Whether you have a stone wall, a natural rock outcropping or a man-made rock garden, choosing the right plants makes your alpine rock garden look like part of the natural landscape. Alpine plants grow in the crevices between the rocks, and those crevices are filled with rich, fast-draining soil. Give your alpine plants the same good soil and they'll reward you with a wonderful focal point for your landscape.
The Christmas rose (Helleborus niger) was the North American Rock Garden Society (NARGS) plant of the month for December, 2010. Hardy throughout U.S. Department of Agriculture plant zones 4 to 8, this 12-inch, evergreen perennial sends up pink-tinged white blooms from fall to early spring. Leave the plant undisturbed once planted. Christmas roses grow best in shady sites with alkaline soil.
The saxifrages (Saxifraga spp.) are a group of alpine plants hardy in zones 2 through 7. They don't tolerate heat or humidity so cannot be grown successfully in the South. Saxifrages form rosettes or tufts of foliage that are topped with red or white flowers in spring or early summer. The cultivar Blood Carpet has red flowers and grows only 4 to 8 inches tall. Tricolour has white flowers with red spots and foliage that's variegated green, white and pink. NARGS recommends the species S. longifolia, a white-flowered form that can be grown from seed. Grow saxifrages in sun to part shade.
Native to the Mediterranean coastline, sea thrift (Armeria maritima) has small, grassy tufts topped with pink flowers in spring and early summer. As you'd expect from a coastal plant, sea thrift is salt tolerant and prefers full sun. It's hardy in USDA zones 4 to 8. A related species, Armeria caespitosa, was NARGS plant of the month in September, 2009.
Despite the odd name, skullcaps (Scutellaria alpina) are lovely plants that are hardy to USDA zone 5. The summer flowers resemble small snapdragons and come in yellow, blue, lavender, pink and white. Another NARGS favourite, skullcaps grow 6 to 12 inches tall and slightly wider. Like most alpine plants, they thrive in alkaline soil and need good drainage.
Not all alpine plants have to be perennials. Plant expert Karen Panter of Colorado State University Cooperative Extension service recommends alyssum (Lobularia maritima), an annual that performs well in rock gardens. The low mats of fine foliage are topped by fragrant, white flowers in spring and summer. A purple variety is also available. Grow alyssums in full sun and shear them back after the first flush of bloom to keep the plants compact.
Another annual recommended by Panter, moss roses (Portulaca) have succulent foliage topped with bright flowers all summer long. The flowers open only when in sunshine, so plant it in full sun for maximum effect. Moss roses thrive on neglect and need supplemental watering only during prolonged drought. This annual reseeds itself freely.