PLANTING AND CARE
Planting times vary according to climate and season of dormancy. In warm areas it’s almost always late fall. Where winters are rugged, it can be during brief semidormancy in early September, but it’s safest in early spring when severe frosts are no longer likely and favorable weather blesses plants at the height of their annual growing power. In temperate sections spring-flowering varieties can be planted from mid-August through October, others in spring.
Lemon thyme and ivy showing results of being trimmed and kept neat
Mr. and Mrs. Alex O’Hare
Strong, healthy, pot-grown plants can be planted throughout the season in almost all areas. They transplant easily, with little disturbance and shock. In fact, whenever we acquire plants during the fall, we keep them in their pots or pot them up, and protect them in the cold frame during the winter. One of my favorite and most knowledgeable rock gardeners tells me that he always pots tender or difficult plants, and keeps them potted until their roots are fully developed, before he sets them out in the garden, no matter what the season.
Soil
Providing the proper soil is vitally important to rock-garden plants, and particularly to alpines. They’re naturally dwarfed by the rough conditions in their native mountain areas. But when they move down to our comparatively lush climates and longer growing seasons, they respond to a too-rich diet by growing too tall and rangy, with too few flowers, and they lose their charm. They do need some nourishment, of course. And they are extremely sensitive to soil textures and drainage conditions. Heavy, soggy soil causes them to rot quickly. Soil that lacks organic matter dries out too fast.
Because of climatic and other variations, I hesitate to set down any “ideal soil mix” for these plants. Instead, I suggest a basic formula that can and should be varied according to the aridity or rainfall in your area, the size and location of your rock or wall garden, the composition of your garden soil, the availability of some ingredients, and the personal preferences of the plant varieties you want to grow. Here it is:
Three parts fairly friable, fertile loam.
Two parts humus—peat, leaf mold, compost—for lightness and fertility.
Two parts stone chips or crushed rock—perhaps the most important ingredient, to keep soil open and cool.
One part sharp builders’ sand, or the gravelly sand used in mixing concrete, to lighten heavy soil and promote drainage.
If your garden loam is sandy or gravelly, of course, you would increase the proportion of humus. If it’s rich in organic matter, you would decrease the quantity of humus. Very sticky, heavy clay soils are best replaced.
Rock and alpine plants are also particularly sensitive to the acidity or alkalinity of soil. This is measured by the pH scale, ranging from very acid pH 4.0, through 7.0 at neutral, to very alkaline 9.0. Rock-garden specialists usually indicate the preferred pH for the plants they sell.
By using a soil-test kit or, better still, sending a sample to your state agricultural service to be tested, you can easily find out whether the soil in your garden is acid or alkaline, and to what degree. Then, to make acid soil more alkaline for plants that need it, mix in small quantities of agricultural lime—the amount depends on the degree of acidity and on the plant itself. Don’t overdo it. Too much lime can do as much harm as good. Alkaline soils are not quite so readily converted to acid, but you can use oak-leaf compost (not completely rotted), acid peat, or pine “duff” (rotted pine needles). Or you can be satisfied with the tremendous number of rockery plants that really prefer alkaline soil. These preferences vary, not only between different plant families, but also between individual members of the same family.
For our small gardens, I’ve found it not too tiresome to provide soils of several types. For specially precious plants, I often mix special, small batches of soil. Then I clean out the planting pocket, make sure that it is tightly closed in with rocks so water won’t wash this soil out or other soil in, and refill it with the preferred mixture.
Watering
Except during rare droughts in spring, when the plants are in active growth, we seldom have to water our rock and wall garden. Our summer rains are usually sufficient for their needs. But whenever or wherever there are long weeks of extreme heat and dry winds in summer, water may be needed. Use a fine spray gently, so that the small plants won’t be washed loose. And let the water soak in long enough so the entire area is moist in depth.
Fertilizing
Some rock plants can take and use fertilizer, in spring; but most alpines are better off without it. For them, we mix a little leaf mold with the gravel mulch when we renew it. For the more ordinary types, we provide a very light feeding of whatever balanced fertilizer we have on hand.
Grooming
In rock gardens (and particularly in very small rock gardens) even slight untidiness or imperfection shows up all too clearly. But I consider it a pleasure to weed and care for the little plants a few minutes every weekend. Faded flowers are snipped off. Weeds are removed with determination, but with gentle protection for any small, weak plants. Spreading shoots that grow long and out of line are cut off, or sometimes uprooted, and removed. Louise Beebe Wilder says it’s an art to keep a rock garden in balance, with the proper proportion of plants against stones. And she is an artist with hers.
In spring, even before the last of the winter mulch is removed, we look for cracks in the soil and for plants that have been heaved out by frost, and press them firmly back into place so their roots can take hold again. We also fill any washed-out pockets with fresh soil mix, and top-dress any low areas. Any rotted leaves and stems and other debris are removed; and the entire garden gets a thin fresh layer of stone-chip mulch.
In fall there’s the general cleanup operation—weeding, pressing in loose roots and packing them with soil, trimming back rambunctious growers, replenishing the mulch of stone chips if needed.
Winter Protection
If you can count on a continuous cover of snow over your rock garden all winter, there’s little you need to do for these hardy little plants. But we can’t; we have the danger of alternate freezing and thawing that can do so much damage. So we provide special protection.
In winter, after the ground is frozen at least an inch deep, we cover the complete garden with salt-marsh hay. (Some types of Fiberglas, and wood excelsior from packing cases, are also suitable, reusable materials). This light, airy blanket keeps plants cold and dark, yet gives them air to breathe. If it threatens to blow away, we hold it in place with a few light evergreen branches.