Answer: A rock garden can consist of almost any plants, depending on the exposure, soil type and amount of light and moisture the area receives. For a hot and sunny slope, try plants such as spring bulbs, thymes, low growing sedums, and creeping phlox (Phlox subulata) along with dianthus and possibly some smaller dwarf or low growing evergreens for winter interest. In a partly shady, cool and moister area you could grow hostas, ferns, astilbe and lady's mantle for example.
Most rock gardens are constructed by (surprisingly) burying up to half of each rock and aligning each rock in such a way that it looks natural. If all the rocks are of the same type (quarried from the same location) and are laid out "facing" the same direction, it helps the area look more "real" instead of like a random pile of rocks that fell off of a truck.
Traditional rock gardens were made using boulders surrounded with gravel and a small amount of compost and then planted with alpines; for our climate this is not always very practical, hence the suggestions above.
Enjoy your rock garden!
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