Rock gardens became popular in the 1800s as a way to show off unusual plants that were not found in most home gardens, namely the kind of hearty Alpine plants that could grow in rocky areas at high elevations. In time, though, the concept of the rock garden broadened to include any garden dominated by large rocks, usually on a slope, regardless of the plants grown therein.
Some gardeners are fortunate enough to have a sloped area on their property that already has interesting rock formations. But you can also create a suitable area by piling topsoil and rocks into the desired shape. An effective rock garden has multiple very large rocks of 100 to 200 pounds or more.
Rock gardens generally have sandy soil, which drains well due to the garden being on a slope. The nooks and crannies around the rocks can easily hide slugs and other pests, and are common areas for weeds to develop, so proper tending of a rock garden includes frequent monitoring of those areas.
As far as plant selection, much of it comes down to your own aesthetic preferences. Choose plants of shapes and colors that mesh in a way you find pleasing. Assuming you are not going to be a purist and limit your rock garden to Alpine plants that one could find growing out from between rocks at high elevations, feel free to plant anything that is suitable for your area's climate and soil. Obviously such plants will vary; for instance, cacti are a very appealing ingredient in many rock gardens in the American Southwest, but wouldn't work in most other areas. As a rule of thumb, plants that are low growing and grow in clumps can be good in a rock garden.
If you're looking for some specific possibilities of plants to consider for a rock garden, here's a small sampling of the many that gardeners have grown with success:
Blue Eyed Grass
Blue eyed grass is a bluish-green grass that reseeds each year and can tolerate cold climates. For several weeks in the spring it is covered with bluish-purple, star-shaped flowers.
Dianthus is a genus of flowers with many varieties of many colors. Among the best for a rock garden is the dianthus alpinus, a low growing species with large, fragrant, pink flowers, that prefers well-drained soils that are not too soggy.
Dwarf Balsam Fir
Many dwarf trees make good additions to rock gardens. The dwarf balsam fir is a squat little tree with stiff branches and a pleasing balsam scent.
Dwarf Hinoki False Cypress
Another tree that fits nicely in a rock garden, the dwarf hinoki false cypress is a very slow growing evergreen tree that reaches four to eight feet in height.
Dwarf Japanese Red Maple
Yet another good tree to consider is the dwarf Japanese red maple, small even for a dwarf tree. Popular varieties include the burgundy lace and the crimson queen.
One of the most familiar and popular flowers in the world, the English daisy is also popular for rock gardens specifically. Its flowers grow in a clump with the yellow ones in the center and the elongated white ones encircling it, giving the appearance that the clump is itself a flower with a yellow center and white petals.
True geraniums grow in tight packed mounds, with dark green leaves and a plethora of little blue, pink, purple, and white flowers. Most varieties of these perennials are winter hardy.
Also called "Indian tobacco" the lobelia plant has a long tradition as a folk medicine. The lower portion of its stem is smooth, and the upper portion is rougher. From summer until frost it is covered with tiny pale blue flowers.
Red Leaf Japanese Barberry
The larger versions of this shrub are considered invasive pests in some areas, but the miniature varieties such as the crimson pygmy are fine for home gardens. The crimson pygmy has bright red leaves on spiny branches, with little yellow flowers that blossom in the spring.
Another good shrub for a rock garden is the Scotch heather, an evergreen with gray, green, or russet foliage, and small pink, purple, or white flowers. The Scotch heather prefers an acidic soil. You have plenty of options to choose that should fit nicely in your garden whether you're looking for a very low or high plant, as they range from two inches to three feet tall.