There's a large maple tree that lives next to my house. It's been a trouper, in spite of old age, ice and wind storms, woodpecker holes, squirrel nibblings, a canker wound, and a barbecue grill rack nailed to one of its limbs (for some unknown reason). I often look out to see how my old friend is doing, and this time I noticed that it is graced with a single vertical stripe of moss growing on—you guessed it—the north side.
This lead me to wonder, is it true that moss only grows on the north side of trees? Here's what I found: In the northern hemisphere, it is generally the rule that in open areas moss does indeed grow more successfully on the north side of trees and other objects (in the southern hemisphere, the opposite is true). This is because moss prefers cool shade and needs moisture to reproduce. The south side of a tree, rock, or other object in the open normally receives more light and heat. Moisture evaporates more quickly there than on the north side, so moss is not as happy in that environment.
However, there are exceptions. For example, if an object blocks air circulation and sunlight from the south side of a tree, and/or a brook or other source of water frequently splashes the tree, then it is possible that moss could grow well on the south side, too.
In a dense forest, where the light is gloomy all around and moisture takes a while to evaporate, the opportunities for moss to set up shop are plentiful. In these areas, you sometimes can see a tree trunk or rock completely covered with a fuzzy, green carpet.
What are your experiences with moss? Have you ever used its location to determine direction? Feel free to share your stories.