Q: I am putting in a natural rock walk, and I'd like suggestions for a low creeper between the stones. Would thyme be good? — Edythe Bertoldo, Blairstown, NJ
A: Thyme could be good, depending on the conditions it will be growing in. If the walk is in full sun all day and on sandy soil (or with underlying gravel and sand), it will probably be too dry. Thyme isn't exactly a desert plant. True, it demands well-drained soil, but it thrives only when there is a regular supply of moisture. However if your soil holds water fairly well (but isn't waterlogged), there are some kinds of thyme that will serve you well. Thyme can get up to a half-day of shade and do fine. If the path is shadier than that, then moss is the best choice. The less you use this path, the taller the creepers can be; but for a path you'll use daily, stick to plants that literally hug the ground. Crushed, broken foliage is not attractive, and something only 3 inches tall will be annoying to walk on. Also, remember that any plant is going to flower at some point in spring or summer, and the flowers will be raised above the foliage, sometimes several inches higher. Culinary thyme (Thymus vulgaris) is too tall. The best candidate is probably wooly thyme (T. psuedolanuginosus), with under a half-inch of gray foliage and pink flowers that rest on top. Or look for a dwarf creeping thyme like T. serpyllum 'Pink Chintz', which stays under an inch. A visit to a good local herb nursery will be time well spent.
Sedums are the most drought-tolerant low-growers. Stay away from the tiniest, Sedum acre, because it is very invasive. Sedum kamschaticum, S. spathulifolium and S. spurium 'Dragon’s Blood' are the best candidates for filling in between your pavers. Since these are taller, they will look best with a yearly trim, ideally right after they flower. (They will recover quickly.) Moss is unbeatable between pavers, and there are thousands of kinds. Growing your own is easy — with some patience and usually a little shade. Since the spores are in the air, if you make the right conditions the mosses eventually move in, usually in about a year. Keep the ground bare: no weeds, no leaf litter. For a lush carpet — which will take longer to fill in — make the soil too acidic for other plants. Moss likes a pH of 5.0 to 5.5, so have your soil tested and then reduce the pH. To take pH down a point, add either yellow granular sulfur or ferrous sulfate. Read the label carefully, as the amount you should use depends on soil type. Sandy soils need less than clay soil. The way you build the path is important, no matter what you're planting. Make it at least 2 feet wide. Install an in-ground barrier along the sides to keep grasses from encroaching. Lay your pavers very tightly in the center, leaving the largest gaps for planting along the sides. To set the stage for moss and to prevent weeds, I suggest acidifying soil in the middle. If you are using perennials or herbs, plant them along the edges and let them find their ways toward the center.