Part 3

When the paths and outer margins have been thus defined, scatter more rocks over the intervening surface, placing them fairly thick but not close together. Next, fill in with soil, packing it firmly and ramming it hard into every crevice. If it fits in with the day's work, it is not a bad plan to water the rock work well in order to pack the soil, and when resuming the labor on the morrow, to add more soil, well pressed down, before proceeding with the second layer of rock.

This second layer should have the rocks placed with the front edge slightly back from that of the lower row in order to form a slope, though an occasional overhang may be fashioned if required for a certain plant known to abhor a drip from above. The construction then proceeds as before, until the desired height is reached. The height is entirely arbitrary, but some points should be at least as high as the line of vision, as one of the great advantages of a rock garden is the pleasure of enjoying some of the typical rock plants without stooping. The rocks used as fillers should overlap here and there to give strength, but care must be taken to contrive plenty of long soil runs. Eighteen inches should be the very least. A plant like the alpine androsace is a tiny rosette, seemingly requiring no more than an inch or two of soil, but its roots are likely to be found following an earth-filled crevice in the rocks to the depth of a yard or so. It is because of this deep penetration of roots that the soil should be packed so very firm; the roots must be in no danger of loose soil or of striking a hidden hollow.

At no point between two stones should the layer of soil be less than two or three inches thick after being packed hard. If an upper stone is likely to bear down too heavily and crush the plant roots, this may be avoided by placing small stones here and there in the layer of soil. The roots will work between these stones, but there must be a continuous, though not necessarily straight, soil run from the front of the rock work to the solid filling of earth. The run should slope downward slightly.

Rocks calculated to simulate a natural stratification ought to be laid on an incline for proper drainage. Such pieces of rock may also be employed sparsely in wedging, and in the making of the so-called "pockets."

These pockets are of prime importance in the construction of a rock garden. They hold the only considerable spaces of soil and are the chief means of colonizing plants, thus providing for pronounced color effects. They should break the slopes and be irregular in size, shape, and distribution. The large ones may be easily subdivided by small stones when the planting is done if a further separation of species is desirable. The soil must slope a little from the top, so that there will be no standing water.

The drainage of a rock garden is of vital importance. There must be plenty of moisture stowed away behind the rocks against the heat of summer, but all excess must be carried away. The garden should drain naturally, as the hills do. If any doubt exists, make a drainage bed of eight inches of clinkers before starting to lay the stones.

The soil should be a good loam with a little peat, and stones varying in size from a mustard seed to an almond. A little manure may be used, but it must be old.

PLANTING THE GARDEN

There are two ways of planting a rock garden. One is to do all the crevice planting along with the building, and the other, of course, is to defer everything until the rocks are in place and the soil thoroughly settled.

The former plan is a singularly appealing, as well as practical, one. There is something fascinating in finishing completely a part of the work as one goes along. The practical advantage lies chiefly in the fact that by this method good-sized plants may be firmly established in crevices at the very outset. The soil in that case should be put part way in the crevice and packed down. Then some loose soil sprinkled on top, and the plant, with the earth well shaken from the roots, unless it has a tap root, laid down horizontally with the crown just outside the edge of the soil. Next spread the roots to follow the soil run; fill up the crevice with more soil, packed well, and follow with more plants of the same kind. Use small stones to wedge plants where it appears necessary. Plants that hang down should be placed in the higher crevices; this must be all thought out beforehand.

As a matter of fact, the planting plan cannot be too thoroughly thought out in advance. At point after point it dovetails with the structural plan, which must accord with the requirements of what may be called the more difficult rock plants--the alpines, some of the ferns, and those plants that fit in well with rock work but demand more than the ordinary garden moisture. The best way is to decide what plants are most desirable in the circumstances, omitting, as a rule, the difficult or "finicky" ones; there will be plenty of time to experiment with those when you have more experience. Make a face plan of the several sections of the rock work and mark on it where the plants are to go. Use numbers, each corresponding to a species.

