Concrete seems the last surface a moss would pick: pale, smooth, baked through in summer and often laced with lime that few plants can stomach. Yet with the right aspect and a little coaxing, mosses will green a concrete path, a rendered wall, a breeze-block screen or a cast urn, turning raw grey into something that looks a generation older. The knack is to grasp what makes concrete awkward, then side with the handful of mosses that genuinely like it.
Why fresh concrete resists it
Two things about concrete hold a moss back. The first is chemistry. New concrete and mortar are strongly alkaline, and for a year or two they leach lime and push up the pH of any film of water sitting on the surface, which a great many mosses will not endure. The second is texture. A steel-floated or shuttered face comes out glassy smooth, offering the rhizoids, the fine threads a moss anchors with, nothing whatever to grip. Add the way an exposed slab dries within minutes of rain, and you have a surface that sheds moisture and spores alike before either can take hold. None of this sinks the project; it simply explains why bare new concrete stays bare for years while an old, weathered, north-facing wall greens over of its own accord.
The mosses that like lime
Success turns on planting the species that treat concrete as home instead of pressing woodland mosses onto it that pine for acid peat. The pioneers of pavement and masonry are precisely the ones to reach for. Wall screw-moss, the small hoary cushion of every mortar joint, actively seeks lime out, and its ways are set out in wall screw-moss (Tortula muralis); silvery thread-moss works its grey-green shoots into the grittiest cracks and shrugs off drought and city grime together. The grey-cushioned pincushions and the various beard and screw mosses fall in beside them. All are tough, sun-hardened urban survivors, and a slurry blended from cushions scraped off an old wall carries exactly these species with it, ready matched to the surface you mean to seed.
Weathering and roughening the surface
Time does half the job if you allow it. A fresh pour is best left through a winter of rain before you seed it, letting the worst of the free lime wash out and the surface pH settle back; a scrub with plain water, or a wipe of dilute vinegar well rinsed off afterwards, hurries that weathering along. Roughness counts for as much as chemistry. If you are casting or laying the concrete yourself, a brushed or bush-hammered finish, or one flicked with sharp sand while still wet, leaves the pitted key that rhizoids need to grip; on an existing smooth face, a going-over with a wire brush or a coarse abrasive pad scuffs in enough tooth to hold a slurry. Cool shade and steady damp finish the preparation, since a surface that stays moist is already part of the way to being colonised.
Seeding it and settling it in
With the surface ready, a moss slurry is the usual method: blend a couple of handfuls of a lime-loving moss with enough rainwater, or buttermilk or plain yoghurt, to make a thin paint, then brush it into the crevices and across the roughened face. The buttermilk can be left out, though its mild acidity and slight stickiness do help the fragments cling while they knit, and there is fuller guidance on mixing and applying it in spraying moss slurry at scale. On flat concrete you may instead press whole cushions down and pin them under a stone or a scrap of wire mesh until they root. Either way, the first month decides the outcome, because the fragments regenerate only while they stay damp; mist daily through any dry spell, and use rainwater when you do, for tap water's lime does the young moss no favours despite the surface beneath it.
Aspect, and living with it
Where you site the work governs success more than any amount of slurry. A face turned north or east, or concrete shaded by a wall, a fence or overhanging planting, holds the damp a moss needs; a slab out in open southern sun will beat every effort short of watering it daily. Level and gently sloping surfaces green faster than sheer ones because they hold the rain longer, which is why a shaded concrete path or the flat cap of a low wall makes the easiest place to start. Where the aim is to give cast pots and ornaments a specifically aged, weathered look there are a few further dodges worth knowing, gathered in ageing pots and ornaments with moss. Once it has settled, a skin of moss on sound concrete asks next to nothing of you beyond an occasional misting in drought and a sweep of fallen leaves, and it harms the concrete not at all, merely lending the hard grey the soft green of somewhere far older than it is.