Building a Rock Garden
A flourishing rock garden gives tremendous interest. In nature, rock plants thrive where the soil drains sharply; where they are exposed to the full rays of the sun; where they are blanketed by snow in winter. It may not be possible to reproduce these conditions exactly, but we must do what we can. Ideally, site it away from the drips of overhanging trees and shrubs. Put it where it won’t be shaded for a large part of the day and where the soil doesn’t collect water after a heavy spell of rain.
A naturally sloping site is preferable to a level one, but a 'table’ rock garden can have great charm. If the ground faces south or south-west, so much the better! A rock garden can be as large or as small as you like, but don’t make the mistake of building it with very small pieces of rock. These look absurd and do not give the impression of arising from a solid base.
Let’s imagine we are constructing one in a sunny part of the garden, sloping down to the lawn. Start by outlining the area with string and canes. If the soil needs draining, remove the top layer and put it on one side for later use, then replace the sticky, badly drained subsoil by a 38cm/15in layer of broken rubble or some other material.
Cover the rubble with a single layer of turves, grass side downwards, to stop the soil that you add later from washing through into the drainage material. Make sure the rock garden is not overshadowed by more dominant features such as a shrub border, garden shed or rose arbour—it must command attention!
It’s tremendous fun recreating in miniature a mountain valley, an upthrusting spur, a winding, tubling stream, an alpine lawn, a flower-decked ravine. But whatever you decide to tackle, it must look right. Lumps of stone placed haphazardly look hideous. If possible, use locally quarried stone. It not only looks more natural but is a lot cheaper than stone which has to be dug a long distance away. Sandstone and limestone mellow quickly and look well. Granite, being very hard, retains a raw harsh look for many years.
If possible, preshape the rock garden first. Mould the pockets (tiers), spurs, valleys and scarp faces with soil before you lay a single rock. Then you’ll have a good idea what the end result will be. If you’ve never laid rocks before, and cannot conceive what that heap of stone will look like, make a model of the garden in bits of polystyrene.
Glue the pieces together and the effect is realistic. Some rocks can weigh a hundredweight or more, so get a friend to help you maneuver them into position. Tool? You will need a spade, fork and a rammer made from a bulk of wood 10cm/4in square 150cm/5ft handle.
Start at the foot of the rock garden. Place a shapely stone in position. This key stone should be tilted backwards slightly. Then it will resemble a rocky outcrop. Sink it one third deep in the soil and ram more soil round so it doesn’t wobble. It must be 'rock firm.’ Then continue with loop or L-shaped tiers of rock, each one slightly higher than the one before.
If you’re uncertain how to achieve this, take a look at a rock garden at a botanic garden or, if you live in a hilly or mountainous district, at a naturally eroded bit of mountainside. You will soon see how to achieve a realistic setting.
Each rock has lines running through it—the strata. Make sure that you align the strata; don’t have some rocks with their strata running horizontally, others vertically. In nature, the strata of an outcrop are always consistent. When all the rocks have been placed, well rammed into position, and all sloping backwards slightly so that rain washes the soil back into the pockets rather than away from them, leave the soil and rocks to settle for a fortnight or so before planting.
You will have chosen dwarf shrubs to punctuate the rock garden; cascaders such as alyssum, aubrietias and saponaria to flow over the outcrops; cushion saxifrages and others to contrast with the trailer and grow over the flat tops of rocks; and some bulbs such as cyclamen to grow up through the carpeters. Start by planting the shrubs. Set dwarf conifers at the base of large rocks and umbrella, or mushroom forms to spread over higher promontories.
Rosette forming ramondas and lewisias hate getting their leaves wet, so plant them with their roots tucked into a vertical fissure. No matter what you plant, so make sure that the roots are well spread out and covered with a good depth of soil.
In their natural state, rock plants are watered and fed continuously by nutrients dissolved in water derived from melting snow, but because the soil is rocky the roots are never waterlogged. We can reproduce this situation by burying a perforated hosepipe in the garden when we are building it.
One end of the pipe is left sticking out and connected to the tap, or another length of pipe, if the tap is some distance away. Then it is possible to supply water at roots level where the plants need it. None is wasted and no soil is washed away.
It also pays to surround rock plants, specially the damp-sensitive, woolly leaved kinds that can rot if too much water gets on to their leaves, especially in winter, with a layer of fine chippings to discourage slugs and wet soil in direct contact with the necks of the plants.
For some odd reason, many people think that rock plants do not need feeding. They do, just like any other garden plants. The best way is to sprinkle a long term fertilizer such as bone meal, round their roots in spring. This will last the whole season, to nourish growth to perfection.