How can I get rid of white spots on my patio?
My natural stone and concrete patio is covered in white spots. I have tried power-washing and bleach, with no luck. The slabs have been down for 15 years and each year get worse. It looks awful. Help! Marlene Mason, Northampton  The white spots are almost certainly lichen, which can also appear as dark green or black spots. Lichen is not a single organism, but two, and sometimes three (fungi, algae and/or bacteria) living in symbiosis. Lichen thrives on porous surfaces such as stone and concrete, into which it can root itself. Lichen can survive in desert and Arctic conditions  no wonder it is difficult to remove. Sponsored  You have two options: change its environment with chemicals or apply heat via a steam pressure cleaner. Sodium