Moss, Lichen, and the Slow Colonization of Hard Surfaces

Lichen is not a plant. It is a composite organism — fungus and photosynthetic partner, usually algae or cyanobacteria, living in a relationship close enough to mutual dependence that neither exists independently in the same form. This partnership is approximately 250 million years old and has proven more durable than most subsequent evolutionary experiments. Lichen colonized bare rock when nothing else could, built the soil that later organisms required, and continues operating on surfaces where nothing more complex will grow.

The biology is strange by normal expectations. Lichen grows measurably but almost imperceptibly — a centimeter of growth may represent years of accumulation. A large patch on an exposed mountain boulder may be older than the civilization that built in its shadow. Lichenometry, the practice of dating surfaces by lichen growth, can estimate when a stone wall was built or when a boulder was deposited by a glacier, using the organism's known growth rate as a clock.

In cities, lichen and moss read as neglect. A mossy stone wall signals an unmaintained property; lichen on roof tiles implies age and deferred upkeep. The aesthetic preference for clean masonry has produced entire product categories dedicated to removing organisms that are doing nothing harmful and a great deal that is beneficial — filtering particulates, retaining moisture, providing microhabitat. The cleaning is primarily cosmetic.

What lichen is actually doing on a stone surface is what it has always done: converting rock to something else, slowly, without urgency, indifferent to human timescales. It was present before the built environment and will be present after it. The cleaned walls will be recolonized. The organisms have longer strategies than the people maintaining the surfaces.

Patience at this scale is not a virtue. It is simply a different relationship with time.