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How To Remove Algae and Black Spot Lichen From Stone

Light green algae is easy to remove.

Dark green algae requires a little more work.

Black spot lichen (or large black areas of lichen) are more difficult and you must follow instructions. These methods are successful in more than 98% of cases. 

Make sure you follow these instructions and ask for help if you need it.

Please see our guide to using pressure washers by clicking here.

What is Algae? It is a simple, usually aquatic, plant that ranges from microscopic to seaweed. It loves warm, damp conditions and will readily grow on stone in the UK due to our weather.

What is Black Spot Lichen? It is made up of two or three types of organism (algae, fungi and bacteria) living in a symbiotic colony. So, whilst it is fascinating, it can ruin the aesthetics of a beautiful external stone very quickly..!

Do not use standard brick or patio cleaner as this is likely to contain hydrochloric acid which can be disastrous. Also, it is not good at removing algae in any case.

For Light Green Algae you can use Algex. This product can have the additional advantage of preventing algae growing for a few months months, depending upon the weather (again, algae loves mild, damp conditions). See instructions on the product page. Note that we use Algex diluted at 1:5 in the UK.

For Dark Green Algae you may require Biorase first followed by Algex. See below and refer to the Technical Instructions on the product pages for full instructions.

For Black Spot Lichen you will require Biorase followed by Algex.

Additional Instructions for Removal of Dark Green Algae and Black Spot.

  • If you are left with "shadows" where the dark marks used to be, you can repeat the Biorase treatment.
  • Alternatively, if the marks are light, you could try Algex after Biorase as follows.
  • Dilute Algex at 1:5 with water.
  • Apply to the area on a dry day. Use enough to wet the face of the stone.
  • You can use a garden sprayer, a wtering can with a "rose" attachment, a brush or a mop to apply.
  • Do not rinse.
  • The marks will usually fade further over a few days.


Related posts
Guidance For Using A Pressure Washer On External Stone and Patios


You need some knowledge to understand how to use a pressure washer in order to acheive the best results and avoid potentially serious damage.


Please read this article for guidance.

View article
Linked products
Biorase - 1L
Biorase - 1L
A cleaner for use on natural stone, wet cast concrete tiles (replica flagstones), pressed concrete tiles, marble, porcelain, ceramic and exterior patios, paths and walls.
Limited stock
Out of stock
£15.48 (inc VAT)
£12.90 (ex VAT)
Supplied in 1 litre containers. Algae killer. For external use. Concentrated and economic. Prevents re-growth for up to one year.
Limited stock
Out of stock
£20.64 (inc VAT)
£17.20 (ex VAT)
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