Laying up the rocks for a rock garden is probably too complex a subject for a short article. Besides, the subject has been covered fairly well in standard books which are available from libraries.
But, whereas the books are likely to emphasize what must be accomplished in this direction, indeed, they tread a little light on the how.
Three Fold Purpose
The surface rocks in an alpine garden serve a three-fold purpose. The most obvious feature is to control erosion.
Most alpine plants require drainage, except bog alpines and a small section demands exceedingly sharp and complete drainage.
Hence, the garden is to be planned in three dimensions. It is an up-and-down proposition. These sharp slopes will erode badly during a cloudburst unless retained within a strong rocky sieve.
The plants should not be too much jammed together— we do not play all the notes of the scale in one chord of music — and the areas between the tiny shrubs and perennials may be mulched with flat slabs of rock, provided the soil is light enough to get air to the roots of the plants.
At the moment, I cannot quote the figures on the evaporation of moisture from each square foot of exposed soil (particularly during summer drought), but it is amazingly rapid.
To slow down this process and to provide conditions more like that of their mountain home, we grow alpines among rocks.
These rocks then keep down the temperatures of the soils about the feeding roots and retain the moisture there, without which the plant must starve.
Yes, those shriveled-up-looking specimens which we associate in our minds with drought conditions, in reality, are hungry.
We do not want our rare mountain elves to go without their daily bread, so proper rock-laying is one of the means of their provision. Soil preparation is the other.
Soil Preparation
While rolling up the rocks to their final position, we apply the soil mixtures over the subsoil base of the garden. Carrying on two or three operations simultaneously may be necessary, especially on a small property.
For instance, the shaping of the subsoil base might be carried forward on a block about 10’ feet square.
The sand, humus, and sifted topsoil blended to nearly cover this area, and the rocks laid about the base of the steepest slopes, applying the soil mixture generously before laying up a higher tier.
There is a wrinkle here, which may help prevent the drying subsoil from losing its form while we are blending the topsoil.
Wherever there is a particularly steep exposure of subsoil, as behind a retaining wall, we may take small rounded stones of a type that will be useless for the finished surface and press them into the face which we have just formed in a kind of veneer.
It does not need to have the strength or solidity of the outer face. So what does it matter if a few of them roll out before we are done?
If they are “mortared” with a little surplus garden loam, they will help to keep a cool root run for the plants of the finished wall face by cutting down further on the surface evaporation.
Stone Veneer
Another important function of this stone veneer is to save on sinking the long, narrow rocks endwise into the bank, which has been recommended, especially on light soils, to keep the main face in place.
It is readily seen that a good many of the blocky rocks of the outer face, when somewhat back-tilted, will bear upon these stones — and thus be less susceptible to frost heaving.
Thus the long dimension of the narrower rocks may more often be used in the plane of the wall face itself. This is more economical.
Strength of Rock Work
Each rock, as it is placed, should be balanced well enough to bear a man’s weight without rocking. Therefore, the skeletal strength of your rock work is of great importance in both planting and maintaining the garden.
The annoying habit of frost, which will sometimes have a good-sized boulder down upon your favorite primula, may be somewhat prevented in summer heat if you are particular about laying the boulders themselves.
Do not hurry. Make perfectly sure the rock fits the place you are trying to fill before going on to the next.
Then take the rock out again and pack the cavity beneath it with your soil mixture. Be certain to eliminate all air pockets that can cause you grief and befuddlement later.
The rocks for the wall should be large of the blocky type. They should blend well in color, but if they are of diverse kinds, be not dismayed.
By lining up like kinds or colors of rock, here is your chance to suggest strata, faults, and other geologic trivia without going to any great extremes or worrying over much about the details.
Acquiring much technical data is unnecessary to build realistic-looking drifts of green rock through the gray. Instead, the artistic urge which underlies all your garden-making will come to your aid.
Indeed, once you have begun, it may be necessary to hold that urge somewhat in check so as not to put too much emphasis on it, for it can be carried too far.
A successful rock garden requires too much energy expended for you to waste any of it!
Function of Rocks
The rock wall is often necessitated by the need to carry the sloping bed behind it up nearer eye level, and it becomes the natural home of all those dwarf plants that like to live practically upside down.
While the bed above is the natural place for dwarf, erect plants, a few of these may properly be brought down into the wall itself.
Here is another point that is seldom touched upon in the books.
Small sloping pockets or basins may easily be provided in the building but are hard to introduce after the rock wall is finished.
The tinier ones may occupy a space between two oversize blocks of the main wall, but the larger pockets should be directly in front of a recession in the wall face.
The recessed portion may be nearly vertical, so it should be most exposed to mossy or lichened rock, or it may be sloping like a tiny boulder-strewn ravine and should be planted.
Primulas, saxifrages, and small bulbs like ixiolirion will thrive in these rocky pockets, which should be filled to the brim with light soil and slope from back to front.
It certainly breaks up the formal man-made appearance of the rock wall and helps blend the rockwork’s various levels into an organic whole.
Third Function of Rocks
This brings us to the third function of rocks: providing a natural-looking setting for alpine plants.
Of course, we cannot usually go to the expense of throwing up a rocky cliff to hang them on, and again, most of the exposed bedrock of our lowland home grounds will be found to be nearly barren, the home of mosses and an occasional pinweed.
However, if we are not blessed with a sandstone or limestone ledge, which is a natural rock wall, we can “make do” with portions of a weathered, old stone boundary wall and at least give something of the substance of the rock ledge if not the form.
Here we may introduce the liveliest colors against the neutral gray of the rock, and with some hope that it will continue from year to year looking better and better.
44659 by Richard J. Darling