It’s not exactly true that moss only grows on the north side of tree trunks. In an open, temperate forest, you may find more moss on the northern side of tree trunks. That’s because the sun shines from the south in the northern hemisphere, so a tree’s north side is generally more shaded and damp – and damp is the way mosses like it.
Mosses need dampness in order to reproduce. Instead of seeds, mosses produce tiny spores. These spores are cast from capsules that only open in moist conditions, and the spores themselves can germinate and survive only if it’s damp.
In a dense, thick forest – where the sun doesn’t penetrate as far – you’ll see mosses on the south, east, and west sides of the trees as well. As for rocks, well, mosses will grow on almost anything if conditions are favorable. As with trees, you might find more moss on the north side of rocks in an open area, but in a shaded forest, chances are good that the rocks will be completely covered.
