Well does it? Or does it only grow on one side? Or does it grow only on the side further from the equator? Is there any truth to this old adage?
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Moss grows best in the shade (and damp, but most relevant here is shade). Because of the curvature of the Earth, in the northern hemisphere the north side of trees is shadier than the south side, so if moss grows on only one side of a tree, it is likely to be the north side. In the southern hemisphere, the whole thing is mirrored so the moss is on the south side. This is, however, nothing more than a rule of thumb, as there are many other factors that affect moss, especially the density of foliage and humidity.
Moss prefers damp locations irrespective of facing.
You need to cut out a few variables before you can use moss as a pretty reliable method to determine North:
With that in mind, choose moss that's growing at least waist height on a vertical, smooth surface.
It's not 100% reliable however. So check around for other mossy regions to get confirmation.
There's some truth to it in that moss prefers shady areas rather than directly sunny ones, so (in the northern hemisphere) since the north is the generally more shady part, you'd assume moss will be more likely to grow on the north of the tree.
For me though, it's nothing more than a curious fact rather than anything to reliably use in terms of orientation or navigation. There's all sorts of other variables that can cause moss to grow or not grow, and like any living organism's placement there's always an element of randomness as well! Lots of things can affect it, from humidity to moisture to the shade other objects cast around it. If you're trying to use this to navigate it could easily give you a false sense of security or make you feel uneasy about your chosen direction if the "moss rule" contradicts it.
So yes, there is some truth behind it and it's nice to know, but not reliable enough to really be of much use.