Every so often someone tells me you can find your way in a wood by the moss, because it grows on the north side of the trees. A real observation sits buried in the old rule, but it has been bent into something far more certain than the moss will bear out. Moss does lean towards one side of a trunk more often than not; whether that side points you north is a separate question, and a shakier one.
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Where the rule comes from
Moss cannot regulate its own water the way a leafy plant can, so it settles and persists wherever a surface stays cool and damp for the longest. On a free-standing tree or an open wall, that is usually the face catching the least direct sun. Because the sun tracks across the southern sky here in the northern hemisphere, the north and north-east side of a trunk sits shaded for much of the day, loses its moisture slowest, and offers moss the steadiest foothold. That is the grain of truth the compass lore grew out of. Watch an isolated oak in the middle of a field and you will often see the green climbing furthest up its northern flank.
Why one tree will mislead you
Trouble begins the instant you trust a single trunk. Several things push moss around with more force than the sun's average position ever manages. Across much of Britain the rain rides in on south-westerly winds, driving water onto the windward face, so the heaviest moss can gather on the south-west side rather than the north. A tree that leans throws its dampest shelter onto whichever side now tips downward. A neighbouring tree, a tall hedge or a building will shade any aspect it pleases. And the moment you step from open field into closed woodland, the canopy shades every trunk fairly evenly, damp lingers through the whole stand, and moss sheets around the full circle with no bias worth reading. The deeper and wetter the wood, the more useless the rule becomes, which is inconvenient, because a wood is exactly where you might wish to use it.
Read the tendency across many trees
To get anything trustworthy out of the old sign, look at the population rather than the individual. Take in a dozen free-standing trees, fence posts and boulders across a stretch of open ground at once, and a bias may show itself, a general drift of the greener, shaggier growth towards one quarter. That aggregate lean carries more weight than any single trunk, and even so it points you to the shadiest, dampest quarter rather than to true north as such. Check it against the position of the sun above all, and against the lie of the land you walked in by. South of the equator the whole thing turns over, since there the sun hangs in the northern sky and the shaded, damp face looks south.
Walls, stones and the same logic
The pattern shows cleanest where nothing moves. A north-facing wall greens over while its sunny south face stays bare, and the shaded foot of a gravestone furs up long before its top. It is the identical preference for cool and damp at work, and on a fixed, upright surface with open sky around it the reading comes out far clearer than on a tree. None of this is really navigation; it is a map of where the sun does and does not reach, which is the thing moss tracks the whole time. The greening of a shaded garden wall, covered in moss on walls, is the same story told on brick.
How much to trust it
As a piece of pub knowledge it stands up: moss does favour the shadier, damper side, and here that is more often than not the northern one. As a way home it is close to worthless on its own. I would never set a course by a mossy trunk, and nor should you if anything much hangs on getting it right. Take it as one soft clue in a reading of the whole scene, weighed against the slope and where the light falls, never as a stand-in for knowing roughly where the sun is. The moss is telling you something true and worth knowing; it simply is not telling you north. The one-line version of this sits among the other moss myths, and if you would rather understand what the moss is actually chasing, whether moss needs sunlight and how moss survives drought both come at the shade and moisture that drive it.