Ron Kushner is a Pennsylvania Certified Horticulturist for Primex Garden Center in Glenside, Pa., a master gardener and the author of A Year in a Rock Garden — An Organic Gardening Guide.

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Rock On
by Ron Kushner    

Want to create a new kind of small-space garden that is the perfect venue in which to try a new palette of plants? Start a rock garden! Here’s how.

Rock gardens can be created easily and are ideal for gardeners with limited space. Exposure should be full sun for at least five hours but partial shade, especially in the afternoon, is fine. The size of the garden can vary from a few square feet to a maximum of 1,000 square feet. Any larger area would require more maintenance than the average home gardener could normally provide.

Traditionally, rock gardens were created to imitate alpine areas throughout the world, generally above timberline, where native plants survived naturally in harsh conditions.

Rock Garden Basics

The soil should contain a fair amount of gravel material so as to provide good aeration with excellent drainage. The lack of sufficient drainage is the number one killer of most rock garden plants. The soil mixture can consist of an even mixture of top soil, “forest products” and a soil amendment containing natural ceramic or shale material (heated to extremely high temperatures forming a lightweight mixture) to assist in drainage. A rich soil composition is not recommended. Unlike perennial and shrub beds, rock-garden soil should allow the plants to struggle a bit. In soil high in organic matter, the most aggressive plant will flourish and take over, requiring an excessive amount of vigilance and maintenance.

Establish the rock garden so that a small hill, or berm above the ground, is created with all sides sloping gently downward. This design forms a kind of raised bed that the plants will thrive in. Rock-garden plants will adapt to a variety of pH levels but 6.6 to 7.0 will accommodate a wide range of available plants.

The design can vary greatly and does not need to follow a particular plan. Some broad guidelines would include the placement of rocks throughout the garden in a manner that “pleases the eye.” Once they are arranged, each rock should be buried by at least one-third of its depth to provide stability and to give it a natural look. The type of stone can also vary, but it should match any existing stone formations in the surrounding landscape as closely as possible. A local quarry or stone yard should be able to supply the appropriate stone in a variety of sizes and shapes. Ultimately, plants will creep around and over the stones so that only portions will be visible.

Another guideline would be to create the look of a scree. A scree develops in nature as stone tumbles down from higher elevations: Larger rocks first, then pebbles and finally dust-like particles. This condition can be created artificially by variously sized rocks plus the use of a stone mulch consisting of stone particles about 3/8 inches throughout the entire garden.

The selection of plants and their placement is the next step in the design process. First, make sure that the plants are not crowded. At least a 4-square-foot space for each plant is a good general rule. This calculation will determine  the amount of plants that can initially  be installed. Flower color and bloom time, mature height and spread, light requirements, foliage, winter interest and hardiness should all be considered. If catalogs are being used for plant selection, check the description and cultural requirements carefully.

A view from the deck in April shows the textures and colors. Rock gardens can look appealing even from a distance when viewed from any angle. 1

Plants for the Rock Garden

Frequently the term “alpine” is used to determine a rock-garden plant. An alpine plant refers to one that grows naturally in an actual mountain setting and was originally named for the Alps in Europe. A “rock plant” or “rock-garden plant” is one that may include alpine plants but would also include any plant of the proper size and characteristics suitable for planting in a rock garden.

When selecting plants, scale must  be considered — the ultimate size of  the plants in conjunction with the size of the rocks. Tiny, prostrate plants and large boulders normally are out of scale as would be fist-sized stones and plants the size of dwarf trees. Once the plants grow and fill out, they tend to blend together in what appears to be more of a woven tapestry than a grouping of different plants.

Members of the stonecrop family (Crassulaceae) are a good choice for rock gardens. There are more than 600 species of Sedum found throughout the northern hemisphere. They have a diverse habit, from creeping to upright, and are hardy in Zones 5, 6 and 7. 

Other suitable plants include Dianthus spp., Armeria spp., Geranium spp. and Thymus spp. Thyme is native to Europe, with a creeping habit forming a ground cover with flowers of red, pink or white in summer. Excellent in hot, dry locations with poor soil.

Plants for the Rock Garden

Firewitch cheddar pinks (Dianthus ‘Feuerhexe’.2
•  ‘Snow Fix’ wall cress (Arabis caucasica ‘Snow Fix’)
•  ‘Pink Lusitanica’ sea thrift/sea pink (Armeria maritima ‘Pink Lusitanica’)
•  Dwarf Hinoki false cypress (Chamaecyparis obtusa  ‘Nana Gracillis’)
•  Dwarf Sawara false cypress (Chamaecyparis pisifera ‘Tsukumo’)
•  Firewitch cheddar pink (Dianthus ‘Feuerhexe’)
•  Myrtle spurge (Euphorbia myrsinites)
•  Bloody cranesbill (Geranium sanguineum)
•  Soapwort (Saponaria ocymoides)
•  ‘Blue Spruce’ stonecrop (Sedum reflexum ‘Blue Spruce’)
•  October daphne stonecrop (Sedum sieboldii)
•  Two-row stonecrop (Sedum spurium)
•  Common houseleek (Sempervivum tectorum)
•  Creeping thyme (Thymus serpyllum)

Chamaecyparis pisifera ‘Tsukumo’ is a miniature, dwarf Sawara false cypress, a curly and thick plant growing only 1 inch a year.1

Dwarf green Hinoki false cypress (Chamaecyparis obtusa ‘Nana Gracilis’) is a conical plant that looks  good in any rock garden.1

Sedum sieboldii is native to Japan, and its blooms are rose-pink from late summer to fall. The common name is October daphne stonecrop.2

Geranium sanguineum ‘Max Frei’ is a low, prostrate plant with red flowers in early spring. It tolerates full sun, even in hot, dry summers.1

Sedum spurium ‘Summer Glory’2

Creeping thyme (Thymus serpyllum ‘Magic Carpet’)2

Maintenance Considerations

The maintenance of a rock garden is considerably easier than traditional perennial or shrub beds and especially easier than caring for edibles.

Regular watering is still required the first year after planting until the plants become well established. From the second growing season on, supplementary water is needed only during periods of excessive drought.

In early spring, remove any brown foliage, dead stems and leaf litter when new growth begins to show. This is also a good time to fertilize; however, never use high-nitrogen fertilizer. A fish emulsion or other fertilizer such as 2-4-1, is all that is required. Rock-garden plants need no fertilizer once established.

Weeds will sprout, especially where mulch is thin, and need to be teased out throughout the growing season. Some plants will require deadheading when their blossoms are spent.

This is the rock garden after “spring cleaning,” now ready for fresh stone mulch and new plantings.1

Insects and other pests are of  little concern in rock gardens — most damage is minimal and there is no need for pesticides.

Fall cleanup includes removing dropped leaves and stems since this debris can potentially harbor fungus throughout the winter. Wait until new growth is showing in spring before pruning any plants.

The beauty of rock gardens is that plants can be added, removed or relocated with little cost or effort. One can feel more relaxed, enjoying gardening on a smaller scale.


1) Photo by Ron Kushner
2) Photo Courtesy of Walter’s Garden, Inc.

From State-by-State Gardening May/June 2013.


Posted: 08/21/13   RSS | Print


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