Rock gardens require specific types of plants to be successful, and a year-round rock garden shouldn't be denuded in winter. Selecting a range of dwarf evergreen shrubs will keep things interesting and provide continuity when annuals finish flowering. Some evergreens blossom in summer, in addition to supplying evergreen foliage. Be careful to not over- or underplant--the garden should be spare, not bare--and plants should be slow-growing and kept pruned so they don't obscure the rocks or smaller plants.
Groundcover links rocks and patches of stone and fills in bare spots. Spring heath ("Erica carnea")and Irish heath ("Daboecia cantabrica") are low-growing evergreen shrubs with the bonus of producing seasonal blooms. Spring heath has needle-like foliage with flowers that range form white to pink to crimson. Irish heath's purple, and sometimes white, flowers bloom all summer, and its foliage is glossy green on top and fuzzy white on the underside. Creeping phlox ("Phlox subulata") has needle-like green foliage and sends out wide swathes of white, pink or lavender-blue flowers in May and June. It reaches only 6 to 8 inches tall and spreads out in unrestricted areas, and also will climb a rock wall.
Dwarf varieties of American arbor vitae ("Thuja occidentalis") reach heights of only about 3 feet but are stars in the garden. These attractive shrubs can be planted singly to show off their evergreen foliage, which ranges from green to yellow. Dwarf germander ("Teucrium chamaedrys") is a low-growing evergreen shrub that produces dark-pink flowers in early summer. It makes a good border plant or low hedge. Offset shrubs with chicks-and-hens ("Sempervivum"), a succulent evergreen perennial that grows from a couple of inches to 6 inches tall. The plant's colours are as pretty as its shape, from the purple leaf tips of "black" chicks-and-hens to the red-leafed species with green tips called "jade rose," to leaves of bright lime green. Hens, the mother plants, produce chicks, new plants, so there are always succulent, drought-tolerant rosettes to enliven the garden.
Dwarf Japanese Juniper
Dwarf Japanese juniper ("Procumbins" cv. Nana) is small but packs a big punch. It has stiff, blue-green needles that shade to purple in the fall and give off a pleasant scent. It's a fairly hardy plant that grows sideways, and although it matures at only 6 to 12 inches tall, it will spread out as far as 5 feet. Use it to fill an empty space in the rock garden, or train it partly over a large boulder or rock wall. Japanese juniper thrives in full sun but will adapt to partial sun, and isn't too fussy about the soil. If it's staked or trained over a high rock, Japanese juniper will grow in an attractive cascade.
Shrubs for Problem Spots
When the rock garden has patches of shade or areas of wet soil, plant something that can easily adapt. Sheep laurel ("Kalmia angustifolia") will grow to 3 feet tall and doesn't mind wet soil. Its foliage is evergreen, and it flowers in mid-June with crimson petals that provide a vivid contrast against the colours of rocks and leaves. Dwarf drooping luecothoe ("Leucothoe fontanesiana" cv. Nana) prefers the shade. It produces evergreen foliage that briefly turns bronze in the fall. Its white summer flowers are small and waxy.
- Cornell University Extension; List of Dwarf Shrubs and Low-Growing Plants Suitable for Foundation Plantings; January 2003
- Washington State University Extension; Rock Gardens and Rock Garden Plants; Sydney McCrea; January 2005
- Colorado State University Extension; Gardening on the Rocks; Peggy Burch; August 2009
- Washington State University Extension: Dwarf Japanese Garden Juniper
- Rutgers University; Herbaceous Perennials (Sempervivum); April 2011