These Gardens Rock

Rock gardens have been cultivated for centuries, reports Gardening Contributor Georgia Raimondi for The Saturday Early Show. The Chinese designed rock gardens more than 2,000 years ago. For centuries, the Japanese and Chinese have admired fine pieces of stone in the same way as we admire venerable trees.

What kind of rocks should you use?

Locally available stone looks most natural and is the easiest to procure and is generally the best value for the money. Stone from other areas usually looks odd and out of place.

Select a variety of stone, preferably with rather flat but irregular faces. Set rocks to form terraces and pockets for plants. Fairly flat pieces can be used at an acute angle, slanting upwards to achieve height.

Design the arrangement of rocks and planting site so that the whole composition provides as natural a setting as possible for the rock garden plants, which are generally considered alpine plants.

Where should you put a rock garden?

A sloping site is more advantageous for several reasons. It has a more natural effect; better able to appreciate the beauty of the alpine plants which are generally diminutive; quick draining. If there is no natural slope, you can create a raised bed or terraced effect with soil and rocks.

Lay the first rocks at the base, trying to keep the strata running in the same direction. Lever the next row into position using levers (a large wooden handle) to move them. As each level is built up, add more of the soil mixture and consolidate around rocks. Make the top reasonably flat, rather than a pinnacle.

Can you use any kind of soil?

Most plants in a rock garden are fairly tolerant of variations in soil, but generally prefer soil that is porous, fast-draining but crumbly and moisture retentive, and not overly fertile. Standard mixture for a rock garden: equal parts soil, compost (for moisture holding and fertility) and fine gravel, coarse grit or builder's sand for drainage.

What types of plants can you use?
Rock garden plants are generally slow-growing plants of relatively small stature. They are called alpine plants because they originate from mountainous regions around the world. They are usually small, hardy, and compact.

• Flowering upright plants, such as primrose, dianthus, veronica, verbena, salvia, dwarf delphinium, dwarf bleeding heart, nepeta (catmint), pin-cushion flower, phlox, campanula (balloon flower), ranunculus, saxifraga, and sempervivum
• Trailing plants, such as vinca and nierembergia
• Creeping ground cover plants, such as sedum (stonecrop), ajuga, and spreading junipers
• Crevice plants, such as thyme and sempervivum (hens and chicks)

In a natural rock garden, it is advisable to avoid placing plants in too regular or geometric patterns. A rock garden should be pleasing to the eye even when devoid of plantings. For a natural effect, avoid plantig too many species with different foliage and flower color in too small a space.

Instead, group together plants of a single kind, leaving enough room between groups so that the plants can spread into irregular drifts, creating a statement with strong foliage and color impact.

Top dress with a generous layer of fine gravel; this will help prevent erosion

Positioning Plants: Position plants while they are still in their pots so you can move them around easily to achieve a pleasing look.

Use a combination of upright, trailing, creeping and crevice plants to achieve a natural look.

On slopes of modest size, it is best to plant the taller plants in the background, ending with tiny tufts of plants in the foreground.

Plant dwarf conifers (miniature shrubs, like evergreens) where they will be silhouetted against a large rock or a rock cropping.

Succulents that grow in rosettes and spread by runners thrive in rock crevices.

Rock plants that have trailing stems (like thyme) should be planted at the top of a rocky ledge so that they will cascade over.

How should you care for a rock garden?

Think of your rock garden as a collection of potted plants, and tend them accordingly. Loosen the soil in each area occasionally with a small garden fork. Most rock plants do well in poor soil, but the occasional addition of manure or compost will give them a boost.

Routine care will include cutting back any leggy plants after flowering, clipping off dead portions, and dividing any plants that become root-bound or too large for their space. Check for insects and diseases regularly. Slugs may be especially pesky because they enjoy the shelter found among the rocks. Never let weeds grow in the nooks and crannies. They will easily crowd out plants in small areas.

Because plants in a rock garden are more exposed than plants in a level bed, they may need more protection in winter. A heavy mulch should be applied before the first freeze.
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