Posted: 11/04/2014 at 19:22
It's absolutely nothing to do with the soil - it's because you have clean air
It tend to grow more strongly on slow-growing plants, so if your shrubs are a bit lethargic, possibly because of cold and heavy clay, then you can perk them up by mulching with lots of organic matter to feed the shrubs and improve the soil structure. That should make the shrubs grow a bit faster and the lichen won't be as noticeable.
However, personally I'm thrilled to see it on my trees and shrubs - when I was a child I only ever saw it in isolated woodland on Dartmoor or West Wales - the only places where the air was pure enough for the lichens to grow.