The general idea is that all the soil shall be concealed, not necessarily at the moment of planting, but at the end of one or two seasons' growth. Unless you are a collector, variety is of little importance. The main thing is that there shall be beauty as a whole, a few marked seasonal effects of color with massed bloom and some green the year round; the garden must never be bare at any time, as nature will show you. Plants clustered here and single there is a good planting rule. Colonies, always of marked irregularity, ought to merge into one another, but they should not so overrun the rock work that no stones are in sight. Not infrequently some of the best effects are obtained where more rock than flowers is seen. A boulder, for example, calls for the contrast of plants, perhaps only a few low-growing ones in a natural pocket, rather than a semi-eclipse. As a rule, plant one hundred of half a dozen or so suitable, and easy, species in preference to fifty or more kinds.

Study at the same time the form of the plants that are to be used; some quickly resolve themselves into a carpet, some never get beyond mere tufts, some always grow straight up, some prefer to hang down, and some have foliage that is evergreen or nearly so. To be more specific, one plant of Saponaria ocymoides will spread out over four square feet of soil, and thus fill completely a moderate-sized pocket, whereas to conceal the same amount of ground three dozen auriculas might have to be used. The same is true of the white rock cress (Arabis albida). So, too, with a crevice. A single plant of one of the trailing stonecrops would fill it, perhaps, when a number of rosettes of the smaller kinds of house leek would be called for.

Tall plants, like the foxglove, may sometimes be used, in a small group, at the end of a bay on the level of the path; but they are best placed behind the rock work, as a background, or as dominating features of the entrance or exit of the garden. At the entrance or exit such bold plants make a good bridge between the rock garden and the outer grounds. Spreading and trailing plants should be placed a foot or more above the path level and most plants with tufts or rosettes of foliage. If the path is broad enough some of the wide-spreading plants may go at the base of the rocks, but the rule there is to use those of moderate spread, with a few tufted plants and some that grow upright, but are not tall, to lend variety. When the path is of flat stones, irregular in both size and placing, this growth should fill all the soil space--even between the stones. Such a path will be found more than worth while, and not as much of an undertaking as it may seem.

Obvious considerations are that plants with a decided hankering after moisture or shade should be favored in the matter of location, though it is astonishing how adaptive many of them are.

Do not plant the weak next to the strong. Unless you are a gardener of eternal vigilance, the weak will have the worst of it before you realize what a mistake you have made.

Finally, do not forget that planting is not the end; it is only the beginning--of planting. So long as the rock garden exists there will always be planting. Normal mortality will necessitate some, there will be thinning out, and time will suggest additions and more or less rearrangement.

And with the planting goes on the continual care, much of which can be done in the course of the daily walk in the garden, and therefore the loss of time will not be felt. Water in case of a real drought, but use a sprinkler, and do not stop until the ground has been soaked to a depth of a few inches. Mere surface watering is bad enough in the ordinary garden; in a rock garden it is a fatal error, as the growth of roots near the top of the soil leaves the plants in no condition to stand the full force of the summer sun.

Go over the garden thoroughly once a year and all the time keep a sharp lookout for weeds. If the soil is heavy, top-dress with grit in the fall. Grit is good for rock plants. Stone chips placed around a plant will prevent too much dampness lodging about the collar in winter. Watch out for weak spots after very heavy rains.

PLANTS FOR A ROCK GARDEN

So many plants are suitable for a rock garden that the range of choice is bewildering. In this, as in the laying out of the garden, advisability takes precedence over pure personal desire, though, very fortunately, it is often not difficult to make the two go hand in hand; a little intelligent thought helps a lot.

To the beginner, no better advice can be given than that which applies to the picking out of the rocks--use the material which is close at hand. This is not, by any means, a mere suggestion to follow the lines of least resistance. It is far more. In the first place, there is always an endless amount of beautiful and suitable plant life to be had without going far afield. Then again, natural harmonious effects in your immediate neighborhood are pretty sure to be appropriate to your grounds. Finally, you can see for yourself how things grow, and as for the hardiness of plants, you have it already tested for you. This refers not alone to the natural conditions; there is a second wide field in the gardens--the hardy gardens--of others, where you can at once choose from the many and learn whether certain plants are too tender or require too much care for your use.

