In the Garden:
Purple verbena is a perfect complement to this granite stone wall.
Plants for Rocky Places
"Something there is that doesn't love a wall."
Robert Frost was certainly not referring to plants, which seem to adore rocks and delight in nestling into every crevice that looks like it couldn't possibly support life. Plants and stone seem to have a natural affinity for one another. Left to their own devices, plants will spread in between paving stones, bravely pop up through cracks in concrete sidewalks, and scramble all over rocks in the wild. With seemingly little soil and water, they grow and flower.
Stone havens offer many benefits for plants. Rocks create their own microclimate. They hold heat and can prolong the growing season for borderline hardy plants. Rocks can both shelter shade-loving plants from the sun and reflect the sun onto sun-lovers. Rock crevices can also trap scarce water for plants.
Taking our cues from nature, we can soften the hard surfaces surrounding us -- from stone walls to walkways and patios -- with mounds of greenery, and let the colors of foliage and flowers can play off the colors in rocks. You can even grow plants directly on top of stone, sans pots. All it takes is soil and edging to contain it, and the right plants.
The weeds that sprout from in between the rocks in a retaining wall suggest good spots for rock garden plants or even drought-tolerant annuals such as portulaca. In a vertical crevice, lodge a stone at the bottom to help contain the plant and soil. Tuck in plants so their roots are lower than the leaves so water will run toward the roots, not away from them.
Even the top of a stone wall can support a miniature garden. You can use smaller rocks to contain the soil and nestle low-growing plants like hens and chicks or sedums into the shallow bed. The limited root space can naturally dwarf the size of the plants. This can work to your advantage if you want to show off a special plant, such as a miniature evergreen that may have more ambitious tendencies.
Paths and Patios
Tiny-leaved plants like thymes, Irish or Scotch moss (Sagina subulata), and blue star creeper (Laurentia fluviatilis) that can take some foot traffic can live in between stones in a walkway or path. Plant dianthus and potentilla to the side of the beaten track so their flowers won't be injured. There are low-growing plants marketed specifically for walkways, such as Stepables and Jeepers Creepers, and some are rated as to how much "stepping" they can tolerate.
Always set the crowns of the plants lower than the top surface of the rocks so they are more protected. Leaves can easily grow back but if the crown is damaged, the plant won't survive. Also make sure the soil level is below the top of the rocks so soil doesn't wash out easily.
Other Concrete and Stone Surfaces
On my To-Do list this summer is to get creative with a rectangular piece of granite I scavenged from the rubble pile at a nearby quarry. I had planned to use it as a garden bench but now I'm envisioning it topped with fun little plants worthy of a close-up view. I've collected some rocks to from a border to contain the soil and plants, but I could also use edging plants such as alyssum.
I will treat this planting as a one-season garden because plant roots will be more exposed than they would be deep in the soil, and it can be difficult to provide enough protection to them over the winter. In the fall I can plant the perennials in the ground and replant the garden next spring.
Soil for Rock-Loving Plants
Many plants that grow amidst rocks are intolerant of too much moisture, so the soil mix needs to provide good drainage. A mixture of equal parts garden soil, builder's sand or grit, and compost is a suitable blend. If the spot is very dry, use more soil/compost and less sand; if the spot receives more moisture, use more sand and less soil/compost. A layer of gravel on the surface will help keep the crowns of the plants dry, which many rock garden plants require.
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