Do you need a beginner’s guide to rock-garden plants? Plants that are typically suited to rocky places are tough characters. Understanding their characteristics will help you select plants for the railroad garden that will work well in the crevices of rocks, creeping along cliffs, or draped down rock faces. They often come from mountainous origins and are accustomed to shallow, gravelly soil and rigorous climates. They are typically small, keeping a low profile in their hostile environment, yet surprisingly, are often evergreen. Other rock plants come from warmer, dry regions of the world. Some of these are silver in color due to tiny hairs on their leaves that help them retain moisture. Still others are succulent and are able to store moisture in their leaf and stem tissues.
Although these plants are tough and will put up with a lot of abuse and neglect, they won’t survive prolonged high humidity or wet feet. Wet or boggy soil will often cause root rot, and hot, humid summers may encourage diseases. If you give these plants good drainage, lean soil, and avoid overhead watering, though, they’ll repay you with year round vigor and long life.
Let’s begin with the soil. A typical recipe for rock-garden soil starts with two parts good garden soil, two parts compost or sphagnum peat moss, and one part fine gravel or course sand. Do not add fertilizer. This mixture is layered over existing soil 2-4″ deep in and around rocks. Make sure there is excellent drainage, as would occur on slopes. If the planting area is elevated but the surrounding ground is flat, first put down a 2″ layer of medium gravel to assure good drainage.
Next, consider location. Rock or alpine plants will likely do well when planted at the top of a wall or rock face. The same plants may struggle or die if planted at the bottom of that same wall (from wet roots). These plants prefer good air circulation and sunshine; they won’t survive if overshadowed by aggressive, larger plants. Many alpine plants are used to being blanketed by snow all winter and may get winter burn if exposed to constant, drying winds. If you live in the north but don’t usually get good snow cover, they’ll do better planted in the lee of a rock where they will still get some sunshine and be protected from prevailing winds. If you live in the south, warm-climate rock plants, such as succulents (see GR, April 2005), might work better for you.
Rock plants in the garden railway
There are many alpine and rock-garden plants that make great garden-railway groundcovers. Others are especially suited to growing in rock crevices and planted stone walls. Some are rambling and look great cascading over a rock face.
We can also look to the herb garden for candidates for our railroad rock gardens. In warmer parts of the country (Zone 6 or 7 and higher), rosemaries (Rosmarianus sps.) are nice plants to replicate scale trees, with their tiny leaves and woody stems; the trailing varieties (Zone 8 and up) are beautiful rambling across rocks or hanging over a wall. Lavender (Lavandula sps., Zones 5-9), lavender cotton (Santolina sps., Zones 5-9), smaller sages (Salvia sps., Zones 4-10), and creeping thymes (Thymus sps., Zones 4-9) are useful rock-garden plants in well drained, sunny gardens. Planting these sun-lovers where they will get reflected warmth and wind protection from a stone wall may allow you to grow them in USDA Hardiness Zones that are borderline or one zone lower than their usual rating.
The combination of rocks and plants has a special appeal. Plants growing on vertical stone surfaces give the appearance of age. They soften the harsh rock face and add interest. Planted walls provide extra growing space for plants, adding vertical interest to the surrounding horizontal areas.There are many uses for stacked-stone walls in the garden railway.
A planted stone wall consists of three elements: flat, dry laid (i.e., without mortar), stacked rocks; rock-garden soil; and appropriate plants. The wall is laid one course at a time, with 1-2″ rock-garden soil layered between each stone course. Soil is also packed into wider vertical crevices. After the soil is layered on each rock course, plants are chosen, their roots teased out a bit, flattened, and planted in the soil. A rock is then settled on top of the soil and root ball as the next course is being laid. Plants are added randomly where gaps between rock layers are wider, or where vertical gaps exist. As each course is finished, rock-garden soil is back-filled behind the rocks to give the plants’ roots something to grow into. The soil is tamped well to remove air pockets. (For more details on planted, dry-laid stone walls see “Rock garden techniques, Part 2” in GR, June 1999.)
During the first year of growth, plants in walls should be watered regularly until their roots get established in the deeper soil behind the rocks. Once established, most rock and alpine plants are quite drought resistant. It is worth mentioning, though, that these plants are not the only life forms that happily make their home in rock walls. In my part of the world, chipmunks and snakes quickly find crevices that are to their liking. If these creatures are a problem in your area, you may elect to build mortar-laid walls.