Rock Garden Do’s and Don’ts
I cannot think of another area of garden-making with as many rules about what you can and cannot do to have a successful garden than rock and alpine plant gardening. They are serious rules which, if broken, ensure failure — you have been warned.
Just as you cannot use old building rubble to build the rock garden, you can and should use local stone because that is what will look natural. Even though what you are about to construct is an artificial element in the landscape, you must strive to make it look natural.
There are five main types of rock. Granite is the hardest and weathers slowly. Slate is the next hardest and is unusual for its gradations of colour from a dark grey-green to nearly black. Limestone is quite soft and weathers well, but you must take care not to grow lime-hating plants, while sandstone is probably the softest type of all. Tufa is a sort of limestone which lime-hating plants will tolerate: it looks like a sponge and does, in fact, hold masses of water.
The site of a rock garden is important. It must not be in the shade or under the drip of overhanging trees. Preferably, it should be in full sun, although part shade is acceptable. A shelter belt of evergreen shrubs or trees on the side of the prevailing wind will give protection from damaging winter winds.
Perfectis absolutely critical, because if there is one thing alpine plants will not tolerate it is sodden . Paradoxically, they do like to have plenty of moisture during the growing season, but the water must not linger. To avoid this, you must provide a free-draining soil with plenty of humus in the compost mix.
Regarding compost, most alpines like lime, so will do well in alkaline soils, while a few must have an acid soil. Generally, lime-loving plants will grow in soil that is slightly acid. Soil alkalinity is expressed in degrees of pH, from 0.14, a pH of 7 being neutral. Above that number the soil is progressively alkaline, below it is acid. The pH can be tested by means of easy-to-use testing kits available from garden centres, so prepare your compost according to the plants you intend to grow.
Composts can be purchased specifically for growing alpine plants and for growing lime-hating alpine plants (this will be identified as ericaceous compost), but you may want to blend your own, especially if you require a considerable quantity. The basic ingredient of compost is loam or clean topsoil, free of weed seeds and soil-borne pests; then moss peat (not sedge peat which is very acidic) or well-rotted; then grit or sand. Never use yellow builder’s sand for horticultural purposes. The proportions are 2:1:1. When blending the compost add a few generous handfuls of bone meal.
Most alpines you purchase will be container-grown, which means that you can plant them at any time of year. However, it is always best to plant either in the autumn or spring, when nature will help you with the watering and when there is less risk of dry hot spells. If you do plant during the height of summer, water the plants daily and give temporary shade for the first few days in their new home.
Most garden centres provide a selection of rock garden stone, but it may be less expensive to go direct to a local supplier or quarry. That way, too, you can be assured of obtaining local stone. The cost of haulage can be quite steep, so the closer to home you obtain your stone, the less expensive it is likely to be.
Always use the same type of stone throughout a rock garden and be sure to obtain a range of sizes. You will need a team of at least three people — using trolleys, wheelbarrows and crowbars — to shift the largest rocks. To a large extent, a successful finish will depend upon how carefully you place the rocks, with an eye to keeping their natural strata aligned and, just as important, their relation to one another in proportion. To do this you will need two people to coax the rocks into the desired placings, while you stand back and, with your expert’s eye, direct operations. Seriously though, in this case, many hands really do make light work and you will need to step back frequently to assess the positioning of the rocks.
All too often I see rock gardens, which look horribly unnatural because each stone is placed in lonely isolation, plopped down and separated from its neighbours by a foot or more of soil in which a miserable little plant appears aimlessly adrift. Fortunately, there is more artistry to creating a rock garden than that. To get a clear idea of what I mean, make a visit to your local botanic garden and study how they have positioned the rocks in carefully arranged outcrops, partially buried in the surrounding soil so that the rocks appear as they would in nature. Also notice how the lines and fissures — the strata — of the rocks all run in the same direction and how each rock is tilted back ever so slightly to assist water run-off. These botanic garden creations will no doubt be much larger than anything you will be attempting, but the rules of construction are the same for five stones as for fifty. Remember, you are trying to mimic nature, so you must either observe what nature does or study what professional rock garden makers have done.