When three stones are stacked in a garden, it's because someone needs a retaining wall to hold back a garden bed. To stack stones three high is usually the best height for a retaining wall, since going higher means the stone wall will likely fall down. On rare occasions a person may have a garden big enough to warrant the use of the standard three stone stack to indicate which direction to go. Trail maintenance workers will use this method to avoid leaving man-made objects behind in nature.
Stones have been stacked by trail guides since the Native Americans began leaving signals for finding good hunting and food sources. Three stone high retaining walls have been used by agricultural villagers since the onset of crop harvesting and kitchen gardens.
Stone stacking is the most cost-effective method for holding back or creating new garden beds. This method is used where there is a need to prevent soil from slumping, or sliding, and because the aesthetic nature of a stone wall has yet to be matched.
A three-foot high rock retaining wall is often used for garden bed and plant retaining purposes. When choosing wall stones, a good mason searches for stones about a foot high and wide, with at least two flat sides, for easy stacking. Today many people use mortar. In older days, mortar was a mixture of lime, clay and sand, sometimes with soil or earth added for volume, and it was bonded together by the process of vigorous mixing. This mortar is just as effective as the mortar bought in stores today.
The decision about if and when to mortar a stack of stones can vary. The degree of flatness and how many sides of the stone are flat contribute to this. Very flat, large, heavy stones when used for a retaining wall need no mortar. The builder can use simple logic and intuition to decide if and when the wall is stable. Most stones that are about a foot by a foot weigh just under 80 pounds. Most strong, healthy adults can lift that weight easily with some effort.
Stones can also be used to store heat. Many healing sweat lodges use stones for this reason. Stones placed strategically in a garden can also increase the soil's temperature and therefore stimulate an increase in plant growth.