Therapeutic horticulture gives people a sense of control and comfort. From what I can tell, it is partially because the brain can recognize certain patterns and repetition, which is easily and presently found in nature. Pattern is a system that is based in memory (Think of how children can pick up on memory/matching games with flashcards), and can help with healing processes of the mind. Pattern is also something that has been present in every culture, yet unique to each one. Here is an amazing experiment done with sand, and the vibrations of sound (this process is called Cymatics, a brief summary can be found here). The patterns correlate with so many things and nature, and a vivid reminder of how everything is so interconnected. Some of these patterns of sand can be seen on the shells of tortoise,and follow the rules of the golden ratio.
Be sure to turn the sound down before you watch the video...Some of the sounds can be really loud!!