Build a Rock Garden

Rock gardens expand a gardener’s planting palette and offer creative challenges to gardening enthusiasts. Ideally suited to hot spots and difficult-to-tame slopes, they take their inspiration from wild flora that sprouts along gravelly mountain peaks and sandy desert floors. Building a rock garden allows gardeners to play with an array of small plant varieties that would likely be lost if planted in traditional perennial borders.

Ed Glover, a member of the Wisconsin-Illinois chapter of the North American Rock Garden Society, has been rock gardening for more than 25 years. He tends his own 120-by-15-foot rock garden at his home and acts as a volunteer rock-garden caretaker at the University of Wisconsin-Madison.

“I like rock gardening because it’s challenging,” Ed says. “It allows me to try to grow plants I’ve seen in the mountains out west. Since the plants are small, you can put in lots of different ones in smaller areas. They supply pretty foliage when not in bloom, and the rocks are interesting, even in winter. I always tell people to start with a small rock garden. If they like it, they can expand it a little at a time.”

Kim Zoss, a horticulturist at the Chicago Botanic Garden, also encourages gardeners to start small. “Beginners might try cultivating a rock garden in a trough, in a nook, atop a small berm or around a water feature,” Kim says. “Rock gardens should look like a natural part of the landscape, so a gardener needs to look at the whole yard to see how a rock garden will fit in. It’s best to place the gardens where you can see them easily and often.”

Rock Garden Location and Construction
As a rule, rock gardens require sunny sites and quick-draining soil. Ed recommends planting alpine-type gardens on sunny, south-facing slopes; xeric-type rock gardens planted with drought-tolerant plants do well where conditions are hot and dry.

“When constructing a rock garden, always build up for good drainage,” Ed advises. “Instead of digging up existing soil, mound new soil atop the ground. I use a soil mix that’s equal parts compost, sharp sand and pea gravel. Dig into the mix to place your rocks, and bury the bottom third of each rock, so it doesn’t look like you just tossed the rocks in.”

Burying portions of the rocks not only provides a natural overall appearance but also encourages plants to stretch their roots to the soil beneath the rocks, which results in healthier plants. Ed suggests spacing rocks to create crevices for tucking in cushion-forming plants or creeping cultivars that will wander over nearby stones. Situate larger rocks so they create microclimates that offer shaded planting sites on one side and sunny spots on the other. Tilt flat rocks to direct rainwater into the soil.

Select a single type of native rock in different sizes and shapes for a cohesive, natural-looking garden. If you’re planting alpine gardens, opt for porous rocks such as limestone, sandstone, shale or tufa; nonporous rocks like marble, basalt and granite work well in desert-inspired gardens.

Rock Garden Plants
Common landscape-design principles – such as including zone-hardy varieties with varying heights, forms, textures and bloom times – apply when creating rock gardens. But, since rock gardening focuses on individual plants, it’s important to not crowd plants and to choose nonaggressive plants that won’t overgrow their designated spots. If you’re including evergreens for winter interest, select miniature varieties with a very slow growth rate.

Ed plants an array of alpine and subalpine perennials, many with cushionlike profiles. Draba aizoides, a mat-forming perennial with yellow flowers, and saxifrage, a silvery-foliaged plant with white, yellow or pink blooms, kick off the flower show in early spring. Creeping phlox and rock cress will blossom next and are followed by summer-blooming dianthus and blue-flowering campanula.

Desert rock gardens supply a good foundation for penstemons, sedums, cacti, low-growing yarrow and hens-and-chicks. The Chicago Botanic Garden’s rock garden combines alpine and xeric perennials, such as ice plant, small sedums, ground-cover potentilla, pulsatilla, moss phlox and dwarf iris with miniature evergreens, such as Teddy arborvitae. Other good rock-garden options include Tom Thumb cotoneaster and blue rug juniper, Kim says.

Perennials are the mainstay of rock gardens, but some gardeners incorporate heat-loving annuals, such as moss rose or creeping zinnia, into their design for season-long color. Kim also recommends tucking miniature spring bulbs amid the rocks for early spring color.

Rock gardening is becoming increasingly popular, and local nurseries are responding to the growing demand for rock-garden plants. Ed and Kim recommend checking local nurseries first—you’ll often find plants larger than those available through mail-order catalogs and varieties specifically suited to your planting zone.

Rock Garden Maintenance
After the plants are in, mulch the entire garden with at least an inch of gravel that matches your rocks and stones. The mulch conserves moisture, keeps weeds down and fashions a suitably stony backdrop for both boulders and plants. Limit fertilizer applications to once a year to keep plants from growing too large. Educate yourself about each plant’s water requirements—too much moisture may cause root rot; too little water may stress out plants. Instead of using a sprinkler, Kim suggests spot-watering each plant to meet its individual water needs adequately.