The black spots that grow on your garden stonework are actually spots of black lichen! The spores from the black lichen start life as tiny dust-like spores growing on the underside of leaves on trees and plants.
These spores are carried by the wind and the rain onto your stonework in their millions. Once they have landed the spores have a two-year germination period. So for the first two-years of your patio’s life it will remain looking really clean. In year three the colour in the stone will start to look dull and look like someone flicked ink across your stonework leaving tiny ink splatters. The reason why they grow on your garden stonework is because they feed off the minerals within the stone. Once they have exhausted an area of the stone of all its minerals they grow in search of more. Not only do they spread across the stone giving spot like appearance, they also burrow down into the stone at a rate of 2mm a year.
Encouraged by warm, damp conditions, these unsightly spots will only get worse if left untreated, ruining the original beauty of your garden stonework.