A trunk furred green to head height, or a green sleeve up the shaded side of an old apple tree, makes many gardeners uneasy: the tree looks as though it is being slowly smothered. It almost never is. Moss on bark is among the most harmless things in a garden, and understanding why settles most of the worry at a stroke.
A guest that takes nothing
Moss growing on a tree is an epiphyte, a plant that uses the tree purely as somewhere to sit. It has no true roots, only fine anchoring threads called rhizoids that grip the surface of the bark, and it makes its own food from light and air exactly as it would on a rock or a roof tile. Nothing passes from tree to moss. It draws no sap, opens no wound and touches nothing living beneath the bark. Ivy, which does haul itself into the canopy and compete for light, is a separate worry; a film of moss on the bark is not. To the tree the moss is a perch and no more.
Why bark turns green
Moss settles where bark stays damp and the air hangs still: the shaded, rain-catching side of a trunk, the lower few feet where humidity gathers, the rough furrowed bark of an old tree that holds water far better than young smooth bark does. This is also why moss seems to single out ailing trees. A tree that is losing vigour carries a thinner canopy, so more light and rain reach its bark, and the moss thickens in answer. The moss has not caused the decline; it is reading the same conditions you could read for yourself. The old belief that moss marks true north rests on this same damp-side preference, though it makes a poor compass, as moss myths and old beliefs sets out.
What you are looking at
The flat glossy mats pressed against a trunk or a fence rail are very often plait moss, Hypnum cupressiforme, the great generalist of bark and stone, which has its own profile in plait moss. Higher up, out on the twigs and younger branches, you tend to find small neat cushions instead, the bristle-mosses and their kin perching in the brighter, breezier air of the canopy. Lichens usually share the bark too, grey and leafy or crusty and yellow, and the two are easily muddled, although a lichen is a partnership of fungus and alga rather than a moss at all. Because the mix shifts from the damp foot of the trunk to the windy top, a single tree can carry several quite different communities at once.
When it is worth a second look
For an ornamental or a woodland tree, moss on the bark asks nothing of you. Old fruit trees are a slightly different case, less because the moss does any harm and more because a heavy fur of moss and lichen usually goes hand in hand with an open, under-pruned, low-vigour tree, and the damp crevices give overwintering pests somewhere to shelter. The remedy is to tend the tree rather than scrub the moss: prune for an open crown, then feed and mulch a struggling specimen, and the bark dries quicker and greens over far less. As for the weight of moss on a branch, often fretted about, it is trifling and never a structural concern.
Removing it kindly, or leaving it be
If a prized specimen or a smartly trained fruit tree really must be cleared, brush the moss off gently with a stiff hand brush while it is damp and lifts easily, and stop at that. Keep a pressure washer well away from living bark, since the jet shreds the thin outer layers and can reach the living tissue underneath, doing far more damage than the moss ever could. The harsh moss killers sold for paths and roofs have no business on a tree either. In most gardens the wiser course is simply to leave it. Bark moss shelters the small insects that feed nesting birds and gives the birds themselves a soft lining for the nest, and it carries good news about your air, for the bristle-mosses crept back onto town trees as the old sulphur smogs lifted, a recovery traced in moss and air quality.