Whether in the mountains or in the plains, traditional rock gardens are generally dry and exposed, with quick draining medium for planting in between the rocks. Rocks absorb heat from the sun and hold it in. Many people have rock gardens for a respite from gardening chores like frequent mowing and deadheading. Plants are selected for their tolerance to drought and heat, in addition to their perennial nature and ease of maintenance.
Choose Plants by USDA Zone
The most important thing to know when choosing plants is your garden zone. If you live in the southwest you may be more familiar with the Sunset Garden zones than the USDA (United States Department of Agriculture) zones. Use a conversion chart to help you determine your zone.
Zones 2 - 10
Douglas Moss Phlox - Phlox Douglasii
These vigorous, spring-flowering plants are perfect for zone 2-9 and forms mounds of narrow, needle-like leaves dark green in color. In a rock garden with many plants with silver gray foliage, the little mounds of dark green offer a nice contrast. In late spring, the plants are completely covered with flowers in pink, purple or mauve. Even a white-flowered variety is available.
With high disease resistance, this repeat performer is practically maintenance-free but a good clipping right after the flowering will keep the mounds thick and shapely. Phlox looks lovely positioned just behind a rock in a sloping garden where it can trail downwards.
Yellow Alpine Alyssum - Alyssum Serpyllifolium
This star of rock gardens thrives in zones 5-9 and has silver-gray leaves and soft yellow flowers. It does well in even very poor soil. Drought resistance makes this shrub ideal for dry and sunny locations. The long flower show in spring adds fragrance to the air. Plants are easy to maintain, as they do not require pruning. Since it has a low and trailing habit, it is best kept to the front of the flower bed where it will happily fill in any empty space.
Rock Cress - Aubrieta Sp.
As the name suggests, these carpet-forming, cascading evergreens are a great addition to any rock garden. Although they do well in sunny locations throughout zones 4-9, they need some amount of moisture to survive. Their cascading habit helps soften the sharp edges of the rocks, contrasting well with mound-forming plants. Free-flowering in spring, in colors ranging from blue and lavender to pink, deep red, and magenta, this is one plant sure to fit into your garden's color scheme.
Coral Bells - Heuchera Sanguinea
These plants with colorful leaves are grown more for their foliage than the flowers. In the rock garden, the coral bells's striking variegations and pink and purple leaves add a splash of color even when the plant is not in flower. These dainty little plants do best in part sun in zones 3-8, and because of their striking foliage should be located in a highly visible location.
Sea Holly - Eryngium Varifolium
Sea hollies, comfortable in zones 4-9, are named so because of their tolerance to salt spray. They take a lot of neglect and do well in hot and dry areas with poor soil adding interesting texture and height to the plantings. The large, branching flower stalks are long-lasting. Their strange beauty is more due to their form than flower color, and they provide incentive for butterflies to visit the rock garden too.
Adams' Needle Yucca - Yucca Whipplei/Filamentosa
The tall spikes of yuccas shoot out in summer from a crown of dark green strappy foliage, bearing little bells of creamy white color. Whether in flower or not, yuccas are a valuable addition to any rock garden in zones 4-10, if space permits. It is a plant-it-and-forget-it specimen that adds architectural beauty to the landscape. Be sure to give this one plenty of space to spread out. Most mature at two feet in width and three feet in height, making them the perfect addition to the back edge of your garden.
Hens and Chicks - Sempervium Sp.
Mainstays of most rock gardens for long, the different varieties of rosette-forming semperviums are drought and deer resistant. Once established in the rock garden, they grow in the most impossible nooks and crannies, multiplying by the dozen. They do put out flower stalks, but the flowers are nothing compared to the highlights of pink, purple, and red on leaves in shades of green and silvery gray, making them great additions for color and texture. Keep these cute little plants towards the front of the garden in zones 2-9, where you can enjoy them the most.
Blue Fescue - Festuca Glauca
This short ornamental grass makes rounded balls of narrow needle-like leaves. Its shape and the typical blue coloration make it a welcome addition in any rock garden in zones 4-8. It needs full sun to achieve its blue-gray color though. The flower stalks that appear in summer should be removed if you want the grass to retain its spherical shape. In cooler areas, the foliage may turn brown but it should be trimmed only in spring, after all danger of frost has passed.
Zones 9 - 12
Pig's Ears - Cotyledon Orbiculata
If you live in warm areas of zone 9-12, you can enjoy these lovely plants. Their large powder-blue leaves provide a nice contrast to the narrow leaves of other rock garden plants and grasses. They cannot stand wet feet, so good drainage should be ensured. In summer, flower stalks carrying salmon-pink flowers arise from each stem, but unless the plant has formed a clump of several stems, their impact may be minimal. Bees and butterflies love them and because of their size, they should be reserved for the back edge of your garden.
Elephant Foot Palm - Beaucarnea Recurvata
This relative of yuccas is not a true palm, but does enjoy the humid temperatures of zone 10-12. Planted in a rock garden, the unusual shape of this plant will capture attention immediately. It is planted with the upper half of its swollen base exposed. Use this interesting plant for a focal point. Drought tolerant and pest-free, the elephant tree palm (also known as the pony tail palm) will not demand any special care from you but will serve as a unique conversation piece in any garden.
Prickly Pear Cactus - Opuntia Sp.
This cactus, also known as a nopal, may be used in the rock garden in zones 10-11 and is particularly well suited for lending a sculptural effect. The fruits of many varieties are edible, hence the name. The flat pads are the planting material. It is a plant-it-and-forget-it specimen, and best left alone too, as the tiny spines called glochids can be really bothersome.
Agave - Agave Parryi
Happy in zones 7-10, agaves make bold accent plants in any rock garden. The small agave varieties are better suited for home gardens. Since agaves are practically pest-free and grow well in poor soil, there's no need to get closer to the plants once they are positioned in the garden. A single plant may produce several suckers around the original one; each sucker is a clone of the mother plant. These can be carefully taken from the main plant and transplanted in a new spot.
Shopping for the Best Plants
Follow these shopping tips to help you select the best plants:
- Know your garden, the amount of sun that reaches your yard, and the what plants will work best in your region before picking a rock garden variety.
- Research the mature size of each plant you wish to place in your garden. This will allow you to purchase and plant accordingly.
- Plants suited for rock gardens are generally dwarf varieties that are hardy and well-suited to poor soil or other harsh conditions.
Nature as Your Guide
Traditional alpine and desert type rock gardens represent the triumph of life force over the harshness of the elements of nature. Art is an imitation of nature, so the best rock garden ideas still come from natural settings. Watch how some plants squeeze into seemingly impossible crevices, while others stand tall and proud among their mostly low-growing compatriots. Besides self-contained little mounds of green, you may find vigorous specimens gleefully tumbling over the boulders, unmindful of their unyielding hardness. Take your cues from nature while selecting plants and arranging them among the rocks.