Rock plants
As a group, rock plants represent an extensive range of hardy perennials, shrubs, and bulbous plants, many of which originate in mountain ranges. Delicate, simple, clear-colored flowers are often prolifically borne in spring and early summer. Many rock plants will flourish in a suitably well-drained site with an appropriate exposure; some require special conditions in the garden or in an alpine house.
What are rock plants?
The term rock plant is often used more or less interchangeably with alpine. In fact, true alpines are native to mountains in temperate, subtropical, and tropical regions, where they grow above the tree line in screes, rocky crevices, and grass. Subalpines occur just below the tree line. Most alpines are compact in habit and rarely over 6in (15cm) tall. This minimizes wind resistance and water loss at high altitudes.
When more broadly defined, a rock plant is any plant sufficiently dwarf in its habit to associate well with true alpines. Many rock plants are found in mountain pasture and woodland or on dry hillsides. A few occur on coastal cliffs and shores.
Adapted to growth in thin, rapidly draining soil, often at high altitudes, most alpines and rock plants will survive extremes of temperature but not excessive moisture. In cultivation, many prefer an environment that reproduces the exposure, light level, and moisture, wind, and soil conditions of their natural habitat. Rock gardens, raised beds, scree beds, and alpine houses will all provide suitable growing conditions. Other less specialized plants, like aubretias (Aubrieta), will thrive in any well-drained site with a suitable exposure.
Ornamental features and uses
Rock plants broadly encompass a diverse mixture of mat- or cushion-forming plants, dwarf shrubs and trees, and bulbous plants. Dwarf coniferous shrubs and trees, such as juniperus communis ‘Compressa’, can provide height and structure in a planting, contrasting with specimens of more open or rounded habit like hebes. Lower-growing, tufted, or mat-forming plants, like sandworts (Arenaria), are useful as a groundcover or an edging.
Some rock plants, like houseleeks (Sempervivum), have small, closely arranged, sometimes hairy leaves, which minimize water loss through transpiration. These provide a varied choice of foliage textures for garden plantings. Specimes like Celmisia semicordata, with its sword-shaped, silvery leaves, will offset rosette-forming, fleshy plants like stonecrops (Sedum) or those with feathery leaves, such as pulsatillas.
Many rock plants are ever green and will provide year-round interest in the open garden. Spaces within a framework of miniature shrubs can be filled with rock plants; their tufts, clumps, or cushions of foliage are often as interesting as their blooms.
Rock plants flower mainly over a relatively short period in spring and early summer, but often produce a profusion of tiny, clear-colored blooms. Autu9mn- and late-winter flowering bulbs, such as some cyclamen or crocuses, may be used to extend the flowering season.
Rock gardens
The most popular constructed environment for rock plants is a rock garden. This is best located in an open, sunny site on a slope, clear from the shade cast by trees and sheltered from cold, drying winds. Where possible, include a source of water, such as a pond or stream, to enhance the basic design.
Construct the rock garden on a bed of coarse rubble, covered with standard rock garden soil. Set rocks into the soil in a natural formation. Pockets between the rocks will accommodate the rock plants and provide them with a deep, cool root run. Top-dress the surface after planting with grit, gravel, or stone chips.
Niches or crevices in a wall, or between paving, can provide growing conditions similar to those of a rock garden. Some rosette-forming plants, such as Lewisia and Ramonda, are best planted on their sides in cracks in a wall, so that water drains away quickly from their collars.
Grow mat-forming plants like Sedum acre and Dianthus deltoides in pockets of soil on the top of a wall, or in vertical cracks, to produce a cascade of color. Use trailing or spreading plants, such as Saponaria ocymoies and Saxifraga ‘Tumbling Waters’, to soften the lines of a boundary wall or the retaining wall of a bank or raised bed.
Beds and containers
Raised beds and scree beds are ideal for growing plants on a level site. Rock plants that prefer very gritty soil and a well-drained root run, like alpine forget-me-nots (Myosotis alpestris), will grow best in a scree bed. If the bed is designed to be a free-standing feature, it should be slightly raised to assist drainage. A scree bed may also be constructed as part of a larger rock garden.
A raised bed can be constructed in a garden of any size and is useful where garden soil is heavy or slow-draining. Its added height improves drainage and brings the attractions of low-growing plants closer to eye-level. Grow species and cultivars that prefer acid soil, such as Cassiope and Arctostaphlos, in a peat bed or bog garden, which may be top-dressed with bark chips.
The retaining walls of a raised or scree bed can be constructed from natural or artificial stone, bricks, wooden logs, or railroad ties. As with a rock garden, all beds should have a rubble base, covered with a suitable soil mix and top-dressed with grit, stone chips, or gravel.
Rock plants will also flourish in containers such as sinks, troughs, and tubs. Provide a generous top-dressing of grit around the bases of the plants to improve drainage and reduce the evaporation of moisture form the soil mix.
Cultivation
In general, alpines and other rock plants prefer an open site in full sun, with moderately fertile soil, a cool, deep root run, and sharp drainage.
With some cushion-forming plants, like Dionysia and Androsace, the rotting of one or two rosettes may rapidly lead to the death of the whole plant. Such species and their cultivars are best grown within the controlled environment of an alpine house or cold frame. Some bulbous plants, like Calochortus, also benefit from a controlled environment since they require protection from moisture when dormant in summer.
In the open garden, shelter susceptible plants from moisture by providing very well-drained soil and a thick dressing of grit around the collar of each plant. Panes of glass or clear plastic can also be propped up over the plants, or they can be covered with an open-ended cloche of glass or transparent plastic.
At planting, incorporate a slow-release, balanced fertilizer into the soil or soil mix. Thereafter, apply a general-purpose fertilizer in spring, if vigorous growth is required. Plants grown in a free-draining scree bed may require watering until established. Rock plants grown in an alpine house, in raised beds, or in containers should be watered regularly. In an alpine house, soak the medium in which plants are plunged, as well as the individual plants. For species and cultivars that resent moisture form above, soak the plunging medium only.
Deadhead rock plants, where practical, to encourage further flowering, and remove withered or damaged growth immediately. Trim plants as required to maintain their neat, compact form, and, where necessary, to restrict their spread.
-Information accredited to The American Horticultural Society of A-Z Encyclopedia of Garden Plants. Page 40-41.
