"Foothill Rock Gardens – A Tapestry of Color"

by Carolyn Singer, Foothill Cottage Gardens
Sierra Heritage, May/June 1990

In the mountains, the melting snowpack reveals rock outcroppings with small tufted plants growing in the crevices. With another winter past, alpine rock plants begin a new season of growth. While the snow is still melting, many of these plants will burst into bloom with flowers of intense brilliance and fragrance. Inspired by nature's tapestry at higher elevations, the foothill gardener can create a rock garden where boulders occur naturally. Rocky soil provides an opportunity, not an obstacle. Many low-growing perennials will flourish on a site that will not support trees or other deep-rooted ornamentals.

Woolly yarrow (left) and gray santolina (right) edge the path in the author's rock garden.

Choose a site where you would like to see colorful spring and summer rock garden plants. Dig into the soil, examining its depth and texture. If the soil is shallow (less than six inches), bring out your wheelbarrow and mix native clay soil with redwood compost or your own compost, to improve the texture and depth. A depth of 10 to 12 inches is ideal.

Amendments such as compost will loosen the heavy clay soil, but do not add more than one-third amendment to two-thirds soil. Clay soil, with its richness of minerals and good water-holding capability, is an asset. Your soil mix should also contain an organic form of phosphorus (raw rock phosphate, soft rock phosphate, colloidal phosphate, or bone meal), since this element is lacking in all native foothill soils. Phosphorus stimulates strong root growth and increased flower production.

Leave buried rocks in place. As they slowly weather, rocks provide a source of minerals. Soils near the rocks will be cool and moist, providing a "cool root run" for plants. Working around large boulders, add a layer of your good soil mix, burying a portion of the base of the rock. Use smaller rocks to retain the soil if it is mounded or on a slope. Eventually the roots of rock garden plants will hold the soil in place and prevent erosion, but begin by placing extra rock on the surface to stabilize loose soil.

Rock garden plants and alpines will grow in shallow soils, but their roots need to penetrate into rocky subsoils. The size of the visible plant is deceiving. An extensive root system develops while the plant is still young, ensuring survival during hot, dry summer days. The root systems of these compact plants may be 10 times the size of the plant growing above the ground. The deeper the soil, the better your plants will grow.

Once the site is prepared, it's time to choose your plants. Many perennials are suitable for rock gardens in the hot summer sun of the foothills, and most of them are deer-proof once established. If you have deer on your property, protect new plantings with netting until the plants have shown visible growth. Wait three more weeks, then remove the netting, but check daily for signs of deer damage. If the deer have been browsing, and testing your planting, leave the net in place for a few more weeks.

Rock cress (Arabis) is one of the most fragrant rock garden perennials, and the first to bloom in the spring. White flowers on delicate stalks cover the evergreen rosettes. Variegated rock cress (Arabis albida 'Variegata' and Arabis Fernando-Coburgii) should have afternoon shade to prevent the white and green leaves from burning in the summer sun.

For fragrance in late spring, choose one of the many species of Dianthus. These grow in the highest elevations of the United States and Europe, and are beautifully adapted to the cultivated garden. Dianthus 'Tiny Rubies' and are spreading, but compact, and will not overwhelm other plants. Their brilliant pink flowers are as fragrant as the carnation (also a Dianthus) bought from your florist. The evergreen foliage adds color to a winter garden.

A drift of mixed colors is spectacular in the large rock garden. The California native yarrow (Achillea millefolium) will brighten the rock garden with its white flowers, but the other species and cultivars add a dimension of color for the entire summer. Pink, peach, salmon, pale yellow, red and lavender blend together as if painted by the artist's brush. This perennial spreads aggressively, so give it room to grow. In the smaller garden, where plants with more restrained growth are desired, Wooly Yarrow (Achillea tomentosa) is a better choice. This Yarrow is low and creeping, with gold or yellow flowers.

Stone cress (Aethionema X Warleyense) is rare and slow-growing, but worth the wait. It is available in nurseries specializing in unusual perennials or rock garden plants. Growing in crevices, or tumbling over rocks, the bright pink flowers appear at the same time as many Dianthus, in May and June. In the larger rock garden, or along driveway edges, Snow-In-Summer (Cerastium tomentosum) covers large areas quickly. Sprays of white flowers cover the gray foliage in late spring. In the most shallow soils, Pink pussytoes (Antennaria dioica 'Rosea'), a California native with attractive silver foliage, looks as good in the winter as in summer. Pink, fluffy flowers on short stems stand above the foliage mat in midsummer. Hikers into the higher elevations will find this in bloom in July and August, pussytoes will tolerate some shade if summer irrigation is minimal.

Stone cress is a rare and slow-growing rock garden plant, but its bright pink flowers, blooming in May and June, are worth the wait.

Because a foothill rock garden does not have the benefit of a melting snowpack or summer thunderstorms, a new planting should be watered every 10 to 14 days. Irrigate established plantings with a good mulch every three weeks. A rocky or gravelly mulch on top of the soil is essential to conserve moisture during the dry season. Spreading a half-inch of sharp gravel over the finished rock garden is ideal, but many gardeners prefer to use an organic mulch such as wood chips or straw. The organic mulches may be too moist for those rock garden plants preferring a dry soil surface. Dianthus and Antennaria mulched with straw may rot during winter rains.

Hike into the higher elevations for inspiration, but leave the native plant material where it grows. A plant dug from the wild will not survive. The wild flowers will produce seed, insuring survival of the species. Our mountains offer a wealth of color, a tapestry to recreate in our own gardens, and many lessons for us. You can find plants for your rock garden in specialty nurseries in your area.