So far as plants native to the immediate neighborhood are concerned, their value to the rock garden of the average person with limited time, who is not obsessed with the idea of growing the rare and curious, cannot be overestimated. And they are so many; more than most realize, and often of an individual beauty not always appreciated in the bewildering profusion of the wild but plainly apparent when an individual, or a little group, is open to close study in a rock garden. Do not make the rather common mistake of thinking that they are too familiar to be interesting; they are never likely to be. And, honestly, can you say in your heart that they are?

For a Connecticut rock garden the Greek valerian (Polemonium reptans) must be purchased, unless a neighbor can spare some from his collection of old-fashioned flowers; there it belongs in that category. But why should you of Minnesota or Missouri deny so beautiful a flower a place in your rock garden, simply because you have only to go to the woods for it? The English enthusiast brings home primroses from the Himalayas, gentians from the Swiss Alps, and Dryas Drummondi from the Canadian Rockies for his rock garden, but he does not fail to take advantage of some of the common things near-by--even the "pale primrose" and the cowslip.

From ferns alone, or from only plants of shrubby growth, a most beautiful native rock garden may be made. And adding small flowering plants, or excluding all else, there are limitless opportunities. It goes without saying that A's rock garden in Maine will not be like B's in Louisiana; but there is no law compelling it to be.

Among the common wild flowers of the East that take on unexpected new beauty when transferred to the rock garden are the celandine (Chelidonium majus), strawberry (Fragaria Virginica), cranesbill (Geranium maculatum), toadflax (Linaria vulgaris), orange hawkweed (Hieracium auranticum), herb Robert (Geranium Robertianum), coltsfoot (Tussilago Farfara), Solomon's seal (Polygonatum biflorum), foam flower (Tiarella cordifolia), bloodroot (Sanguinaria Canadensis), and some of the violets. These are but a few names, and random ones at that. Some of them, the coltsfoot, cranesbill, celandine, and toadflax, spread too rapidly, but by careful watching and not allowing the seed to ripen, they may be kept within bounds. There are many such plants that will take all the room in sight if they are allowed to, and they must be watched closely, or else discarded altogether. Some of them answer a good purpose by giving the rock garden a quick start, after which they may easily be reduced or thrown out altogether. There need be no compunction about discarding. Certain plants, like certain friends, you enjoy having for a visit, but do not care to see remain forever and a day.

Annuals as a class are not desirable for the rock garden; for one thing, the care of renewal is too great. Biennials are almost as much care, but in each case there will always be exceptions that are a matter of individual preference. Few, for example, would have the heart to reject the dainty little purple toadflax of Switzerland (Linaria alpina), just because it is a biennial. The main dependence, however, must be placed on perennials--the plants that, barring accidents, last indefinitely. These should be mostly species; if horticultural, do not use the bizarre--Darwin tulips, for example, or the Madame Chereau iris. Nor, with rare exceptions, should double flowers be used. A double daffodil looks horribly out of place, while the double white rock cress (Arabis albida) will pass.

The easy rock garden plants, where the material is not taken from the wild, are to be found in most of the large hardy gardens of the East. Some of them are natives of Europe or Asia, and more than is commonly suspected are at home in other parts of the United States. Among the best of these for carpets of bloom are Phlox subulata, Phlox amna, Aubrietia deltoidea, maiden pink (Dianthus deltoides), blue bugle (Ajuga Genevensis), white bugle (Ajuga reptans), woolly chickweed (Cerastium tomentosum), creeping thyme (Thymus serpyllum), dwarf speedwell (Veronica repens), Saponaria ocymoides, alpine mint (Calamintha alpina), and pink, white, and yellow stonecrops (sedum). All of them fairly hug the ground. There are other plants that form a carpet of foliage, but the flower stalks rise higher. These include white rock cress (Arabis albida), the permissible double buttercup (Ranunculus acris fl. pl.), the also permissible double German catchfly (Lychnis viscaria), another double flower, "fair maids of France" (Ranunculus aconitifolius), Carpathian bellflower (Campanula Carpatica), grass pink (Dianthus plumarius), Iris pumila, crested iris (Iris cristata), Christmas rose (Helleborus niger), Phlox divaricata, Phlox ovata, Phlox repens, foam flower (Tiarella cordifolia), Veronica incana, Alyssum saxatile, Saxifraga cordifolia, and various avens (geum).