Lichens are non-parasitic, plant-like organisms; they are, in fact, composite organisms – a species of fungus growing with a species of algae and there are nearly 2,000 species of lichen in the UK.
Lichens prefer areas with clean air and moist or damp conditions – although they will also grow in much drier conditions.
They colonise the bark of trees and shrubs, as well as fences and other wooden structures, rocks and even paving slabs. They do not harm the plants they grow on – although some people think they are a disease. But because they more commonly grow on plants that are already stressed, weak or lack vigour, many people think they are the cause of the problem.
While they can look attractive, and are often encouraged on hard surfaces to help age and even improve their overall appeal, or as part of Japanese gardens, on paths and patios they can become slip hazards.