Here is a wide list to select from-the problem is not so much what to take as what not to use.
Azaleas: Any of the hardy deciduous azaleas are available, but it is easy to overdo them. In most types of landscaping, azaleas and rhododendrons should be used in masses; in rock gardening a single plant often better answers the purpose.
Dogwood: The native Flowering Dogwood (Cornus florida), is in character of growth admirable for the rock garden; it is possible to keep the trees at any height by pruning for lateral growth.
Forsythia: Most varieties rather large and coarse. Suspensa litboldi is the lowest growing and most graceful.
Goldflower (Hypericum moserianum) : Good for yellow flowers in midsummer; variety Buckleyi is more dwarf and spreading.
Highbush Blueberry (Vaccinium corymbosum): The natural growth of this fine native shrub is often such as a Japanese gardener might spend a lifetime in achieving; few plants are more picturesque.
Rhododendrons: Almost too large and too heavy even for the background, excepting in rock gardens of large size. The Myrtle rhododendron (myrtifolium), Wilson rhododendron, thriving in sun or shade (both of which are hybrids of r. minus) and ovatum, are dwarf alpine forms.
Roses: Several of the taller-growing species, not mentioned in the preceding list, such as Hugonis and the Redleaf Rose (r. rubifolia) with its small starry blossoms, carry the spirit of the rock garden.
Tamarix: I have never seen this shrub recommended for use in rock gardening, but it is particularly pleasing and effective; its feathery sprays of pink blossoms may be enjoyed from spring until late summer by using the several varieties; africana, the earliest; gallica, blossoming in midsummer, and indica in late summer.
Witch-hazel: Our common Witch-hazel (hamamelis virginiana) frequently found growing on rocky hillsides, unfurls its graceful little tasseled yellow bells in late October or even in November, and in January or February the Vernal Witch-hazel (h. vernalis) bursts out impatiently; the Chinese Witch-hazel (mollis) follows a bit later; the Japanese is impartial, and flowers either in latest autumn or in earliest spring; plant against evergreens; semi-shade will do.