A new terracotta pot or a freshly cast stone ornament announces its newness, all sharp edges and bright clean surfaces. A skin of moss is the quickest way to take the edge off and lend the weathered, long-settled look that otherwise takes years of weather to earn.
The slurry method
The technique is the same moss milkshake used for graffiti and walls. Blend a couple of handfuls of clean moss with enough buttermilk or natural yoghurt, thinned with water, to make a loose paint, then brush it over the surfaces you want aged. The dairy helps it cling and feeds the moss a little while it takes hold. Paint it into crevices, around rims and over textured areas where moss would naturally settle, rather than evenly all over, for a believable result. It is worth matching the moss to the job: a low, tight-growing kind gathered from a wall or paving will suit stone better than a loose, feathery woodland moss.
What it works on
Porous, rough materials take moss best. Terracotta, unglazed ceramic, natural stone, concrete and cast reconstituted stone all give the moss something to grip, and their surfaces hold a film of moisture that helps it survive the first weeks. Smooth, glazed or sealed surfaces are far harder, since there is nothing for the rhizoids to hold and the slurry simply slides off. Pick a piece destined for shade and damp, because the same conditions that grow moss anywhere apply here; an ornament in full sun and wind will stay stubbornly clean whatever you paint on it.
Slow, and never guaranteed
It is fair to say at the outset that this is a slow business and not a certain one. The slurry is really a way of putting moss where you want it and giving it a fighting start, not a paint that turns a pot green overnight. Some pieces take within a season, others never do, and a dry summer or a change of position can undo months of progress. Treat it as an experiment worth repeating rather than a guaranteed finish, and try more than one piece so that at least some come good.
Other ways in
The slurry is not the only route. Simply standing a porous pot in a shaded, damp spot, out of drying wind, will let moss arrive on its own over a year or two, and a north-facing position under trees does much of the work for you. You can also press small pads of living cushion moss into damp crevices and mist them until they knit on, which is slower to spread but surer to take than the painted-on kind. A scatter of soil or old potting compost smeared into the surface first gives spores and fragments something to settle in.
Keeping the patina
Once painted or planted, keep the piece in shade and mist it regularly for the first few weeks while the moss establishes, exactly as for any new moss. After that, a position that stays cool and damp will maintain the patina with little help, while a dry spell will send it dormant and dull until the moisture returns. Over a season or two the moss thickens and the piece looks as though it has stood in the garden for decades, which is the whole idea. The underlying method is set out in the slurry and spraying guide.