I have always loved seeing lichen on stone walls and in churchyards but until I read Katherine Swift’s The Morville Year I had no idea how fascinating it really is:

A lichen consists of not one but two organisms living in symbiosis: a fungus, capable of withstanding extremes of temperature and drought but unable to photosynthesis, and an alga, which – Dalek-like – inhabits the protective structure provided by the fungus and in return manufactures enough food for its own needs and those of the fungus.  Every lichen partnership is unique, and neither partner can survive without the other.

There are some seventeen hundred British lichen.  Many churchyards have well over a hundred, cohabiting on individual gravestones in what lichenologist Dr Vanessa Winchester calls ‘miniature, self maintaining gardens’.   Churches and churchyards are important havens for lichen, even where stone is common, because the churchyard may include gravestones made from different sorts of stone, not necessarily native to the area, each of which will support a different population of lichens.

Lichens are extremely sensitive indicators of environmental pollution: a rich bloom of lichen means clean air.  They also grow infinitesimally slowly – half a millimeter a year is fast – so lichen growth is also an indicator of age.  Many lichen will be as old as the tombstones on which they live.  So don’t scrub them off (it is a misconception that they destroy the stone) but do keep the surrounding undergrowth cut, as lichens need sunlight in order to thrive

So now I know…