Now you’ll place plants next to rocks or in crevices between rocks. Choose plants for sun, part shade, or shade, depending on the specific conditions of your garden. As you place the plants, cover any bare areas with smaller stones. Because the site is sloped, start at the bottom of the site and work upwards.
Limit the number of different types of plants you use, which is more natural and pleasing to the eye. Don’t cover the garden completely with plants—the rocks should be visible. Leave enough space between plants so that they can spread out as they grow. Consider the ultimate height of plants for both their placement and the scale of the garden. You may not want plants that protrude too high above the rocks, or you may want a mix of low and taller.
Consider color, too. Granite grey looks good with colors from cool blues to warm pinks and bright white. Sandstone, however, may look better with plants limited to a warm palette. Finally, fill your scree bed with gravel, pea gravel, or a combination of small-scale materials. It’s optional, but if you have full sun, you may also want to plant alpines and groundcovers in the scree.
Porous rocks such as sandstone or hard limestone add a weathered look—and are easy to split if necessary.
Because moving stones is so labor intensive, you’ll want to use rocks from the site—or at least from close by, to save on transport. If you are bringing stones in, put some thought into your choice. Porous rocks such as sandstone or hard limestone add a weathered look—and are easy to split if necessary. Craggy tufa (soft limestone) provides nooks for plants to grow in, but may break down eventually. Hard granite won’t have the same character, but its jagged forms add drama. Smooth boulders and river rock also have soft edges. Moss- or lichen-covered stone adds to a naturalistic, unplanned look.
Gravel—crushed stone—ranges in size from 2″ to 4″ to pebbles ¾” or smaller. Some sizes come polished, but unpolished has a more native aesthetic. Use a mix of sizes under plants and between boulders. Take into account your regional rock types. To order, know the square footage of coverage as well as how many inches deep the gravel bed will be.
Rock Stars: which plants thrive
Stones retain heat, and so rock gardens can be too dry for some plants, so opt for heat-loving ones. Ranging from tiny specimens to cascading plants to grasses, alpine plants are a rock-garden favorite. Cascading plants such as dwarf aster, candytuft, alyssum, sedum, rock cress, and snow-in-summer are a must because they’ll spill over hard edges, softening the look. Specimens such as creeping thyme, soapwort, creeping phlox, and blue star creeper spread out, providing groundcover.
To avoid a garden that is bare in the winter, choose some evergreens such as Cole’s prostrate (a type of hemlock), dwarf cotoneaster, and dwarf mountain pine. For spring color, incorporate a variety of bulbs such as crocus, miniature daffodils, freesia, grape hyacinths, dwarf iris, and snowdrops.
Low-growing trees such as dwarf conifers and acer (Japanese maple) may add just the right amount of verticality to a ground-hugging rock garden. Herbs work well, too—try catmint, thyme, or verbena. Little Bunny dwarf fountain grass, blue creeping sedge, Japanese forest grass, blue oat grass, and blue fescue have fluffy plumes and whispy strands to create year-round impact. Alpine plants aren’t the only option. In the right climate, succulents such as hens-and-chicks, blue chalk sticks, Eche-veria glauca, agave, and many cacti are visually arresting.