Moss is one of those weird, old plants that you can just tell comes from a different time. The fossil record isn't all that expansive for them because of their fragility, but there is evidence that they were around at least four hundred million years ago, and could have evolved long before then. They are unique in that they reproduce with spores, sort of like a fungus (though they aren't closely related). They also don't have a vascular system, which separates them from most other plant life.
I associate moss with the swamps of my childhood in Northern Minnesota. There are places where moss covers every surface, sometimes many feet thick, in the peat bogs. Moss can be useful stuff. My parents store fresh carrots over the winter in a sphagnum lined cellar. It can also be used, in the form of peat, for a fuel—though that is pretty rare these days in the Western world. For most of us, moss is just that green fuzz that tends to grow on things after a while. When it grows in our lawns, on concrete, or on roof shingles, it can be quite destructive. When this occurs, we need to think about getting rid of it.
Getting Rid of Moss
Physical removal will probably be necessary. If you have an area of moss that is bothering you, killing and removing it is going to be your first step. Killing the moss can be done physically, chemically, or a combination of both. Look for chemical moss killers that contain ferrous sulfate, or use a cryptocidal soap. Physical removal of moss from a lawn can be done with a rake or shovel. Using a tiller or a dethatcher will make the job a lot easier. If the moss is on a roof or other architectural surface, a scraper or brush might be more appropriate. Be careful not to damage the surface any more than necessary. Remember, moss spores are in the air all the time and could return, if the conditions are right.
A healthy lawn will prevent moss from taking over. Moss tends to take over lawns that are not properly maintained. They will fill up dead patches caused by poor fertilization, poor aeration, and compaction. If your lawn area is thick and healthy, there will be no place for the moss to grow. Yearly aeration treatments, along with a basic fertilizer, can keep your lawn healthy. Adding new soil and reseeding any dead spots will help prevent moss from returning.
Shade is a haven for moss. At least in the Northern Hemisphere, mosses and lichens are famous for growing on the North side of trees in a forest. They like a little bit of light, but not direct sunlight. The same is true in your yard or on your house. If there is a spot that is being invaded by moss, I am willing to bet that it is in the shade most of the day. Trimming some branches from nearby trees might help to keep moss from becoming a problem. In places closer to the equator, where it might be wetter and warmer, moss grows everywhere. Attempting to get rid of moss in that kind of environment might be impractical.
Moss likes acidic environments. Places that have a lot of decomposing plant material tend to get acidic. While acidic soil is great for blueberries, it is also a place where moss thrives. But it isn't good for your lawn, and that's why moss will win in an acidic environment. Keeping your lawn clear of large amounts of decaying matter will help keep your yard free of moss, in the long run. You can also adjust the pH of your lawn by adding a base, like garden lime.
A damp lawn is a mossy lawn. Moss grows very well in bogs and swamps. If your lawn is losing the battle to moss, it might be too wet. Some basic landscaping might be necessary to provide adequate drainage for runoff from nearby roofs or other water sources. Combined with the other factors listed here, your moss problem should abate.
Moss likes to grow in shady, wet, acidic places. In the average yard, that means around the base of a shady tree or up a brick retaining wall. If you want to keep your tree, and have opened up the ground level to as much sun exposure as you can, and moss is still growing, maybe you need to think about just going with it? I really like the look of a rock garden skirting a tree. Add a pond with a fountain, and some mossy rocks of the non-rolling variety. You've built yourself a little piece of the grandeur of nature. All without having to fundamentally change the dynamics of your yard.
If you don't want to get rid of that shady, wet spot by cutting down trees and digging drainage paths, any moss you remove will come back. Moss spores are flying around all the time, and will only sprout in the correct conditions. If you don't change the conditions, you will never get rid of the moss.