There is a patch of lichen on a dry-stone wall near where I walk most mornings. It has been there, as far as I can tell, for at least thirty years. Grey-green, roughly the size of a dinner plate, utterly unbothered by frost, heat, driving Pennine rain, or the occasional sheep rubbing against it. I have watched storms strip bark from mature oaks and shift roof slates clean off farmhouses. The lichen simply carries on. It does not grow quickly. It does not make a fuss. It just persists, clinging to the stone with a tenacity that, the more you think about it, becomes genuinely astonishing.
Lichen is not a single organism. It is a partnership, a quiet alliance between fungi and photosynthetic algae or cyanobacteria living so closely together that they effectively become one thing. The fungal partner provides structure and anchors the whole arrangement to whatever surface it has chosen; the algae or cyanobacteria manufacture sugars through photosynthesis and feed the colony. Neither could survive in that environment alone. Together, they can colonise bare rock in the high Arctic, crumble ancient ruins in the Sahara, and turn the shaded north face of an oak tree in the English Lake District into something resembling a miniature alien world.
What Makes Lichen Such a Formidable Surface Coloniser?
The key to understanding lichen as a lichen surface coating is to appreciate just how hostile the environments it chooses actually are. Bare rock has no soil, no moisture retention, no nutrients to speak of. Temperature swings on exposed stone can be dramatic; a dark rock face in summer sun can reach 60°C before cooling rapidly after sunset. UV radiation at altitude is punishing. Lichen handles all of this through a combination of biological tricks that materials scientists are only now beginning to fully catalogue.
One of the most important is the production of secondary metabolites, compounds known collectively as lichen acids. These organic acids etch microscopic pits into rock surfaces, giving the fungal threads, called hyphae, something to grip. It is, in essence, chemical anchoring. The lichen does not simply sit on the surface; it chemically bonds with it over time. Once established, the thallus (the body of the lichen) can absorb water rapidly during rain or heavy dew, then lose virtually all of it during dry spells and simply wait, sometimes for years, in a state of suspended animation, without dying. This is called poikilohydry, and it is a capability that has no real equivalent in human-made protective coatings.
Beyond the anchoring chemistry, many lichen species produce compounds that act as natural UV screens. Parietin, the vivid orange pigment in the common Xanthoria parietina lichen you will see on rooftops, churchyard walls, and coastal rocks all across Britain, absorbs ultraviolet light before it can damage the photosynthetic cells beneath. It is, functionally, a built-in sunscreen. Other species produce antifungal and antibacterial compounds, protecting the colony from competing microorganisms. The whole system is remarkably self-contained.
Ancient Ruins and Living Armour
Walk around almost any ancient monument in Britain and you will see lichen. Stonehenge’s sarsen stones carry it. The dry-stone field boundaries of the Yorkshire Dales are mantled in it. Mediaeval churchyard headstones across Somerset and Shropshire are slowly being consumed by it. Conservators have a complicated relationship with lichen on heritage stonework. On one hand, certain species accelerate physical weathering through their acid production and the mechanical pressure of hyphae penetrating stone pores. On the other hand, some research suggests that a well-established lichen crust can actually slow surface erosion by binding loose particles and reducing the direct impact of rain and wind.
Historic England has published guidance on managing lichen on listed structures, acknowledging that blanket removal is rarely the right answer and that the relationship between lichen and ancient stone is genuinely nuanced. The presence of slow-growing crustose lichens, in particular, is sometimes treated as a sign that a surface has been undisturbed for a very long time, a kind of biological timestamp for conservators.
What Materials Scientists Are Learning From Lichen
Here is where things get genuinely exciting for anyone who thinks about surface protection for a living. Researchers at several UK universities, including groups at the University of Sheffield and University College London, have been studying lichen biology with a very practical goal in mind: understanding how its attachment and protection mechanisms might inform the design of new coatings and adhesives.
The poikilohydric property is of particular interest. A coating that can repeatedly cycle between wet and dry states without cracking, delaminating, or losing adhesion would be enormously valuable for outdoor applications. Most conventional coatings fail at precisely this point; the repeated expansion and contraction caused by moisture uptake and release eventually causes micro-cracking and loss of adhesion. Lichen simply does not have this problem. Its structure accommodates the movement without losing integrity.
