, so we can recommend the right type of stone. There are a number of pavings we recommend for damp and shady gardens. These include porcelain paving, which is virtually non-porous. Riven Indian sandstone and granite are also good choices. Or why not take a look at composite decking and clay pavers, both of which absorb very little moisture?

View from above of small rear garden with Silver Grey Porcelain paving, clay pavers and firepit.
Silver Grey porcelain and Charcoal Grey clay pavers are good choices for a shady, north-facing garden, as in this design by Catherine Clancy, built by Bes Landscapes.

Use an impregnating paving sealant

Using an impregnating paving sealant will reduce algae growth.  This seals the surface of the paving and prevents algae from establishing itself easily. 

There are a few common misconceptions about sealing patio paving.  One is that sealing natural stone paving will make the surface of the stone glossy.  This can be the case when using cheap, topical paving sealers.  However, we always recommend Dry Treat Stain-Proof, which you can apply yourself. This is what we use for our pre-sealing service. It doesn't alter the colour or texture of the stone and inhibits algae.

Find out more about why sealing your patio might be right for you.

How to remove algae from a patio

Bottle of Lithofin Algex, algae remover and preventer
An annual application of Lithofin ALGEX will help to keep algae at bay over the winter.

While algae does look ugly, it won't cause any long-term damage to your natural stone or porcelain patio. It's easy to shift, so there is no reason to panic. 

Algex is a maintenance cleaner that breaks down algae and lichens. What we like about this product is that it stays on the stone, almost as a barrier, and every time it rains, it dilutes again and penetrates the stone a little more. This means it continues to work at reducing the algae's ability to get a foothold over the winter season.

Apply Algex to clean stone after it's been thoroughly swept with water or jet washed. You can use a watering-can to apply this product, but a low-pressure sprayer is preferable to allow for better coverage and penetration. We recommend an annual application as part of maintaining your patio.

Discover more ideas on how to get the most from a damp, shady garden

Post updated: August 2023