A mossed boulder, or a run of stepping stones gone soft and green at the edges, carries a settled, long-standing look that no freshly laid stone can fake. It is one of the more achievable garden effects too, since bare rock is close to a moss's ideal home once you understand what the plant is really after. The catch is that you cannot hurry it, and the wrong rock in the wrong corner will stay stubbornly bald however hopefully you water it.
Why bare rock suits moss
Moss asks almost nothing of the ground it sits on, because it draws water and minerals in across its whole surface instead of through roots, as the question of moss and roots explains. A stone that would starve a rooted plant is therefore no obstacle at all; what matters is the texture of the surface. Rough, pitted, slightly porous rock holds a film of moisture and gives spores and fragments somewhere to lodge, while glassy, polished granite sheds both water and settlers and greens over painfully slowly. Weathered stone that has already gathered a little grime and grit is far readier than a fresh-cut face. Because most mosses lean either towards lime or firmly against it, the chemistry of the rock quietly decides which species will ever thrive: sandstone, slate and old brick suit the acid-lovers, whereas limestone and mortar carry a different, lime-tolerant set.
Choosing the stone and the spot
Aspect governs the outcome more than anything you do afterwards. A north or east face, shaded through the hottest part of the day and slow to dry after rain, is where moss will hold; a baking south face defeats all but a handful of drought-hardened cushions. Site your stone near a pond, a downpipe or a shaded wall, where the air stays damp and barely moves, and you are already halfway there. Pick rock with plenty of texture, sandstone, tufa, weathered concrete and rough-hewn granite all take moss well, and set it low enough that rain splash and ground damp reach it. Tufa, a soft porous limestone, is almost a cheat, holding water like a sponge so that mosses colonise it unusually fast.
Transplanting living moss
Moving moss that is already growing on stone is the surest route. Lift small patches from a wall top, an old paving slab or a rockery where it grows thickly, with permission and without ever stripping a wild site bare; the etiquette is laid out under collecting moss responsibly. Match the source to your rock, for moss taken from acid stone will sulk on limestone and the reverse holds just as firmly. Press each patch onto the chosen surface, working it into hollows and against the grain, then hold it there. On a vertical or sloping face a criss-cross of dark thread, a smear of clay slip or a scrap of net pinned over the top keeps the pieces in contact until their rhizoids grip, which takes a few weeks. Several small cushions knitted onto a boulder look more natural than one large sheet, and they creep out to meet one another in time.
The slurry method, and its limits
The much-shared trick of blending moss with buttermilk or yoghurt and painting it on has its uses, though it proves less reliable on rock than the makers of videos imply. The principle is sound enough: the blitzed fragments regenerate, and the milk leaves a mild, faintly acidic film to see them started. It does best on porous, shaded, dependably damp stone and fails on anything that dries hard between waterings. Think of it as a way of seeding fragments into cracks and pits rather than a coat of instant green, keep it misted daily, and give it weeks before anything shows. The full recipe, and why it disappoints roughly as often as it charms, runs through moss graffiti.
Settling it in
Whichever method you choose, the first month or two decides the rest. Newly laid moss has no anchorage and can dry out within hours on an exposed stone, so it needs to sit in shade, sheltered from wind, and be watered lightly every day or two until it grips, ideally with rainwater or cooled boiled water rather than hard tap water, which leaves a chalky bloom; there is more on this under watering moss. Brush fallen leaves off before they smother a patch, and pull out any seedlings that root in the damp. After that the work falls away almost to nothing. An established mossed rock looks after itself for years, flushing green after rain and fading to a dull grey crust through dry spells, and it only deepens and softens as the seasons pass.