The most amazing feature of moss is that it is able to grow on any surface, provided that the humidity in the air is correct. It actually prefers surfaces low in nutrients, since an excess of it may damage the plant. Due to this, there is no need for soil for it to grow abundantly, just a constant humidity of around 80% and sunlight is enough for it to grow naturally.
The most common symbolism associated with moss is the smallness of humanity in contrast with the greatness of nature. Therefore, many people choose to let it grow freely in respect and remembrance of the nature of man.
In Zen gardens, the meaning goes a little bit further. The ability of this plant to cover large areas during an extended period of time, is greatly referred to the ability of the human mind to achieve it’s natural state under patience and perseverance. It is also aesthetically used in the design of such gardens as islands, where rocks are mountains and sand the water.