Who hasn't marveled at a dandelion that thrives as it grows from a tiny crack in the pavement? Or more improbably, a tree growing from a rocky outcrop? It makes us stop and reconsider what we know about growth requirements for plants. Many plants are adapted to growing in such extreme environments, and the contrast of soft foliage and flowers with stone leads to interesting and striking results. Crevice gardening allows gardeners to get creative as they showcase beautiful plants in a dramatic setting; it is ideal for someone who is ready for a new gardening adventure.
One advantage of a crevice garden is that it requires only small amounts of water. A variety of plants will work; what they have in common is that they are adapted to thrive in dry conditions, so too much water can lead to deadly bacterial and fungal infections. Another consideration, particularly relevant now, is that this type of garden is more fire safe than a bed mulched with wood chips or bark.
Structure and Site
To create the structure for a crevice garden, flagstones, slate, rocks, or even found materials like slabs of broken concrete are placed like plates in a dish drainer, with the small spaces in between filled with a fast-draining medium. Hypertufa, which is a lightweight do-it-yourself material made from a blend of Portland cement, peat moss, perlite, and water can also be used to make slabs or containers. Because peat is a non-renewal resource, some gardeners use coconut fiber in its place.
As you can see from the illustration, while the roots may reach into the soil below, the surface is rocky and dry, so the garden is not conducive to weeds taking hold (though as we know only too well, weeds are good at growing in even unhospitable environments).
The structure of the garden can be made to simulate a natural-looking rocky outcrop, or it can be strikingly modern or a charmingly whimsical miniature environment. An ideal place could be a small, awkward, overlooked area in your yard that isn't suitable for a traditional garden—maybe a small patch in a side yard, or next to a driveway, or even in place of a retaining wall. Beautiful crevice gardens can also be created in containers—which have the advantage of being able to be moved to a new location according to need—and this is a good option for those with a small yard or just a patio. Wherever the garden is located, good drainage is essential. This can be achieved by planting on slopes, in raised beds, or in containers. Because most plants suitable for crevice gardening are small, raising them closer to eye level makes them easier to see and appreciate. A variety of heights also makes the arrangement more interesting and aesthetically pleasing.
Deeper containers generally work better than shallow ones because they allow moisture to drain quickly from the surface, so the crowns of the plants remain dry while the roots stay moist. You can see how this works with a kitchen sponge. Saturate a rectangular sponge and hold it horizontally until gravity pulls out enough water that it stops dripping, then turn it to a vertical position. You will see that more water easily runs out in the vertical position. Thus, shallow containers tend to retain moisture more evenly throughout the soil than deep ones, though, of course, it is also true that smaller containers dry out more quickly, whatever the shape.
Many plants suitable for crevice gardens grow well in desert climates as well as alpine environments, but in hot climates, it may be necessary to place them on the east side of a building, tree, or shrub to prevent the rocks from getting too hot. Keep in mind that denser, heavier natural stones retain more heat than porous, lightweight material such as volcanic rocks or hypertufa. Placing a patio umbrella where it can shade the plants from the intense afternoon sun can also help prevent overheating. In hot climates, planting in the fall through early spring allows plants to become well-established before the summer heat starts.
Soil and Fertilizer
The most important quality of the soil used for crevice gardening is that it should drain well so that moisture is pulled away from the crown of the plant. The crown is the area most susceptible to attack by pathogens, and it will rot if it is kept moist. The right soil mixture is key, because pores in the soil—the air spaces between the particles of soil—allow the roots to get both the air and the water they need. Soil with smaller pores, such as clay, retains more water than soil with larger pores.
A common mixture used in crevice gardens is 1/3 soil, 1/3 sharp sand (the kind with jagged, irregular shapes), and 1/3 gravel or grit (such as the kind sold for chickens), which gives the plants a mixture of pore sizes. As gravity pulls the water down, the surface will stay dry, but the roots will have the moisture they need. It's important that the roots do not sit in water though; this can be deadly. Because containers dry out more quickly than the ground, a soil that retains a bit more moisture is used in containers.
Another consideration is the pH level of the soil. Most plants—particularly alpine plants—suitable for crevice gardening will thrive in soil that is neutral to slightly acidic, but there are exceptions. Many plants that have evolved in dry steppe or desert climates—including baby's breath (Gypsophila species), candytufts (Iberis species), and saxifrages (Saxifraga)—prefer alkaline conditions. You're stuck with the basic soil in a garden site, but with a small raised bed or a container you can use a soil mixture suitable for the plants you wish to grow. So just as you would group plants with similar water needs together, grouping plants with similar pH preferences together is advisable.
In addition to needing little water, crevice gardens also have the advantage of requiring little to no fertilizer. Too much fertilizer can lead to growth that is too loose and lush, rather than the tight and compact growth that would occur in their natural environments. Container-grown plants do need some fertilizer, since their roots can't reach down into the soil below, but it is best to avoid over fertilizing by applying it at a rate of about half what is recommended on the label.
Plants
Which plants can be grown in a crevice garden? More than you might think. The Denver Botanic Garden's Rock Alpine Garden includes over 3,000 unique types of plants. Their website gives an extensive list of these plants—including their origin and growth requirements—in a very useful searchable database, providing not only valuable information but inspiration for gardeners.
Plants suitable for crevice gardening come from a variety of genera: Cyclamen, Daphne, Dianthus, Draba, Eriogonum, Festuca, Gentiana, Hosta, Iris, Lewisia, Penstemon, Polygala, Primula, Pulsatilla, Saxifrage, Sempervivums, and Tulipa. With some research, you can find which will grow best in your environment. The North American Rock Garden Society has detailed information about suitable plants.
Many of these plants are native to Europe and Asia, but some California natives will work too: bitter root (Lewisia rediviva), Cascade desert parsley (Lomatium martindalei), coyote mint, (Monardella odoratissima), dark throated shooting star (Primula pauciflora), Davidson's penstemon (Penstemon davidsonii), Idaho fescue (Festuca idahoensis,), Sulphur buckwheat (Eriogonum umbellatum), prairie flax (Linum lewisii ), yarrow (Achillea millefolium), and others. Calscape is a wonderful source for information on these native plants.
You might be wondering how to get a plant started in a crevice. Because of the narrow spaces involved, plantings are usually bare root or small rooted cuttings, or plants can be started from seeds. High Country Gardens has additional information on planting.
If you are curious about crevice gardening, remember that you can try it on a small scale without making a major investment in time and energy and still receive strikingly beautiful results. These gardens require little water or fertilizer, and while no garden is maintenance-free, they are low maintenance. The resources listed below can help you get started.
Books:
Tychonievich, Joseph (2016). Rock gardening: Reimagining a classic style. Timber Press.
Seth, Kenton, & Spriggs, Paul (2022). The crevice garden: How to make the perfect home for plants from rocky places. Filbert Press.
Other resources:
Denver Botanic Gardens: Rock Alpine Garden