There is a stretch of dry stone wall on a lane near me that I have drawn perhaps twenty times, and I am still not finished with it. At a glance it reads as grey rubble. Sit down beside it for ten minutes, though, and the grey resolves into a dozen colours — sulphur, rust, sage, slate, a cold blackish green — and the rubble resolves into a community. Stone walls are among the most underrated habitats in Britain, and they are conveniently everywhere: field boundaries, churchyards, garden walls, railway cuttings, the backs of old industrial buildings.

A habitat built by hand

What makes walls so rewarding is that they are vertical, free-draining and often lime-rich. Rain runs off rather than pooling, so the surface dries quickly after a shower, and the stones themselves weather at different rates depending on what they are made of. A limestone wall in the Cotswolds, a gritstone wall in the Peak District, a slate wall in Snowdonia and a flint wall in Norfolk each carry a noticeably different community of species.

The two faces of the same wall are also worth comparing. A south-facing face is warmer and drier, and tends to carry more lichen. The north-facing face stays damp for longer and is usually greener with moss. Even along a single face, there is a gradient: the base is splash-fed and shaded, the middle is the main growing zone, and the top coping stone is exposed, sun-bleached and often colonised by the toughest crusts of all.

What you are actually looking at

Lichens are not plants. They are a partnership between a fungus and an alga or cyanobacterium, and they come in three broad growth forms that are genuinely useful to know when you are sketching:

• Crustose — painted onto the stone, impossible to lift without a knife. Look for the map-like outlines of Rhizocarpon or the cracked mud of a Lecidea.
• Foliose — leafy and lobed, with edges you can peel back. Xanthoria parietina, the common orange lichen, is the classic example and glows on nutrient-enriched stone near farmland.
• Fruticose — shrubby or cup-shaped, such as the little grey-green Cladonia cups that appear on the tops of older walls.

Mosses divide more simply into acrocarpous species, which grow upright in tight cushions, and pleurocarpous species, which creep and branch into feathery mats. Grimmia pulvinata makes grey-green cushions with white hair points; Tortula muralis is the common wall screw-moss with translucent hairs; Bryum argenteum forms silver-green tufts in pavement cracks; Hypnum cupressiforme sheets the shaded mortar in fine, combed layers.

Kit worth carrying

You do not need much. A x10 hand lens is the single most important item, because it turns a smudge into architecture. Beyond that:

• A soft pencil (2B or 4B) and a small cartridge pad for quick tonal studies.
• A pocket watercolour tin with a decent ochre, a viridian or sap green, burnt umber and Payne's grey.
• A water brush and a small spray bottle — spritzing moss makes its colour deepen dramatically and shows you its true green.
• A kneeling mat or folded coat, since you will be crouching at the base of the wall.
• A notebook for dates, weather and aspect, which matter more than you would expect.

Getting texture onto paper

Begin with a light pencil silhouette of the stone shapes, then block in the mortar and shadow before you touch the growths. Lichens sit on top of that structure, not instead of it.

For crustose lichens, work dry: scumble pale grey-greens with a nearly dry brush, then stipple darker flecks into the surface. For foliose species, draw the lobes as overlapping shapes and save the brightest colour for the growing edges. Xanthoria mixes well from cadmium orange with a touch of burnt umber; the greener crusts come from yellow ochre softened with viridian or a little Payne's grey. Leave paper white for the sunlit faces of the stone — the highlights do more for the illusion of hardness than any amount of grey.

A useful discipline is the quadrat study: mark out a five-centimetre square on the wall with your fingers and draw it at actual size. You will find more in that square than in a whole metre of wall.

Keeping a wall diary

Return to the same wall through the year and your notebook becomes a record rather than a sketchbook. Note the date, the weather of the preceding days, and whether the stone is wet, damp or bone dry. Lichens are visible in every month, but their colours are richest after rain, when the thalli swell and the greens turn almost luminous. Mosses look their best from autumn through early spring; in a dry August they shrivel to brown threads and seem dead, until ten minutes of drizzle brings them back. Watching that revival happen, pencil in hand, is one of the small privileges of this kind of looking.