Once you've decided to go ahead and design a rock garden - after you've thought it out and designed it -- the first thing you'll have to do is lay down a solid foundation for it. Although it will take more time, it is recommended that you dig up the area around where the rock garden will be. This is because the majority of plants and flowers used in a rock garden need soil that is both quick-draining and water-retentive [source: Ward]. For a raised bed, experts recommend that you dig down approximately three feet (91.44 centimeters).
Once you're done excavating, the drainage layer needs to be put in. The function of this first layer is to absorb moisture that is coming from the soil. Experts suggest that this layer should be mixed with piece of old bricks and concrete, clunky rocks and even pieces of broken clay pots. This layer should fill up to one-half of the trenches that you have dug up. The next step is to lay down the sand layer. Since it is water-permeable, the sand's purpose is to keep the topsoil propped up. In general, it's recommended that you separate the drainage and soil layers with a few inches (approximately seven centimeters) of coarse sand [source: Hessayon]. Putting in the all-important soil layer of your rock garden is the next step in the process. The trick is to use a soil that's uniquely suitable to the plants and flowers that you intend to have in your rock garden. Experts usually recommend the following combination for rock garden plants: one part topsoil, one part small gravel and one part humus, peat or leaf mold.