A skin of moss over the soil of a bonsai does real work as well as looking right. It keeps the root zone cool and evenly damp, stops the surface washing out when you water, and gives the planting that settled, long-established look that takes years to fake otherwise.
Choose the right moss
You want a fine, flat, tight-growing species that hugs the soil, not a tall or shaggy one that competes for the eye with the tree. Low feather mosses and the flat plait mosses are ideal. Collect small amounts from paths, walls and pots rather than stripping a single patch.
Applying it
The simplest method is to press small pieces of fresh moss firmly onto the dampened soil surface, butting them together with no gaps, then water gently. The slurry method also works well on bonsai: blend moss with a little water or buttermilk and paint it over the soil, where it regenerates over a few weeks.
Do not let it work against you
Moss holds water, which is the point, but on a tree that likes to dry between waterings it can keep the surface too wet and hide what the soil is doing. Keep an eye on watering, lift a corner now and then to check the soil beneath, and trim the moss back if it starts creeping up the trunk. On the show bench, a neat moss surface is the finishing touch; off-season, give the tree normal airflow.
Matching the moss to the tree
The moss should sit beneath the tree in scale and in feeling, never compete with it. On a small shohin bonsai a coarse, shaggy moss looks absurd, like long grass round a model; choose the finest, tightest carpet you can find. On a larger, rugged tree a slightly bolder moss reads well. Aim for variation too: a single flawless green sheet can look artificial, whereas a mix of tones and a hint of bare, mossy soil at the edges looks like ground that has simply aged. Many growers keep a tray of spare moss going so there is always a patch to lift from before a show.
Through the year
Moss on bonsai is best treated as a seasonal dressing rather than a permanent fixture. It looks its part in the cool, damp growing season and for display, but a tree sealed under moss through a wet winter can stay soggy at the worst time. Many keepers lift or thin the moss off-season to let the soil breathe and to watch the surface, then re-dress before the tree goes on show. That rhythm also lets you correct watering habits the moss would otherwise mask.