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 earn 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, and both spread into a close, even skin. Collect small amounts from paths, walls and pots rather than stripping a single patch, and take from several places so no one spot is scarred, as set out in collecting moss responsibly.
Why it helps the tree
The benefits come from how a moss lives. It has no true roots, only rhizoids that anchor it, so it does not compete with the tree for anything in the soil; it simply sits on the surface and takes up water across its whole body. A damp moss layer shades the compost, slows evaporation and buffers the temperature swings that a shallow bonsai pot suffers in sun and wind. It also breaks the force of the watering can, so fine soil is not splashed out of the pot. What it gives back visually, the sense of aged ground, is a bonus on top of genuinely useful shelter for the roots.
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 to settle them. The slurry method also works well on bonsai: blend moss with a little water or buttermilk into a thin paint and brush it over the soil, where it regenerates from the fragments over a few weeks. Firm contact is what matters; moss that sits loose above the soil dries and lifts. The method is covered in full in the slurry and spraying guide.
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 compost 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 at the soil.
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. The same eye for scale governs the accent plantings called kusamono.
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. Keeping a spare tray of moss growing in a shaded corner means there is always a fresh patch to lift from, and that rhythm also lets you correct any watering habits the moss would otherwise mask.