The chemical anchoring mechanism is also attracting attention. The idea that a surface treatment might actively etch and bond to a substrate at a molecular level, rather than relying purely on mechanical adhesion or surface tension, opens up possibilities for coatings that bond more durably to difficult substrates like wet concrete, rough stone, or weathered timber.
There is also growing interest in the antimicrobial properties of lichen-derived compounds. Usnic acid, found in several common lichen species, has demonstrated antibacterial activity in laboratory conditions. For exterior coatings intended to resist algae, mould, and biofilm build-up, this is a potentially significant lead. The challenge, as always, is isolating the compound in sufficient quantities without harvesting wild lichen unsustainably, and then stabilising it within a coating formulation. Neither problem is solved yet, but the direction of research is promising.
Lichen in the Workshop and in the Field
I find it curious that some of the most sophisticated questions about surface adhesion and protection are being answered by something growing quietly on a damp wall. Craftspeople and woodworkers have always paid close attention to natural surfaces; anyone who has spent time preparing timber for finishing knows that the condition and texture of a surface determines everything that happens afterwards. Even something as straightforward as choosing the right panel saws for breaking down timber accurately is part of the same broad understanding that good surface preparation begins long before any coating touches the wood.
Lichen, in a sense, has been teaching that lesson for hundreds of millions of years. It prepares its own substrate, modifies the surface chemistry to suit itself, and then applies a living coating that is flexible, self-repairing, UV-resistant, and drought-tolerant. It is the product of evolution working on a problem that human engineers are still trying to crack.
Why Lichen Matters Beyond the Laboratory
Lichen is also an important ecological indicator. Because it absorbs moisture and nutrients directly from the air and rain rather than from soil, it is extremely sensitive to atmospheric pollution. The near-disappearance of many lichen species from British cities during the industrial era of the nineteenth and twentieth centuries is well documented. Their gradual return to urban environments, including central London and Manchester, is one of the quieter good-news stories of improved air quality in Britain over the past four decades. The BBC has reported on lichen as a bioindicator for pollution monitoring, noting that lichenologists now map species distributions to track air quality improvements in ways that no instrument can quite replicate.
So the next time you are out on a hillside in the Cairngorms, or walking a coastal path in Pembrokeshire, or simply passing an old churchyard wall, have a proper look at the lichen. Notice the colours, the textures, the variety of forms. Some are flat and crusty, painted directly onto the rock as if sprayed on. Others are leafy and lobed, almost like tiny succulents. A few hang in long grey-green strands from the branches of old trees in the wetter Atlantic woodlands of the west coast. Each one is a working prototype for a surface technology we have not yet managed to fully replicate. Remarkable, really, for something that most people walk straight past.
Frequently Asked Questions
Is lichen harmful to stone walls and buildings?
It depends on the species and the context. Some lichens produce acids that slowly etch stone, accelerating weathering over decades. However, well-established lichen crusts can also protect surfaces by binding loose particles and reducing direct rain impact, so conservators assess each case individually rather than removing lichen automatically.
What is lichen actually made of?
Lichen is a symbiotic organism formed from a partnership between fungi and photosynthetic algae or cyanobacteria. The fungal component provides structure and anchors the colony to its surface, whilst the algae or cyanobacteria produce sugars through photosynthesis to sustain them both. Neither partner could survive alone in the same environment.
Why does lichen grow so slowly?
Lichen grows slowly because it relies entirely on nutrients absorbed from rain, dust, and air rather than soil. Most crustose species grow only a fraction of a millimetre per year. This means a lichen patch of significant size on an old wall or stone can represent decades or even centuries of undisturbed growth.
Can lichen survive extreme cold and drought?
Yes, remarkably so. Lichen can lose almost all of its water content and enter a state of suspended animation during droughts or freezing conditions, then rehydrate and resume normal biological activity within minutes of rain. This ability, known as poikilohydry, is one of the properties that materials scientists find most interesting.
Where can I find lichen in the UK?
Lichen is widespread across the UK and found on almost any stable outdoor surface: dry-stone walls, churchyard headstones, rooftiles, rocky coastlines, tree bark, and mountain rock faces. The richest lichen diversity tends to occur in the wetter, cleaner air of western and northern Britain, particularly in Pembrokeshire, the Scottish Highlands, and the Lake District.