Moss and lichen
Moss and lichen are micro-organisms which adhere to a solid surface. Both require moisture to grow and reproduce, so thrive in damp, sheltered or shaded areas. Poor drainage conditions contribute to growth on paths and hard surfaces, moss particularly likes a pitted surface that allows it to set down roots. Growth tends to be more significant in autumn and winter, but can happen at any time if the surface is persistently damp and shady.
In the built environment moss and lichen can grow on many hard exterior surfaces such as tiles of roofs, stone or brick façades, block paving, flag stones and wooden cladding, decking and furniture. They can make exterior terraces, pathways and steps extremely slippery and potentially hazardous.